Handbook p11 electromagnetism

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Electromagnetism Handbook P11

“ I happen to have discovered a direct relation between magnetism and light, also electricity and light, and the field it opens is so large and I think rich. �

-Michael Faraday (1791-1867)

1

HANDBOOK P11 ELECTROMAGNETISM OVERVIEW OF HANDBOOK ABL

CONCEPT

ABL 1 ABL 2 ABL 3 ABL 4

Magnetic Effects of Electric Current Mechanical effect of electric current Electromagnetic Induction AC and DC Dynamo

NO OF ACTIVITIES 4 2 3 2

TIME (min) 70 45 55 45

ABLs WITH REFERENCE TO STANDARD S.No. 1 2

STANDARD XI X

RELEVANT ABL ABL 1 and ABL 2 ABL 3

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PAGE NO

2

LIST OF FIGURES, CHARTS AND WORKSHEETS S. No

Name

Page No

Note to Instructor: All the figures in this handbook are for the Instructor’s reference only. The Charts need to be printed and shown to the learners during the course of the activity. Worksheets need to be printed out in advance for the learners. The number of worksheets required is mentioned in the Material List.

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3

ABL1: MAGNETIC EFFECTS OF ELECTRIC CURRENT Activity

1.1

Learning Objective Demonstration of the magnetic effect of electric current and also Ampere’s swimming rule

1.2

To study the distribution of magnetic field around a straight current conductor.

1.3

To study the magnetic field distribution at the centre of a circular conductor carrying current.

1.4

To study the distribution of magnetic field due to a solenoid carrying current.

Key Message  The current carrying conductor produces a magnetic field around it.  The direction of the field is given by Ampere’s swimming rule.  The magnetic field around a straight current conductor comprises of concentric circular lines of force of the field everywhere in a plane perpendicular to length.  The direction of the field lines is given by right hand clasp rule.  The field strength is maximum near the conductor and decreases on moving away  A current through a circular coil of a conductor produces a strong, nearly uniform magnetic field at the centre.  The two faces of the coil can be identified as North and South poles.  A current carrying solenoid which is an electromagnet behaves similar to a bar magnet.  A current carrying solenoid can magnetize a ferromagnetic rod placed inside and thus produce artificial magnets.

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Time (min)

15

25

15

15

4

ELECTROMAGNETISM Background information to the Instructors Electric current, a drift of free electrons through a metallic conductor created a sensation and excitement because of the heating effect and the in turn light, that lead to the invention of electric bulb, electric heaters¸ and several other devices. But the discovery of magnetic effect while helping to understand what exactly the current does also leads to invention of electromagnets and artificial magnets which find immense applications in scientific and industrial field. The fact that magnetic effect or magnetism is due to current i.e. electrons in flow made scientists to think of the possibility of seeing the reverse effect-that is producing current in a conductor from a magnet. Finally, the scientists (Faraday, lenz ) succeeded in their efforts to induce electric current in a conductor by varying the magnetic field surrounding it – the effect known as electromagnetic induction.

The study of producing magnetism by electric current and electric current in a conductor by a magnet, the two reversible effects, combined into one is what we today call as “electromagnetism”. It has three sections: i) Magnetic effect of electric current ii) Mechanical effect of electric current and iii) Electromagnetic induction

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5

ABL 1.1 LEARNING OBJECTIVE: Demonstration of the magnetic effect of electric current and also Ampere’s swimming rule ADVANCE PREPARATION Material List S.No 1 2 3

Material Oerested model 4 Battery box with 4 batteries Magnetic compass with pivot

Required quantity 1 per class 1 per class 1 per class

Things to do Place the Oerested model on the table in the clear vicinity of all the students.

Safety precautions N.A SESSION Link to previous activity In earlier classes/ where magnetism is introduced you might have learnt that  A freely suspended magnet always comes to rest in north-south direction.  The pole of a magnet which points towards north direction is called North Pole or north seeking pole and the pole of a magnet which points towards south is called South Pole or south seeking pole.  Like poles repel and unlike poles attract each other.

Procedure  Place the magnetic needle on the pivot and set the stand below the conductor.  Wait till the magnetic needle settles in North-South direction.  Adjust the position of the conductor to be parallel to the magnetic needle (N – S direction)  Pass a current through the conductor in S - N direction.  Observe the movement of the magnetic needle.  Reverse the direction of current and observe the direction of deflection of magnetic needle.  Repeat the experiment placing magnetic needle above the conductor.  Repeat the experiment using more number of cells.

UNDERSTANDING THE ACTIVITY Leading questions  In which direction does the magnetic needle rest when no current is passed through conductor?

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What happens to the needle as you pass the current? Why does the needle deflect?

What happens to the needle when you reverse the direction of current?

What difference do you find when the needle is kept above the conductor and pass the current?

What do you observe when the number of cells in the battery is increased?

Is there a way to determine the direction of the magnetic field?

Discussion and explanation

Oerested model

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Magnetic needle below magnetic needle above the conductor The needle comes to rest in the direction of earth’s magnetic field. It’s North Pole pointing to geographical north and South Pole pointing geographical south.

When the current is passed through the conductor, the magnetic needle is deflected. It indicates that the current through the conductor has produced magnetic field causing its deflection.

A charged particle in motion produces a magnetic field around it. A conductor has a number of free electrons moving about in random directions. When the ends of the conductor are connected to battery, current flows through; it means free electrons drift in the direction opposite to direction of current. The magnetic fields produced at any point outside due to all such drifting electrons have the same direction and they add up to generate a net (non-zero) magnetic field. Magnetic field is a force field and has direction. The North Pole deflects in the direction of the magnetic field produced by the current.

When the direction of current is reversed, the field direction is reversed. The direction of forces on N-S poles is reversed. As a result the needle deflects in opposite direction.

When the needle is positioned above the conductor, the direction of the field is reversed and the North Pole deflects in opposite direction.

Using larger number of cells helps to pass larger current and thus a larger magnetic field.

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7 The direction of the field indicated by the deflection of the North Pole is determined by Ampere’s left hand swimming rule.

Ampere’s left hand swimming rule: Imagine a person swimming along the direction of the current facing the magnetic needle, the North Pole is deflected towards his outstretched left hand.

KEY MESSAGES  A current in a conductor a stream of electrons. Produces a magnetic field. The field around the conductor is the total field of all the electrons constituting the current.  The direction of the field is given by Ampere’s swimming rule.

Learning check: 1. Are magnets readily available in nature?

2. Can current conductors replace bar magnets?

3. Does a stream of proton generate a magnetic field around? If so, do the protons in the conductor contribute to the magnetic field around a current conductor?

Time: 25 min

ABL 1.2

LEARNING OBJECTIVE: To study the distribution of magnetic field around a straight current conductor. (In a plane perpendicular to the length of the conductor) ADVANCE PREPARATION Material List S.no 1 2 3 4

Material Field around a straight conductor model Battery 12V Connecting wires compass needle

Required quantity 1 per class 1 per class 2m 1 per class

Things to do: Place the model on the table in the clear view of all the students. Explain the parts of the model to the students. Divide the students into 5 to 6 groups with 6 students in each group. Call a group near the table and instruct one of them to demonstrate the experiment on instruction from the teacher.

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8 Safety precautions: N .A SESSION Link to previous activity  In the previous ABL it is learnt that a current when passed through a conductor always produces a magnetic field around it. Here in is studied how this magnetic field is distributed around the conductor.

