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OCR GCSE Physics A · J249
OCR J249 · P3.1b Check the specification (PDF) (opens in a new tab)
You may have seen a balloon cling after being rubbed on clothing, or felt a tiny shock after walking across a carpet. These are effects of static electricity: charge builds up on an object rather than flowing continuously around a circuit. Rubbing two different surfaces together can separate charge. Electrons move from one surface to the other, leaving the surfaces oppositely charged; rubbing does not create charge from nothing. The detailed account of which surface gains or loses electrons belongs to the next lesson.
The charge remains where it builds up most readily on an insulator, such as a plastic rod or rubber balloon, because charge cannot move easily through it. On a conductor such as metal, charge moves easily and will usually flow away through your hand if you hold it while rubbing it. An isolated conductor can nevertheless retain charge: the metal dome of a Van de Graaff generator is supported by an insulator. Its moving belt transfers charge to the dome, allowing a large build-up.
Charged objects can push or pull one another even when there is a gap between them. Suspend a charged plastic rod so it can turn freely, then bring a second charged rod close without touching it. If both carry the same type of charge, the suspended rod moves away: like charges repel. If they carry opposite types, it moves towards the other rod: unlike charges attract. The movement before contact is evidence of an electrical force acting at a distance. A charged rod attracting an uncharged piece of paper is a useful static-electricity observation, but attraction alone does not show that both objects are charged.
As charge builds up, the electrical difference between a charged object and a nearby object can become large enough for charge to cross the air gap. Charge then moves suddenly through the air: this brief discharge is a spark. It may be visible and make a crackling sound, as when a charged Van de Graaff dome sparks to a nearby conductor. The spark is not charge being made; it is charge that had built up being released.
Rubbing can leave charge on an insulator. Charged objects exert forces across a gap, and a large build-up can discharge as a spark.
Rubbing surfaces separates charge. Static charge builds up on insulators because it cannot move away easily; charge usually escapes from a conductor that is connected to earth. A sufficiently large build-up can discharge suddenly across an air gap as a spark.
Like charges repel and unlike charges attract, even before the objects touch. Repulsion shows that both objects are charged; attraction alone does not.
When describing an attraction or repulsion test, state that the charged objects move while they are still apart: the force acts without contact.
Two objects attracting does not, by itself, prove that both are charged: a charged object can also attract a neutral one. Repulsion is evidence that both objects are charged.
Distinguish charge building up on an insulator from a spark, in which charge suddenly crosses an air gap.
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Static electricity
The build-up of electric charge on an object, rather than a continuous flow of charge through a circuit.
Insulator
A material, such as plastic or rubber, through which electric charge cannot move easily.
Conductor
A material, such as metal, through which electric charge can move easily.
Spark
A brief discharge in which electric charge passes through air between objects.
Put your knowledge into practice — try past paper questions for Physics A
Static electricity
The build-up of electric charge on an object, rather than a continuous flow of charge through a circuit.
Insulator
A material, such as plastic or rubber, through which electric charge cannot move easily.
Conductor
A material, such as metal, through which electric charge can move easily.
Spark
A brief discharge in which electric charge passes through air between objects.