Current Electricity In the old times scientist thought that current was flow of positive charge from the positive terminal of the battery to the negative terminal of the battery. So many rules in electricity depend on this concept. Later on they found out that this was not the case. They found out that current is flow of electrons from the negative terminal of the battery to the positive terminal of the battery. But it was too late so many rules were already made depending on the older concept and does it really matter which way charge flows: positive to negative or negative to positive; we still get electric current, bulbs light up and motors work. So for the sake of not changing so many rules already made the scientist decided to leave the old concept as it was and called it the conventional current. So conventional current flows from positive to negative and electrons flow from negative to positive terminal of a battery.
Definition of current 1.
It is flow of charge (electrons).
2.
It is the electrical charge passing any one point in a circuit in one second. Or simply charge per unit time. The unit of current is Ampere (A).
The Ampere can be defined in the following way:
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From the second definition we get
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Charge (Coulombs, C)
Time (s)
Electromotive Force When you turn on an electric light, current flows in the wire. The current does not flow out from the switch – it is already in the wire – connecting the lamp to the power supply via the switch simply gives it the energy to flow. This energy can come from a variety of sources – kinetic as in a dynamo, a chemical reaction in a cell, light falling on a photoelectric cell, heating the junction of two metals in a thermocouple, sound in a microphone. When an electric current flows electrical energy is converted to other forms of energy such as heat, light, chemical, magnetic and so on. If you were to connect two wires with a bulb and join them the bulb does not simply light up. Why? This is because the electrons present in the wire do not have the energy to flow. If you were to connect the two wires with a bulb and join them to a cell/battery the bulb will light up. Why? This is because the cell has given electrical energy to electrons and they are able to move through the complete circuit passing through the bulb, converting electrical energy to light and heat energy as the bulb lights up.
Definition of Electromotive force 1. It is the energy given to electrons by the source to move around the complete circuit. 2. It is work done by electrons when they move around the complete circuit. 3. It is chemical energy in the cell converted to electrical energy in the electrons.
Potential Difference When electrons pass through the bulb their electrical energy is converted to heat and light energy. This is known as the potential difference.
Definition of Potential Difference 1. 2. 3.
It is energy used by the electrons to pass through a component such as a bulb. It is work done by the electrons whey they move through a component in a circuit. It is electrical energy of electrons converted to other forms of energy.
Formula
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đ?‘˝=
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đ?‘Ź Energy (J)
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The unit of Electromotive force is J/C or Volt. Charge (C)
Definition of volt When 1 Joule of energy is provided to 1 Coulomb of charge, 1 volt of 1 V = 1 J/C
CELLS IN SERIES
Total Voltage = 1.5 + 1.5 + 1.5 + 1.5 = 6 V
CELLS IN PARALLEL
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=
Total Voltage =
1 1.5 1
+
1 1.5
+
1 1.5
=2
= 0.5 V
2 NOTE: The voltage used up is more in series so the batteries die out much earlier than if they were arranged in parallel. It is better to connect cells in parallel because they will work for longer time. Another reason is that in parallel the amount of current generated in the circuit increases. You will learn about this when you study resistance.
Resistance
The free electrons in a metal are in constant random motion. As they move about they collide with each other and with the atoms of the metal. If a potential difference is now applied across the metal the electrons tend to move towards the positive connection. As they do so their progress is interrupted by collisions. These collisions impede their movement and this property of the material is called its resistance. If the temperature of the metal is raised the atoms vibrate more strongly and the electrons make more violent collisions with them and so the resistance of the metal increase (see later). The resistance of any conducting material depends on the following factors: (a) the material itself (actually how many free electrons there are per metre cubed) (b) its length (c) its cross-sectional area and (d) its temperature
The resistance of a given piece of material is connected to the current flowing through it and the potential difference between its ends by the equation:
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�= Resistance (Ί)
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Current (A)
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Definition of Ohm
The unit of resistance is ohm (ď —).
Ohm’s Law “The voltage is directly proportional to current provided the temperature remains constant.�
The gradient of a V/I graph is resistance. If the ratio of voltage to current remains constant for a series of different voltages the material is said to obey Ohm's Law. This is true for a metallic conductor (wire) at a constant temperature. It is known as an OHMIC CONDUCTOR. A material which does not obey Ohm’s Law is known as a NON-OHMIC CONDUCTOR. It is important to realise that Ohm's Law only holds for a metallic conductor if the temperature is constant. Resistance increases with temperature so if it is not kept constant then the graph will not be a straight line.
Graphs of ohmic and non-ohmic conductors
Wire (ohmic conductor)
filament bulb (non-ohmic) As the bulb gets warmer the resistance increases so the current 1 gets less. đ?‘… đ?›ź đ??ź
thermistor (non-ohmic) As the thermistor gets warmer the resistance decreases (this is this is an exception) so the current increases.
Experiment: To verify Ohm’s Law Apparatus: resistor, voltmeter, ammeter, variable resistor, cell, switch. Diagram:
Method: 1. Arrange the apparatus as shown. 2. Set the variable resistor to a maximum and then close the switch. 3. Measure the voltage and current across the resistor R. 4. Reduce the resistance of the variable resistor to obtain different values of voltage and current. 5. Record your results in a table. Precautions: (ATP POINTS) 1. Always set the variable resistor to maximum before closing the switch. REASON: If the switch is closed while keeping resistance low, a large current will enter the circuit and may damage the ammeter and other components. Setting the variable resistor to maximum will make sure that minimum current enters the circuit. 2. Record your readings with interval of time. That is, take a few readings then open the switch. After a while again close the switch and take more readings and so on. REASON: This will ensure that the circuit wires and components do not heat up. Opening the switch will allow the circuit to cool down before you continue to take readings. Resistance is directly
proportional to temperature so it is important to keep the temperature constant if we want to keep the temperature constant. 3. You must make sure that the contacts are ‘good’. This means that the joints in the circuit must be made firmly. Any loose connections may lead to inaccurate reading. 4. You must check that there is no rust or dust on the wire especially at the joints. Rust/Dust will increase the resistance.
Series and parallel resistors In this section we deal with the mathematics of more than one resistor in a series or parallel circuit.
Two resistors in series 1. 2.
Current (I) is same in both R1and R2. Voltage divides as V1 and V2. V = V1 + V2
Where R is the total/effective resistance of the two resistors in series. For three resistors in series the combined resistance is:
Two resistors in parallel 1. Voltage is same in both R1 and R2. 2. The current divides in the branches as I1 and I2. I = I 1 + I2
Where R is the total/effective resistance of the two resistors in parallel. For three resistors in parallel it is:
Notice that two resistors in series always have a larger effective resistance than either of the two resistors on their own, while two in parallel always have a lower resistance. This means that connecting two or more resistors in parallel will increase the current drawn from a supply.
Resistors in parallel – an alternative formula The formula for two resistors only in parallel may also be written as:
By Shafaq Hafeez shafaq@physics.com.pk