PROCEDURE  Spray a thin layer of iron filings or dust on a white paper spread over the horizontal platform of the model. Observe how the filings are aligned.  Pass a steady current through the straight conductor by connecting a 12V battery.  Gently tap the platform and allow the iron filings to align freely. Observe the pattern of the alignment.  Take a compass needle, place it at different points along a circular path around the conductor and each time mark the direction (with an arrow head) in which the north pole of the magnetic compass needle rests.  Switch off the current. Disturb the alignment of the iron filings. Pass the current through the conductor in opposite direction by interchanging the polarities.  Gently tap the board and observe the alignment of iron filings.  Using a magnetic compass mark the directions of North Pole by placing it at different positions around the conductor on the platform.  Compare the direction of alignment the North Pole in the two cases before and reversing the direction of current.

UNDERSTANDING THE ACTIVITY Leading questions  What happens when the current is passed through the conductor?  Why do the filings form a symmetric pattern when the board is gently tapped?  What kind of geometrical pattern is formed?  Can we assign any direction to the closed lines of the symmetric pattern?  What is the purpose of marking the direction of the North Pole by taking the compass around the conductor?  What difference do you observe in the direction of the lines in the pattern, before and after reversing the current?  How these lines are dispersed (density at regions) near as well as away from the conductor?  What can we infer from all these observations?

Discussion and explanation  The iron fillings are distributed randomly before the current is passed.

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On passing current a magnetic field is produced around the conductor. Each piece of iron filing is magnetized. When gently tapped, they become free and tend to align along the direction of the magnetic field.

The iron filings form a geometrical pattern of closed concentric circles. They are called field lines.

These lines are dense close to the conductor suggesting a large field strength that decreases on shifting away from the conductor.

The magnetic field around the conductor in a horizontal plane can be visualized as comprising of concentric field lines which are dense near the conductor. From the knowledge of the compass needle, (the direction North Pole points) the direction of the field is indicated by an arrow mark associated with each closed circle. When the current through the conductor is upward the field lines are anticlockwise and when the direction of the current is down ward the field lines are clockwise.

The direction of the magnetic field lines is given by right hand clasp rule.

Clasp the conductor in the right hand and stretch thumb pointing to the direction of the current in the conductor. The direction in which the fingers encircle the conductor indicates the direction of the lines of force of the magnetic field.

KEY MESSAGES  The magnetic field around a straight current conductor comprises of concentric circular lines of force of the field everywhere in a plane perpendicular to length.  The direction of the field lines is given by right hand clasp rule.  The field strength is maximum near the conductor and decreases on moving away.

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10 Right hand clasp rule: Clasp the current conductor in the right hand such that the thumb is stretched in the direction of the current. The direction in which the fingers encircle the conductor gives the direction of the field.

Time: 15 min

ABL 1.3

LEARNING OBJECTIVE: To understand the magnetic field distribution at the centre of a circular conductor carrying current.

ADVANCE PREPARATION Material List S.no 1 2 3 4 5 6 7 8 9

Material Circular coil model Battery 6V Rheostat Ammeter 0-500mA Key Connecting wire Magnetic compass Card board 12cmX8cm Iron filings

Required quantity 1 per class 1 per class 1 per class 1 per class 2 meter 2 or 3 1 per class 50 gm per class

Things to do: Place the circular coil model on the table in clear visibility for all the students.

Safety precautions: N .A SESSION Link to previous activity In the earlier activity you have seen the distribution of magnetic field around a straight current conductor. Here you are studying the distribution at the centre of a circular conductor of current.

PROCEDURE  Connect the battery, key, rheostat and ammeter in series between the free ends of the coil.  Spray iron filings on the card board all around the coil.  Gently tap the board and observe the iron filings.  Switch on and pass about 1.2 to 1.5 A of current through the coil. (Adjust the rheostat for varying the current)  Gently tap the board and observe the pattern formed by iron filings.  Place the compass needle on the card board at some distance from the coil, and mark the direction of the N pole. Shift the compass and place it on the previous mark. Again mark the North Pole. Repeat the trials by going around the conductor. A set of points are marked around conductor which lie along a circle. Associate an arrow in the direction of North Pole.

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11  Shift the compass near to the other side and repeat the same.  Place the compass at the centre of the coil and mark the direction of north pole.

UNDERSTANDING THE ACTIVITY Leading questions  How do the iron filings sprayed on the board appear, before the current is passed?(gently tap)  What pattern do you observe on tapping the board, when the current is passed through the coil?  What do the lines of alignment of iron filings indicate?  Are these magnetic field lines equally dense in all the regions? If not discuss.  Is the direction of current at A same as that as that at B?  i) Compare the magnetic field around a straight conductor and circular conductor? ii) Which of these arrangements do you prefer to produce uniform magnetic field? iii) In which region is the magnetic field uniform?  What is the direction of the magnetic field at the centre of the coil?  If the coil behaves as a magnet, is it possible to identify the poles?  Can you increase the strength of the magnetic field at the center?

Discussion and explanation  Before the current is passed through the coil iron filings lay scattered randomly on the board.  On passing current, due to the magnetic field produced around the conductor, the iron filings get magnetized. Each iron filing behaves as a tiny magnet.

When the board is tapped gently the tiny magnets align along the lines of force of the magnetic field which are concentric circles (called field lines). The pattern observed shows the way the magnetic field lines are distributed in a plane perpendicular to the plane of the coil passing through the centre.   

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The field lines are concentric circles around the sections A and B. The circles spread out on moving away from A or B. The north pole of the compass indicates the direction of field lines.

A magnetic field is said to be uniform if the field lines are equally dense, straight and parallel.

Clearly, the magnetic field produced by the coil is uniform at the center and non-uniform away from the center of the coil.

When the compass needle is positioned at different points around the coil, the North Pole always points in the direction of the field.

The concentric circular field lines are associated with an arrow which is the direction of the field at the place.

When directions are marked the field lines pattern can be looked upon as shown in figure.

The directions of the field lines shown by compass are in agreement with right hand clasp rule.

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The current leaves the board at A (upwards) and enters the board at B.(down words) The field lines are dense at the centre. Over a small region at O, the field lines are almost parallel. It means the magnetic field at the centre is nearly uniform and perpendicular to the plane of the coil.

If the current in the coil is clockwise the field at O is directed away. If the current in the coil is anticlock-wise the field at O is towards the observer.

The end face of the coil where the field lines come out behaves as North Pole and the face where field lines enter behaves as South Pole.

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In the case of a straight conductor the lines are dense near the conductor. The field is strong near the conductor and becomes weaker and weaker on going away.

In the case of a circular coil the field due to all the parts reinforce at the centre. The field is much stronger at the centre of the coil. Smaller the radius larger is the field. Larger the number of turns and current, larger is the field at the centre.(BαI, Bαn, Bα1/radius) The field lines are nearly parallel at the centre. Magnetic field at the centre is nearly uniform. The compass shows the needle resting along the axis of the coil (see figure).

KEY MESSAGES  A current through a circular coil of a conductor produces a strong, nearly uniform magnetic field at the centre.  The two faces of the coil can be identified as North and South poles.

Know more: 1. The current in a coil produces a magnetic field at the centre. (B ∝ I) stronger the current, larger the field.

This property is used in designing T.G which is used to measure current.

Learning check: 1. What kind of a magnetic field is produced when an AC current is passed through the coil?

2. Suppose a coil has 20 turns. The current in first 10 turns is clock wise and the next 10 turns is anticlockwise. What is the nature of magnetic field at the centre?

3. A current of 2A produces a magnetic field of 0.4 units at the center. What current is required to produce a field of 0.8 units?

4. A wire of 1m is wound into a coil of 10 turns. Another wire of same length is wound into coil of 20 turns. When a same current pass through them, which coil produces a larger magnetic field at the center?

5. Compare the field produced by the two coils in question 4?

ABL 1.4

Time: 15 min

LEARNING OBJECTIVE: To study the distribution of magnetic field due to a solenoid carrying current.

