Static Electricity: Electric Charge

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Electric Charge

Q = total charge on a body

q = charge on an individual particle

Indicates if a body has an excess or a deficiency of electrons

Scalar quantity

Unit: Coulomb (C)

1 Coulomb is the amount of charge that passes any point in a circuit when a current of 1 ampere flows for 1 second

Like charges repel each other; unlike charges attract each other

Methods of Charging

Charging by contact

Charging by induction

Charging by contact

Involves the contact of a charged object to a neutral object

The substance with a weaker attraction will lose some of its electrons to the other substance

Rubbing polythene or Perspex rods with a cotton cloth causes the polythene rod to gain electrons from the cloth, whereas the Perspex loses electrons to it

The polythene rod becomes negatively charged

The Perspex rod becomes positively charged

Charging by induction

If you bring a charged insulator (a negatively charged polythene rod) close to a conductor (a metal sphere on an insulated handle), the charge on the rod will cause the opposite charge to be attracted to it, and the similar charge to be repelled

If you then earth the sphere (by touching it), the negative charges will flow to the ground

Then, you remove the earth and subsequently the charged rod

The positive charges spread out and you are left with a positively charged metal sphere

In this diagram, a positively charged rod is touched against a neutral gold leaf electroscope

The positive charges on the rod attract the negative charges and repel the positive charges on the electroscope

The negative charges are cancelled out by the positive charges on the rod, leaving everything positively charged

The rod is then taken away, leaving a positively charged electroscope

Conductor = any substance through which electric charge can flow

Insulator = any substance through which electric charge cannot flow

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