STATIC ELECTRICITY Static electricity is charging the surface of a material where the charges do not move within the material itself. The charges stay on the surface of the material. There are two types of charges: positive (+) and negative (-).
Law of static electricity (electrostatics):
LIKE CHARGES REPEL UNLIKE CHARGES ATTRACT
FORCES ON LIKE CHARGES +
+
-
REPULSION
-
REPULSION
FORCES ON UNLIKE CHARGES +
ATTRACTION
CHARGING AN OBJECT 1.
BY FRICTION (rubbing two surfaces together)
EXAMPLE 1: Polythene / ebonite rod rubbed with a dry woolen or silk cloth makes the rod negative and the cloth becomes positive.
Polythene rod
-
-
Silk cloth
Rub the two together
+ + + + + +
separate the two
EXPLANATION: The polythene rod and the cloth both are neutral (uncharged) before they are rubbed together. During rubbing, electrons from the cloth’s surface travel onto the polythene rod. The polythene rod becomes negatively charged as it has gained excess electrons from the cloth. The cloth on losing electrons becomes positively charged. EXAMPLE 2: Perspex / cellulose acetate rod rubbed with a dry woolen or silk cloth makes the rod positive and the cloth negative.
Perspex rod
+ + + + + +
Rub the two together
Silk cloth
+ + + + + +
-
- -
- -
separate the two
EXPLANATION: The Perspex rod and the cloth both are neutral (uncharged) before they are rubbed together. During rubbing, electrons from the Perspex rod’s surface travel onto the cloth. The cloth becomes negatively charged as it has gained excess electrons from the rod. The Perspex rod on losing electrons becomes positively charged.
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2. CHARGING BY INDUCTION: When a charged rod is brought near an uncharged object such as a piece of paper the charges on the rod induce / create opposite charges on the surface of the paper by the force of attraction. As unlike charges attract, the piece of paper jumps up to the rod and sticks to it. It falls off after sometime when the two charges are neutralized.
Positive rod ( Perspex)
+++++++++++ - - - - - - - - -
++++++ +++++ Piece of paper
EXAMPLE 1: CHARGING A METAL SPHERE BY INDUCTION. Step 1. A charged rod is brought near the metal sphere. The negative charges are repelled to the far end of the sphere leaving to behind positive charges at the end near the rod. sphere.
Step 2. The metal sphere is earthed with a wire or by touching with your finger. The electrons travel down the earth leaving behind positive charges on the
`
_ _ _ _
+ + + +
+ + + +
e-
Earthing
Step3. The earth wire is removed. _ _ _ _
+ + + +
`
Step 4. The charged rod is removed. The positive charges spread on the surface of the sphere as like charges repel.
+ + + +
NOTE: 1. The spheres have an insulating handle / stand which prevents the charges from flowing back onto the sphere once charged. 2. The number of charges on the rod and the sphere must be the same. Earthing: it is providing a path for the electrons to go down to the earth or come up from the earth to the charged object.
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EXAMPLE 2: CHARGING TWO SPHERES WITH OPPOSITE CHARGES. Step 1. Touch the two spheres together and bring a charged rod near one end. The negative charges on the sphere near the rod are repelled to the far end of the second sphere leaving behind positive charges. _ _ _ _
+ + + +
Step 2. The two spheres are separated each one with an opposite charge.
-
-
+ + +
-
-
+ -
3. CHARGING BY TOUCH. EXAMPLE 1: When a metal plate is attached to the positive terminal of a battery, the plate becomes positively charged. The electrons on the metal plate X are attracted to the positive terminal of the battery resulting in an excess of positive charge left behind on the plate. The positive plate attracts electrons from the negative terminal of the battery to plate Y. So plate Y becomes negatively charged.
metal plates
10 V battery
EXAMPLE 2: When a negatively charged rod is touched on to a metal sphere, the electrons travel to the metal sphere and make it negatively charged. Similarly, when a positively charged rod touches a metal sphere, electrons from the sphere travel onto the rod to neutralize the positive charges. This makes the metal sphere lose electrons and so it becomes positively charged. ELECTRIC FIELD: It is a region around a charge in which other charges experience a force. NOTE: Lines of force are drawn outward from positive charge and inward towards a negative charge.
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Examples of electric fields
HAZARDS OF STATIC ELECTRICITY: 1. 2. 3. 4.
Electric shock Fire Explosion Lightning
USES OF STATIC ELECTRICITY: 1. 2. 3. 4. 5. 6.
Photocopy machine Spray painting cars Insecticide spraying on plants Precipitators Gold leaf electroscope. Lightning conductor.
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Lightning conductor (rod): If lightning falls it travels through the rod to the ground thus preventing the building.
How does lightning occur? It occurs when electrons from a low lying cloud induce a positive charge on the top of a building. The electrons then jump off from the cloud on top of the building causing a lightning bolt. This is how lightning occurs.
Principle of the lightning conductor The lightning conductor (rod) does not only prevent the building when struck by lightning but in fact it prevents lightning
from striking the building in the first place. As a storm cloud comes above a tall building it repels electrons from the lightning conductor to the ground through the copper strip. The top of the lightning conductor has spikes which become positively charged as electrons are repelled to the ground (induction). The air in between the cloud and the conductor is ionized as electrons from it are attracted towards the positive spikes of the lightning conductor. The lightning conductor is thus neutralized in this way. The positive ions of air left behind are attracted towards the cloud as shown. The cloud is also neutralized in this way. Both the conductor and the cloud are neutralized thus no lightning occurs.
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Precipitator It is a metal plate placed inside a chimney that eliminates smoke particles (ash and carbon) from waste gases as they rise through the chimney. The waste gases that come out of the chimney are much cleaner and less smoky causing less air pollution. The detail working is explained below in the diagram.
NOTE: you need to memorize at least one use of static electricity in detail. A question on use of static electricity can come in paper 2 section B with 4 to 5 marks.
CONDUCTORS: Those materials which allow electrons to flow through such as metals. INSULATORS: Those materials which do not allow electrons to flow through such as plastics, glass and wood.
By Shafaq Hafeez shafaq@physics.com.pk