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14 Material List S.no 1 2 3 4

Material Solenoid model Connecting wire Soft iron core Compass needle

Required quantity 1 per class 1 meter per class 1 per class 1 per class

Things to do: Arrange the model on the table. Make sure, a group of 15 to 20 students can see the field patterns at a time. Students groups in turn can see the field pattern.

Safety precautions: N .A SESSION Link to previous activity: In the previous activity students have learnt the field distribution around a circular coil. Here the field is studied around a solenoid which is nothing but a series of connected circular turns spread over a length.

PROCEDURE a)  Connect the free ends of the solenoid to a 6 volt battery. Spray iron filings on the board. Gently tap the board. Iron filings lay scattered.  Switch on the current. Gently tap the board. The iron filings form closed magnetic field lines.  Switch off the current and disturb the alignment of the iron filings. b)  Insert the iron core into the solenoid. Switch on the current.  Gently tap the board. The iron filings form closed magnetic field lines.  Place the compass needle near one end face of the solenoid. Observe the direction of the North Pole of the needle.  Shift the compass needle nearer to the other end face of the solenoid observe again the direction of deflection of North Pole of the needle.

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15 UNDERSTANDING THE ACTIVITY Leading questions  What is a solenoid? When is it called a long solenoid?  What does the alignment pattern of iron filings indicate?  What is the significance of arrow marks done with compass?  Is the magnetic field uniform? If not, where is it very strong? And weak?  How is the field inside the solenoid?  Compare the field produced by the solenoid with the field produced by a bar magnet?  What difference in the field pattern do you observe on inserting the iron core into the solenoid?  What is the advantage of a solenoid over a bar magnet?

Discussion and explanation a)  A solenoid is a long cylindrical coil of several turns of insulated copper wire. The planes of each of these turns are parallel. Length of the coil is 10 to 15 times the radius for a long solenoid.  When a current is passed through, each circular turn produces a magnetic field around it. The fields due to all the turns reinforce and produce a stronger field.  The iron fillings get magnetized and align along the direction of the field. Their pattern represents the shape of the field lines. The field lines come out of the face at one end and enter into the face at the other end. When a compass needle is placed near one end face, if the North Pole turns away from the face, the field lines are leaving the face. This end acts as North Pole. At the other end the North Pole of the needle turns towards the end face. The field lines enter the solenoid through this end and this end acts as South Pole.  All the field lines are closed. The field lines leaving the North Pole spread out and enter the South Pole and pass through the solenoid from South Pole to North Pole (air or core). The field is very strong and uniform in the middle inner region of the solenoid. In the region outside the solenoid and off the axis the field is weak. b) 

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When the iron core is introduced, it gets magnetized. The field around the solenoid due to magnetized iron core and current coil becomes much stronger everywhere. The field pattern remains same. The field lines spreading out at north face entre into the South Pole face and pass through iron core from South Pole to North Pole.

The strength of the magnetic field can be increased by i) increasing current, ii) increasing the number of turns per unit length, iii) decreasing the radius and iv) by using a core of larger permeability.

The field around a solenoid is similar to the field around a bar magnet.

The solenoid can be used to produce artificial bar magnets or itself can be used as electromagnet.

A solenoid is advantageous over a bar magnet because the field strength can be varied conveniently.

KEY MESSAGES  A current carrying solenoid which is an electromagnet behaves similar to a bar magnet.

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16  A current carrying solenoid can magnetize a ferromagnetic rod placed inside and thus is useful to produce artificial magnets.

Knowing more:  A steel bar when placed in a current carrying solenoid for certain length of time gets magnetized.  The advantage of using a current solenoid as a bar magnet is that its magnetic field can be increased by increasing the current through it or by inserting a ferromagnetic rod through it.  Solenoids are used in television and several display devices.

Learning check: 1. Place a steel bar inside the solenoid and pass current for about an hour. Remove it from solenoid and suspend it freely. In which direction does it come to rest?

2. Is the magnetic field at end faces same as the field at the centre?

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17

ABL 2: MECHANICAL EFFECT OF ELECTRIC CURRENT Activity 2.1

Learning objective

To demonstrate the mechanical effect of an electric current – A force is exerted on a current conductor placed in a magnetic field.

2.2

Understanding the working of an electric motor – application of mechanical effect of electric current.

Key messages

Time (min)

 A mechanical force is exerted on a current conductor when placed in a magnetic field.  Force is maxi-mum when the conductor is perpendicular to field direction. No force is exerted when the conductor is along the field direction.

20

 As a result of the mechanical force exerted on each edge of a rectangular coil a couple is produced that rotates the coil in the magnetic field.

25

TOTAL

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18

Mechanical effect Back ground information for the Instructors In an electric motor, fan, grinder, mixer electric scooter, we make things (wheel, fan blade) to move by sending an electric current through them. Moving an object means kinetic energy involvement. These devices therefore illustrate the conversion of electrical energy into mechanical energy. Their working is based on ‘mechanical effect’ of electric current.

What is mechanical effect?

A charge in motion produces a magnetic field around it. Whenever a charged particle moves through a magnetic field, as a result of interaction between the two magnetic fields, a force is exerted on the moving charge. The force is maxi-mum when the charge moves at right angle to the field.

If positive and negative charges are made to move in a magnetic field, they experience forces in opposite directions.

The direction of the force is reversed either by changing the direction of motion or the direction of external field.

When a positive charge moves at right angles to a magnetic field the force is exerted in a direction perpendicular to the plane containing direction of motion and direction of the field.

Force on a current conductor: A current is a stream of electrons moving in a direction opposite to that of current in a conductor. When placed in a magnetic field all the electrons experience a force perpendicular to I and B. In other words the conductor is lifted up or down the plane containing I and B. the magnitude of the force, F = B I l when the direction of current is perpendicular to field (B). Force is a vector. How do we find the direction of current?

Direction is given by Fleming’s left hand rule:

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19 Stretch the thumb, fore finger and middle finger of the left hand in mutually perpendicular directions. If the fore finger points to the field direction (B), middle finger points to the direction of current (I) then the stretched thumb gives the direction of force.

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20 Time: 20 minTime: 15 min

ABL 2.1

LEARNING OBJECTIVE: To demonstrate the mechanical effect of an electric current – A force is exerted on a current conductor placed in a magnetic field.

ADVANCE PREPARATION Material List S.no 1 2 3

Material Fleming’s left hand rule model Battery box Connecting wires

Required quantity 1 1 1

Things to do: Use the model to demonstrate the mechanical effect to the entire class. Divide the class into 5 to 6 groups. Demonstrate to each group separately.

Safety precautions: N .A SESSION Link to previous activity: PROCEDURE  Suspend the brass rod with the help of connecting wires horizontally between the two poles of the magnet.  The free ends of the connecting wires are joined to the terminals which are connected to the battery. Switch on and pass the current. The brass rod suddenly experiences a push in a direction perpendicular to the direction of the current in brass rod and direction of magnetic field (B and I).  Reverse the direction of the current and observe the brass rod.  If possible increase the strength of the current and observe the effect on movement of the rod.

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21 UNDERSTANDING THE ACTIVITY Leading questions  What is the direction of the magnetic field?  In which plane do you find the field direction and current direction?  Why does the rod move as we pass current?  What do you observe when the direction of the current is reversed?  How do you determine the direction of the mechanical force exerted on the brass rod?  On what factors does the force depend upon?  Does the force depend on direction of current (rod) with respect to the field?  For which position of the rod is the force maximum or minimum?

Discussion and explanation  The magnetic field between the poles is directed from N Pole to South Pole. The current is along the rod. Field and current are in the vertical plane.  When current is passed through a conductor, each electron in motion experiences a magnetic force in a direction perpendicular to the direction of I and B. The resultant of all these forces acts on the conductor and pushes it in a direction perpendicular to the plane containing B and I. If the current is from P to Q, the force on the conductor is to the right.  The resultant force pushes the rod in a direction perpendicular to the plane containing rod and field direction. Form Fleming’s left hand rule, the force on the brass rod is perpendicular to this plane and along horizontal. (to the right)  On reversing the direction of current or field, the direction of the force is also reversed.

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The magnitude of the force is given by Lorentz equation, F = B I l sinθ.

Force increases when the field B becomes stronger.

Force increases when the current is increased.

Force increases as the length of the conductor increases.

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Force also increases as the inclination of the conductor with respect to the field increases. i) It is maximum when θ = 90 i.e. field and conductor are at right angles. ii) it is minimum when θ = 0 i.e. the conductor is placed with its length parallel to field direction.

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22 

Therefore, the force on the conductor is different for different inclinations of the rod in the field. It is maximum when the brass rod is perpendicular to field. Force is minimum for smaller inclinations and is zero when the rod is parallel to field.

KEY MESSAGES  A mechanical force is exerted on a current conductor when placed in a magnetic field.  Force is maxi-mum when the conductor is perpendicular to field direction. No force is exerted when the conductor is along the field direction.

Learn more:  The mechanical effect of electric current finds application in i) measuring instruments such as moving coil galvanometers, ammeters and voltmeters and ii) motors, fans etc,.  Let A and B be two long straight conductors carrying a current in the same direction. ‘A’ produces a magnetic field in which B experiences a force and in the same way B produces a magnetic field in which A experiences a force. The two conductors experience a mutual force of attraction (F).  If the direction of current is reversed in one of them, the two conductors experience a repulsive force.  Let A and B be the two long straight conductors carrying a currents in the same direction. A produces a magnetic field in which B experiences a force and in the same way B produces a magnetic field in which a experiences a force. The two conductors experiences a mutual force of attractions.  If the direction of current is reversed in one of them the two conductors repel each other.  We have seen in the case of a motor, a current passing through a coil sets it to move freely. If on the other hand, if the coil is not free, it comes to rest after rotating through a certain angle θ. The magnitude θ depends on the strength of the current or potential difference. Usually a needle fixed to the coil rotates and the tip is made to move over a calibrated scale. Moving coil galvanometer can measure currents as small as 10 -6 A  Imagine a study current passed through a rectangular conductor. The direction of the current in one arm is opposite to the direction of the current the other parallel arm. They both experience equal and opposite forces in strong magnetic field. A couple is thus created which acting on the rectangular coil tends to rotate. This is the principle of electric motor.

Learning check: 1. A thin aluminum rod is suspended between the two poles of a horse shoe magnet with length parallel to N-S. A current is passed through the rod. In which direction does the rod move?

2. For demonstrating mechanical effect is it necessary that the current carrying conductor is made of ferromagnetic material?

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23 3. In the figure ABCD is a rectangular coil. What happens when a very strong current is passed through?

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24

ABL 2.2

Time: 25 min

LEARNING OBJECTIVE: Understanding the working of an electric motor – application of mechanical effect of electric current.

ADVANCE PREPARATION Material List S.no 1

Material Simple model of a motor

Required quantity

2 3 Things to do: Safety precautions: N .A SESSION Link to previous activity A current passing through a conductor placed in a magnetic field is acted upon by a force.

Interactive demonstration: The teacher will place the working model of the motor on the table and also display a chart showing the parts of the motor. He leads the demonstration by eliciting a few questions. (The answers may be given by the students, if not teacher prompts them in providing answers) 1. What is an electric motor?

It is a device that helps objects or machines to move.

Because it uses electric current to run the motor. It converts electrical energy into mechanical energy.

2. What is the principle on which it works?

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25 It works on mechanical effect of electric current.

3. Can you name the three important parts of the motor i) A current source (battery) ii) A suitably shaped conductor coil (Armature) iii) A strong magnetic field in which the conductor is placed (magnets).

4. Why do you think these parts are very important?

A current when passed through a conductor (armature) placed in a magnetic field causes the conductor to move.

5. Can you identify the ‘conductor’ in the diagram?

Yes, it is labeled as ‘armature’. It is in the form of a rectangular coil with a very large number of turns wound on it.

6. Can you see the magnets and battery?

Yes. (Children show the N and S Poles of a permanent magnet and the battery.)

7. Where is the coil positioned? Why is it shaped as a rectangular plane coil?

What is the cause for rotation?

The coil is suspended in the magnetic field between the poles. The plane of the coil is perpendicular to the field initially or when at rest. When the current passes through the coil it flows along AB in one direction and along CD in opposite direction. Both the edges ( AB and CD) are in the same field.

Therefore AB and CD experience equal and opposite forces that forms a couple which rotates the coil in anti-clock wise direction. BC and AD also carry current but they are parallel to field. No force acts on them.

8. Can we change the speed of rotation?

Yes. The rate of rotation depends upon i) number of turns, ii) inertia of the coil, iii) strength of the current and iv) strength of the field.

When a motor is given to you, we can change the speed only by altering the current because other things are already fixed.

9. What are the other parts?

The other parts are brushes and split rings.

10. Why are brushes used for?

A brush is a larger surface of a conductor that provides electrical contact while the coil is rotating (B1 and B2). They are connected to the battery.

11. What are split rings?

It is a thick, short copper ring split into two equal half’s (P and Q). Each of these split rings is connected to the free ends of the armature.

These split rings in contact with brushes help the flow of current through the external circuit.

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26 12. How does the motor work?

Look at the diagram. Assume the plane of the coil A B C D is horizontal. The current from battery entering at B1 through split ring P passes along A-B-C-D and back to B2 through split ring Q and then to battery.

Edge AB gets a downwards force, edge CD gets an upward force (From Fleming’s left hand rule).

The two forces forming a couple rotate the coil in anti-clock-wise direction.

After a quarter revolution the slip rings slide past the brush and are not in contact with brushes. No current flows. But due to inertia of motion it keeps rotating. Next, after half revolution the split ring P now comes in contact with B2 and q is in contact with B1. The current through CD and AB are reversed. The edge AB which has current from B to A is pushed up and the edge DC with current from DC is pulled down.

During first half, AB is pulled down and CD is pushed up, in the next half AB and CD have interchanged positions, CD is pulled down and AB is pushed up. The coil keeps rotating in the same direction. i.e. (anti clock wise) 13. How are split rings useful?

The split rings after each half rotation in contact with B1 and B2 alternately help to reverse the currents in the edges AB and CD so that the coil rotates in the same direction. Splits-rings act as a commutator.

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27 ď ś As a result of the mechanical force exerted on each edge of a rectangular coil a couple is produced that rotates the coil in the magnetic field.

Do it yourself: Making of a simple motor Wind an insulated copper wire around the tube. Remove the coil form the tube. Hold all the turns in the coil together and twist the two ends of the wire around the coil. This makes a circular coil in which two ends are along the diameter. The ends act as axle. Remove insulation of two ends of the wire. Fix safety pins at the two ends of the battery cell with the help of rubber band. Insert the two ends of the coil into holes of safety pins. Place the magnets on the cell, below the coil. Give a slight jerk to the coil. It begins to rotate. Observe the direction of rotation.

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28

ABL 3: ELECTROMAGNETIC INDUCTION Activity

Learning objective

Key messages

Time (

3.1

Demonstration of electromagnetic induction and arriving at Faraday’s laws of E.M.I .

 An emf or current is always induced in a coil whenever the magnetic flux linked with it is changed or there is relative motion between the magnet and the coil. (I law)  The magnitude of the induced emf is directly proportional to the rate of change of flux (II law).

These two laws states constitute the laws of EMF.

15 min

3.2

Demonstration of generating an Alternating current

 AC current or voltage is produced by alternately increasing and decreasing the flux linked to coil.  In our country AC supplied has Vo = 220 volt and f = 50 Hz.

25 min

3.3

Understanding and demonstrating the phenomenon of mutual induction (coil – coil experiment)



To study the design of a transformer and understand it’s working.



3.4

It is possible to induce a current or emf in a coil by changing the current in another coil nearby. This is mutual induction.

A transformer is a device that alters the voltage of an AC. A step up transformer increases the voltage of an AC.  A step down transformer decreases the voltage of an AC.  In both the cases the frequency of AC is not altered. There is ideally neither gain nor loss of energy though there is always a finite loss in practice.

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15 min

29 Background Information for Instructors Magnetic Flux The effect of a magnetic field on a conductor placed in the field is understood in terms of a quantity called magnetic flux.

If a coil of area A and turns n is placed in a magnetic field B with its plane 1 ar to the field, the magnetic flux linked with the coil is Φ = nBA

In this position the number of field lines through the coil is maximum. When the coil rotates though an angle θ, the plane of the coil is inclined in the field and the number of field lines through the coil decreases. The flux through the coil decreases. Φ = nBA cos θ. When the plane of the coil is parallel to the field, no field lines pass through the coil. The flux is zero.

Thus, as the coil rotates in a magnetic field, the flux through the coil varies from 0 to maximum twice in each of rotation. Remember, the field around the coil is the same.

Magnetic flux can be changed either by 1. Changing the field B or 2. Rotating the plane of the coil.

Sometimes, the magnetic flux is also understood as the number of field lines passing through the coil.

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30

ABL 3.1 Time: 15 min

LEARNING OBJECTIVE: Demonstration of electromagnetic induction and arriving at Faraday’s laws of electromagnetic induction.

ADVANCE PREPARATION Material List S.no 1 2 3

Material EMI model Galvanometer Powerful bar magnet

Required quantity 1 1 1

Things to do: Place the EMI model (Faraday model) on the table. Ensure, the model is visible to all the students. If the number is large, demonstrate this activity group wise.

Safety precautions: N .A SESSION Link to previous activity In ABL 1.1you have observed that a current passing through a conductor produces a magnetic field.

Here in is demonstrated the induction of current (or emf) in a conductor with the help of a magnetic field.

PROCEDURE

 Connect the two ends of the coil to a sensitive galvanometer.  Take the given bar magnet and push the North Pole briskly into the coil. Observe, the galvanometer needle deflects on one side of zero.  Keep the bar magnet still inside and observe that the galvanometer needle shows zero deflection.

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31  Take the magnet away from the coil briskly. Observe, the needle of the galvanometer deflects on the other side of zero.  Reverse the magnet and now push the South Pole into the coil. Keep the magnet still inside coil and pull it out of the coil. Observe the galvanometer deflections in each case and compare with earlier observations.  Now, push the magnet into the coil and pull it out briskly repeatedly. Observe, the needle of the galvanometer deflects alternately on either side of the ‘zero’.  Repeat the experiment by i.

Using coils of larger number of turns provided on the board and ii.

Using stronger magnets  Observe what happens by keeping the magnet stationary and moving the coil towards and away from the magnet.

UNDERSTANDING THE ACTIVITY Leading questions  Do you have any current source in the experimental set up?  Why do you use galvanometer? What is the significance of using a centre tap galvanometer?  What is happening when the magnet is pushed in or pulled out or remains still? Explain, why?  What difference do you observe when the magnet is pushed in or out, by reversing the ends (Poles)?  Explain what happens when i) rapidity of moving the magnet is slowed? ii) The number of turns in the coil is increased? And iii) a stronger magnet is used?  Which is the phenomena of “electromagnetism” that is demonstrated here?  How do you increase the magnitude of the induced current?  Are the results same when magnet is fixed and coil is moved “to and fro” with respect to the magnetic field?

Discussion and explanation  Our model has an electrical circuit that includes a conductor coil and a galvanometer. There is no known source of electric current. The action is within the coil initiated by moving the magnet and galvanometer is used to detect the current. It has a centre zero scale to enable the detection of current flowing either way.  On moving the magnet briskly towards the coil the galvanometer deflection records a “current” induced in the circuit.

The bar magnet has a strong magnetic field around it (fig 1). The field is stronger nearer the poles and fades out on moving away from it. When the magnet is held near the open end of the coil the field lines flush through the turns of the coil. We say there is a magnetic flux linked with the coil (Flux = number of lines passing through the force of the coil).

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32 When the magnet is suddenly pushed into the coil, the field and the flux (number of lines passing through unit area) increases. The increasing flux drives the free electrons from one end to the other end of the coil. There is a certain amount of current flowing through the galvanometer causing the

deflection of the needle on one side of the centre-zero.

 





When the magnet is held stationary inside, the field and the flux through the coil remain steady and the induced current vanishes.

Again when the magnet is suddenly withdrawn, the field lines and flux penetrating the coil decrease. This decreasing flux again drives the free electrons in the wire in the opposite direction.

There is again certain amount of current flowing through the galvanometer causing the deflection of the needle in the opposite direction.

When the magnet is held stationary the current vanishes because the flux or field is constant and is not changing.

These deflections due to currents induced are transient. The currents last as long as the magnet is moving or flux is changing. When the magnet is at rest the current is zero.

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33 

 





 

When South Pole instead of North is pushed into the coil, (the direction of the field reversed, the change in flux drives electrons to flow in a direction opposite (to that of when North Pole is pushed in).

When the magnet is pushed in and out alternately the direction of the current induced is also reversed alternately.

When the magnet is moved in or out briskly, the magnetic flux linked with coil changes at faster rate. The current induced is larger as indicated by larger deflection. If the magnet is moved very slowly, very small current is induced due to small rate of change of flux.

Increasing the number of turns or using a stronger magnet or increasing the area of the face of the coil increases the flux and results in increased induced current. The results are same whether the coil is at rest and magnet is moved or magnet is at rest and coil is moved to and fro. All that is required is i) magnetic flux and ii) change in flux brought about by the relative motion between the coil and the magnet.

The results are same when the magnet is fixed and the coil is moved ‘to and fro’ with respect to the magnet. The magnetic flux does not change when there is no relative motion and flux changes when there is relative motion.

This phenomenon in which an emf and hence a current is induced in a coil by varying the magnet flux linked, is called electromagnetic induction. The induced emf lasts as long as flux is changing.

“The relative motion between the coil and the magnet which produces a change in magnetic flux through the coil is the cause for the induced emf or current in the coil”.

KEY MESSAGES  An emf or current is always induced in a coil whenever the magnetic flux linked with it is changed or there is relative motion between the magnet and the coil. (I law)  The magnitude of the induced emf is directly proportional to the rate of change of flux. (II law) These two laws states constitute the laws of EMF.

Learning more: 1. Suspend a coil in a strong magnetic field between the poles of a magnet. Because of the field lines penetrating through the coil, there is a magnetic flux linked with the coil. On rotating the coil about an axis perpendicular to field lines, the magnetic flux linked with the coil changes continuously. An emf is induced in the coil.

2. E. M. I is the method employed universally to generate electricity, be it in Hydroelectric or thermal or nuclear or gas based power stations.

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34

3. A current coil behaves as a bar magnet. Replace the magnet by a current coil in the activity 3.1. By moving the current coil ‘to and fro’ from the other coil, an emf is induced in the latter.

4. Faraday’s laws of E.M.I: An understanding of the results discussed in this section leads us to the well-known ‘Faraday’s laws of electromagnetic induction.

1st law: An emf is always induced in a coil whenever there is a change in magnetic flux linked to it.

The emf lasts as long as the flux is changing.

Explanation: when the coil is at rest the magnetic flux linked with it is constant or fixed. No emf is observed, when the coil is rotating the flux is continuously changing and there is always induced emf.

2nd law : The magnitude of emf induced is directly propositional the rate of change of magnetic flux.

Explanation: If the coil rotates faster the flux changes at faster rate. The emf induced is larger.

Magnitude of emf, E = change a flux/time Learning check: Assume that induced emf is equal to rate of change of magnetic flux.

1. The magnetic flux linked with a coil changes from 100 SI units to 20 SI units in 100ms. What is induced emf ?

2. In a coil of resistance of 5Ω, a bar magnet pushed suddenly into it increases the magnetic flux from 0.4 units to 4 units in 0.1 second. What is the induced current?

3. If in the above, the magnet moves in 10 seconds to produce the same change in magnetic flux, what is an induced emf? And current?

4. In the figure here, a conductor coil is displaced from A to B in a uniform magnetic field, there is no induced emf in it. Why?

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35 5. In the diagram above assume the coil is fixed and the magnet (both poles) are rotated around. Is emf still induced in the coil?

6. How do you increase the induced emf in the experiment described above?

Time: 25 min

ABL 3.2

LEARNING OBJECTIVE: Demonstration of generating an Alternating current.

ADVANCE PREPARATION Material List S.no 1

Material Electromagnetic (AC), Induction model

Required quantity 1 per class

Things to do: Place the model on the table in the vicinity of the entire class. Explain the parts of the model. The teacher shall ask a student to come to the table and demonstrate the working.

Safety precautions: N .A SESSION Link to previous activity: PROCEDURE ďƒź Show the bar magnet to all the students. Hang it at the lower free end and release.

ďƒź Ask the remaining students to watch the direction of deflection in the galvanometer as the magnet oscillates every time dipping in and out of the coil.

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36  The demonstration is done to all the groups.

UNDERSTANDING THE ACTIVITY Leading questions  Why does the direction of deflection of the needle reverse periodically?  What kind of current is this?  Where do we come across such current?  What are the characteristics of this kind of current (AC)?

Discussion and explanation  When the bar magnet dips into the coil the magnetic field and flux linked to the coil increase, there is an induced current.  When the bar magnet is getting out of the coil the magnetic field and flux linked to the coil decreases there is again an induced current but in opposite direction.  The galvanometer deflections thus reverse alternately.  The current or emf induced is alternating (AC).  In either direction the current is not steady but varies from 0 to maximum and then to zero and reverse its direction. The variation from 0 - Im - 0 – (Im) – 0 is called a cycle of variation.

1. Period: It is the time for one cycle of variation ( T = - - - - sec) 2. Frequency: It is the number of cycles of variation in one second ( f = - - - Hz) 3. The maximum value of the current during each half cycle of variation is called peak value. (Io) Our domestic mains have 220√2 volt peak voltage and 50Hz frequency in our country.

KEY MESSAGES  AC is a current or voltage produced by alternately increasing and decreasing the flux linked to coil.  In our country AC supplied has peak voltage Vo = 220√2 volt and f = 50 Hz.

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37 Time: 15 min

ABL 3.3

LEARNING OBJECTIVE: Understanding and demonstrating the phenomenon of mutual induction (coil – coil experiment).

ADVANCE PREPARATION Material List S.no Material 1 2 3

Mutual induction model Sensitive Galvanometer Suitable iron core, connecting wires

Required quantity 1 per class 1 per class

Things to do: The teacher divides the class into 5 to 6 groups. He shall draw a circuit diagram on the board and explain the role of different components or circuit elements. He will impress upon the students that the primary and secondary coils are not electrically connected but separated.

Safety precautions: N .A SESSION Link to previous activity An emf is induced whenever the magnetic flux linked with a coil changes.

PROCEDURE  Connect the coil marked primary to the AC mains. Introduce a sensitive galvanometer across the ends of the coil marked (S) secondary.  Close the key. Observe what happens?  Switch off and replace the galvanometer by a AC voltmeter (multi meter) and measure the voltage across.  Repeat the trial by changing the number of turns of the secondary.  Redo the experiment by introducing a soft iron rod into the primary.

UNDERSTANDING THE ACTIVITY Leading questions

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38

       

Why are the two coils named primary and secondary?

Is there any electrical contact between the two coils P and S? What kind of voltage is introduced in the primary coil?

What do you observe in the secondary, when the key in the primary is closed?

What is the effect of changing the number of turns?

What is the effect of introducing a soft iron rod into the primary coil?

Identify the cause and effect in this demonstration?

What phenomenon of electromagnetic induction do you learn from this activity?

Discussion and explanation  A pair of coils (not connected) in this experiment are termed primary (P) and secondary (S). The coil in which we introduce an alternating current is called primary and the coil in which a current is induced as a result is called secondary.  The turns are of insulated copper wire. They (P,S) are either placed very close or wound one over the other coaxially not in contact.  The primary is connected to an alternating current source. It gives a periodically fluctuating current or voltage.  On closing the key, a current passing through the primary coil produces a magnetic field around it.  As the current is changing continuously the magnetic field also changes periodically. The magnetic flux through the secondary coil surrounding the primary senses the changing magnetic field and magnetic flux. As a result of electromagnetic induction a current is induced in the secondary.  The current in the secondary is also alternating and of same frequency as primary current. But the emf induced in the secondary depends on the number of turns in it. For larger turns of the secondary coil the emf induced is larger.  Emf induced in secondary α number of turns in the secondary. 

 

Vs α n s When a soft iron bar is introduced in P, it gets magnetized and enhances the magnetic field and variations of magnetic field around the secondary. As a result larger emf is induced when soft iron core is introduced.

This phenomenon, in which a varying current in one coil induces an emf in another coil not in contact, is called mutual induction.

Varying current in primary producing varying magnetic field (or flux) is the cause. The emf induced in the secondary coil is the effect.

KEY MESSAGES  It is possible to induce a current or emf in a coil by changing the current in another coil nearby. This is mutual induction.

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39

Learning check: 1. Can we interchange primary and secondary and yet demonstrate mutual induction?

2. Assume P and S are not wound over one another, but held near each other. Can you still demonstrate mutual induction?

3. If we place some iron sheets between primary and secondary, does it increase or decrease induced emf?

4. If we take primary and secondary farther, does the induced emf increase or decrease?

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40 Time: 25 min

ABL 3.4 (Optional activity)

LEARNING OBJECTIVE: To study the design of a transformer and understand it’s working.

ADVANCE PREPARATION Material List S.no 1 2 3 4

Material Opened up transformer model A multi meter Key in the primary circuit AC mains supply point

Required quantity 1 per class 1 per class 1 per class 1 per class

Things to do: The teacher shall place the transformer model on the table in the clear vicinity of all. Draw the circuit design on the black board. Explain the various parts of the transformer (shown them in the model) and also their functions.

Primary coil with leads: - When connected to mains AC voltage is introduced. It is this AC which is to be transformed (Vp).

Secondary coil with leads: - It is the region where the emf is induced. The emf measured across secondary leads is transformed voltage (Vo).

Rectangular iron core (Laminated): - It is the material that is magnetized by the primary current. It provides the variable magnetic field (as primary varies) necessary for inducing current in secondary.

Multimeter or AC voltmeter: - To measure the voltages in P and S coils.

Safety precautions: Children are instructed not touch or come in contact with the terminals of primary or secondary.

SESSION Link to previous activity The transformer works on the principle of mutual induction.

PROCEDURE Having explained various parts of the circuit, the teacher does the demonstration. He will,  Switch on the primary  Using a Multimeter measure the input voltage (Vp) across the primary and current in primary (Ip).  Measure the voltage across the secondary leads as Vs and current in secondary (Is).  Note down the number of turns in the primary and secondary and calculate the turn ratio TR = ns/np  Calculate the ratio (Vs/Vp) Agastya International Foundation. For Internal Circulation only.

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41 Compare T.R and (Vs/Vp)  Calculate input power Pi = Vi Ip and output power, Po = Vs Is and ratio Po/Pi S.No

Vp

Ip

Vs

Is

Np

Ns

Ns/Np

Vs/Vp Pi = VpIp Po=VsIs

UNDERSTANDING THE ACTIVITY Leading questions  What is input voltage?  What are the characteristics of this input voltage (V p)?  What kind of voltage is measured across the secondary terminals?  What are the characteristics of the voltage in the secondary or output (V s)?  Are the primary and secondary circuits electrically connected?  How do you explain the origin of voltage in the secondary?  Is the Vs greater or less than Vp? On what factors does it depend upon?  What is the role of the laminated core?  What are step-up and step down transformers?  The transformer which is used is it step up or step down?  When it gives a higher secondary voltage, is the output power greater than the input power? Does it not appear that principle of conservation of energy is violated?  What is the efficiency of the transformer?

Discussion and explanation  A transformer is a device used to increase or decrease the voltage of an AC without changing the frequency. It works on the principle of mutual induction.  The voltage to be included in the primary is input voltage and is the one whose voltage is to be altered (Vp). It is mains AC whose voltage is 220V and frequency 60 Hz.  When switched on, the current passing through the primary turns produces a strong magnetic field around it. The iron core is magnetized and the secondary is in the magnetic field of the core. As the current in primary alternates the magnetic field (flux in the secondary) also fluctuates at the same rate. There is no electrical contact between P and S. The fluctuating magnetic field (or magnetic flux) induces AC emf in the secondary. The voltage induced in secondary also alternates with the frequency same as primary AC. If Np and Ns are the number of turns of primary and secondary Vp α Np and Vs α Ns.



∴ Vs/Vp = Ns/Np For step up transformer T.R >1 Ns > Np ∴ Vs>Vp. For step down transformer T.R <1 Ns < Np ∴ Vs<Vp. The transformer we have employed is step-down transformer.

The power given to primary is , Pi = Vp Ip



The power obtained in secondary is, P0 = Vs Is For ideal transformer, it is found, Pi = Po - the principle of conservation of energy is not violated.

VsIs = VpIp Vs/Vp = Ip/Is

when

Vs>Vp then Ip>Is.

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42



When the voltage is decreased in secondary, the current there in increases.

The efficiency of the transformer is ratio of output power to input power.

Efficiency, η = P0/Pi for ideal transformer P0/Pi ∴η=1 Because of i) heating of primary and secondary coils ii) eddy currents, iii) hysteresis of the iron core and iv) some leakage of flux, the output power is less than input power, P0<Pi η <1 In practice, Vs/Vp may be less than T.R (η < 1) because of energy losses (stated above).

KEY MESSAGES:  A transformer is a device that alters the voltage of an AC. A step up transformer increases the voltage of an AC.  A step down transformer decreases the voltage of an AC.  In both the cases the frequency of AC is not altered. There is ideally neither gain nor loss of energy though there is always a finite loss in practice.

Learning check: 1. When the AC from mains (220V, 60 Hz) is introduced into the primary, the voltage recorded in the secondary is 3300V. What is the turn ratio?

2. The turn ratio of a transformer is 5. The input voltage and current are 220V and 5.3A. What are the voltage and current in the secondary?

3. Given Vp = 220V , Ip = 5A , Vs = 2200V and Is = 0.4A. What is the efficiency of the transformer?

4. The efficiency of a transformer is 0.8. The input power is 2200 watt. If Vs = 4400 V what is secondary current. What is output power?

5. What is power for an electric circuit?

6. Can a transformer be used to vary DC voltage? Why?

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ABL 4: AC AND DC DYNAMO Activity

Learning objective

Key messages

Time

4.1

Understanding the principle and working of AC and DC dynamo.

 Dynamo converts mechanical energy into electrical energy.  AC dynamo converts mechanical energy into AC current.  DC dynamo converts mechanical energy into DC current.

25 min

4.2

To understand the working of an induction coil.

The induction coil provides very high voltage DC pulses from low voltage DC, working on the principle of mutual induction.

20 min

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44 Time: 25 min

ABL 4.1

LEARNING OBJECTIVE: Understanding the principle and working of AC and DC dynamo.

ADVANCE PREPARATION Material List S.no 1

Material AC and DC dynamo models

Required quantity 1 per class

Things to do: Place the AC dynamo model in the table in clear view of 8 to 10 students. Divide the class into 3or 4 groups of 8 to 10 students in each group. Demonstrate the working of dynamo to each group by turn, the teacher shall draw a diagram of the Ac dynamo on board and explain the different parts and also their importance.

Safety precautions: N .A SESSION Link to previous activity It is learnt in previous activities that a current or emf is induced in a coil whenever the magnetic flux linked with it is changed. (Moving or rotating a coil causes change in flux) a) AC dynamo: PROCEDURE  Set the plane of the coil perpendicular to the field direction.  Make sure the galvanometer is connected across the brushes B 1 and B2 and the slip rings S1 and S2 are in contact with B1 and B2.  Rotate the coil steadily with the help of the handle.  Observe the deflections of the galvanometer during first half and second half of each cycle of rotation.  Continue observing for each cycle for 4 to 5 minutes.

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UNDERSTANDING THE ACTIVITY Leading questions  Is there any current source introduced in the circuit?  Do you observe the deflection of the galvanometer when the coil is at rest?  Why does the galvanometer show deflections when the coil starts rotating?  Why do the deflections in the galvanometer reverse the direction after each half cycle of rotation?  Is the deflection (or current) steady?  What kind of current is induced in the coil?  How can we induce larger current or voltages?

Discussion and explanation  The ends of the coil are in contact with slip rings S1 and S2 which from a closed circuit with the galvanometer through contacts with brushes B1 and B2. Clearly there is nothing such as battery or cell in the circuit.  When the plane of the coil is perpendicular to the field (I) a maximum number of field lines penetrate through the face ABCD. The magnetic flux linked with the coil is maximum. The coil is at rest in the field. There is no change in flux. There is no current in the circuit. (no deflection of galvanometer)  As the coil starts rotating the plane of the coil is more and more inclined to the magnetic field. The flux starts decreasing giving rise to “induced current”. When the coil rotates through half cycle (positions I -> II -> III) the flux reaches a minimum (zero) after quarter cycle (θ = 90 0 position II) and then increases to maximum at the end of half cycle. (Position III, θ=180 0). During this half cycle induced current or emf increases from zero, reaches a maximum and then becomes zero. The galvanometer swings to one side and then comes to zero.

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



 

 

During first half when the coil starts rotating in the anti-clock wise direction AB moves down and CD moves up. The current induced in the coil is from B→A→D→C which flows C→S2→B2 →G→B1→S1→B. The induced current through G is along B2 G B1.

During second half of rotation (position III →IV→V→I) the flux through the coil again decreases reaches ‘zero’ at the end of ¾ of the cycle and becomes maximum after one cycle.(IV, θ = 360 0). The emf induced also increases, reaches a maximum (at θ = 270 0) and again become zero. Since the edge AB moves upwards and CD downwards, the direction of the current induced is opposite to that in first half.

That is C→D→A→B in the coil and along B→S1→B1→G →B2→S2→C in the external circuit. The current through G is along B1 G B2.

The current induced in the coil alternates as shown in the figure for one complete cycle. The sane continues for each successive cycle.

The current is not steady. It is called as AC or alternating current.

Larger emf’s or current can induced by i) rotating the coil at faster rate II) rotating the coil in larger magnetic field or III) by increasing the number of turns in the coil.

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b) DC dynamo: (A DC dynamo has the parts same as AC with only difference that the ends of the coil are connected to two semi cylindrical split rings S1 and S2.)

PROCEDURE Same as in Ac dynamo UNDERSTANDING THE ACTIVITY Leading questions  Why do the deflections in the galvanometer shown are on the same side of zero?(unlike in AC dynamo)  Is the deflection (or current) steady throughout?  What kind of current is this induced current?  Can this current be directly utilized?  How can we induce larger currents?

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48 







The ends of the coil are connected to split rings S1 and S2 (together called commutator). During first half cycle S1 is in contact with B1 and S2 is in contact with B2. After half revolution S1 makes contact with B2 and S2 makes contact with B1. This arrangement helps to reverse the current in the second half cycle so that the direction of current through G is always along B2 G B1.

When the plane of the coil is perpendicular to the field( position I) a maximum number of field lines pass through the face ABCD. The magnetic flux linked with the coil is maximum. The coil is at rest in the field and there is no induced current as same.

As the coil starts rotating (say the anti-clock wise direction) the plane of the coil is more and more inclined to the magnetic field. The flux through the coil starts decreasing giving rise to induced current in the coil.

When the coil rotates through half a cycle (positions I→II→III θ= 0 to 90 0 to 1800) the flux decreases reaches a minimum (at θ = 900 ) and then increases to become maximum (at position III, θ = 180 0).

Since AB is moving down and DC is moving up, the induced current follows the path B→A→D→C in the coil and C→S2→B2→G →B1→S1→B in the external circuit. The current during this half cycle increases from zero (at θ = 00) to maximum (at θ = 1800) and then decreases to zero again ( at θ = 1800).

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





During the next half cycle (Positions III→IV→V→I) again the flux decreases, reaches a minimum (zero at θ = 2700) and then reaches maximum (V, θ = 3600 or 00). Now since AB is moving upwards and DC is moving down words the current induced in the edges of the coil is along DCAB (opposite to that in I half). But the S1 and S2 are now in contact with B2 and B1. Therefore the current in the external circuit is B→S1→B2→G B1→S2→C. The current through G is again along B2 G B1. The current during this half cycle increases from ‘zero’ (at θ = 180 0) to maximum (at θ = 2700) and again becomes zero (θ = 3600).

Nevertheless, the current the G is always in the same direction (B2→G→B1). The induced current for one complete cycle is DC but pulsating or fluctuating. The same repeats for each cycle.

This current can be made steady using a fitter circuit and then used. Stronger current can be induced using stronger magnetic fields or larger number turns in the coil or by rotating the coil at faster rate.

KEY MESSAGES  Dynamo converts mechanical energy into electrical energy.  AC dynamo converts mechanical energy into AC current.  DC dynamo converts mechanical energy into DC current.

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ABL 4.2

Time: 20 min

LEARNING OBJECTIVE: To understand the working of an induction coil.

ADVANCE PREPARATION Material List S.no 1 2 3

Material Working model of an induction coil A low tension battery A few connecting wires

Required quantity 1 per class 1 per class 2meters per class

Things to do: The teacher shall write a neat labeled diagram of the induction coil model being used. He shall explain the different parts and their use.

Primary (P): A few turns of thick copper wire wound on a soft iron core.

Secondary (S): A secondary coil of very large number of turns of insulated copper wire wound over the primary.

Output terminals: The ends of the secondary coil are connected to T1 and T2 . Across these terminals the output voltage is drawn or measured.

Make and break arrangement (M): It helps to make and break the primary circuit alternately. When on closing K the current flows through the primary and the soft iron core is magnetized. The soft iron head H is attracted to the core(C) causing the break or losing a contact with M. The primary current stops, the iron core are demagnetized releasing H. It makes contact with C and closes primary. Thus the primary circuit is opened and closed alternately.

Safety precautions: N .A SESSION Link to previous activity A current varying in one coil induces an emf in the neighboring coil.

PROCEDURE  Connect a low tension DC (about 20V) in the primary.  Close the key K.  Measure the required high voltage across T1 and T2.

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UNDERSTANDING THE ACTIVITY Leading questions  What is an induction coil?  On what principle does it work?  How do you account for the induced high voltage across T 1 and T2?  Is the output across T1 and T2 a DC or AC? Why?  On what factors does the magnitude of the output voltage depend upon?  Is the output voltage a pure DC?  What would happen if the make and break arrangement is absent?

Discussion and explanation  An induction coil is a device that helps to obtain a very high DC voltage from low voltage DC.  It works on the principle of mutual induction.  When the primary circuit is closed the iron core is magnetized. The flux linked with the secondary or the magnetic field around it suddenly increases. There is an emf induced in the secondary in one direction.  The soft iron core attracts H and breaks primary. The current becomes zero. The field and flux around secondary drops to zero suddenly. This decrease in field and flux again induces an emf in secondary in opposite direction. Since the breaking of the circuit is faster than the ‘making’, the emf induced at the time of break is very much larger than the emf induced at the time of make.  If we ignore the smaller emf’s induced at ‘make’ then large voltage pulses induced at break in the same direction constitute the DC output across T1 and T2.  The ratio of the number of turns in the secondary to the number in the primary (n s/np) determines the magnitude of output voltage.  Output voltage is pulsating DC.  In the absence of make and break arrangement there is steady DC current. The field and flux do not change. No emf is induced.

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KEY MESSAGES ď ś The induction coil provides very high voltage DC pulses from low voltage DC, working on the principle of mutual induction.

Learning check: 1. What is a fluctuating DC? 2. When the voltage in the secondary becomes very large, is the current also very large?

Learn more: The induction coil are ready sources required to operate cathode ray tubes, discharge tubes, X-ray tubes etc. . .

Transformers where are the transformers useful?

Almost all the electronic gadgets in use are semiconductor based and require very low voltages for operation. Our mains is 220V, 6.3A source. Using a step down transformer the supply voltage 220V is lowered to desired value (40V, 60V,. . . . .). TV, Refrigerator, mobile chargers, computers, laptops, all are fitted with source step-down transformers, In industry where heavy duty electrical devices like industrial furnace are used 220V is stepped to 10000 to 20000V, using transformers.

In a power generating station, the power generated is of small voltage and large current. If this current is transmitted as it is through cables, most of the power is lost in the form of heat. Almost nothing reaches the end user. To prevent the transmission loss, the power generated is stepped to very high voltages using a step-up transformer. When the voltage is stepped up by 100 times the current decreases by 1/10000 or 1/(100)2 times. This high voltage small current AC is transmitted through the cables. At the other ends where the power is received the high voltage low current AC is converted to low voltage high current AC and then supplied to the domestic circuits.

Uses are plenty and the list can run into a book.

Maglev trains: These trains capable of speeds 450km/hr - 650 km/hr are basically electromagnetic trains. There are no conventional engines or steel tracks.

The bogies are provided with fixed magnets at the base. The runway is a guide way which has series of coils all along the path. When a large current is passed through the coil they behaves as electromagnets. The polarity at the top of the coil is same as the pole of the magnet at the base of the bogie facing the coil. Because of the repulsion between the poles the bogie is lifted 6 to 20 cm in air.

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53 Another electrical circuits propels the train. The train moves in air above the guide way. The moving surfaces are not in contact and thus friction is eliminated.

Induction heaters: These are the devices where the heating element is not in contact with the vessel or specimen. The specimen to be heated (must be a metal or magnetic material) is placed inside a coil. When an AC current is passed through the coil, a strong magnetic field inside the coil starts fluctuating. Due the constant change of magnetic flux associated with the metal specimen in the cavity, currents are induced in the body metal. These currents producing heating effects resulting in the increase in temperatures are called eddy current. Induction furnaces also make use of the same principle.

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