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Electric charge (1)

rhis person has been charged up. Her hairs a!! carry tbe same type of :harge, so they repel each other.

Electric charge, or ‘ekctricity’, can come from batteries and generators. But some materials become charged when they are rubbed. Their charge is sometimes called electrostatlc charge or ‘static elecrridty*. It causes sparks and crack]es when you take off a pullover, and if you slide out of a car seat and toucli the door, it may even givc you a shock.

Negative and positiye charges

Polythene and Pcrspex can be charged by rubbing tliem with a dry, woollen dotli.

When two charged polythene rods are brought dose together, as shown belo w, tliey repel (try to push each otlier apart). The same thing happens with two charged Perspex rods. However, a charged polythene rod and a charged Perspex rod atlract each other. Expcriments like this suggest tliat there are two different and opposite typcs of electric charge. These are called positive (+) charge and negative (-) charge:

Like charges repel; unlike charges attract.

The closer the charges, The greater the force betwecn tliem,

nueleus:

$ proton

@ neutron

Where charges come from

Everytliing is madę of tiny particles called atoms. These have electric charges inside them. A simple model of the atom is shown on the left. There is a central nueleus madc up of protons and neutrons. Orbiting the nueleus are much lighter electrons;

Electrons have a negative (-) charge,

Protons have an eąual positive (+) charge.

Neutrons have no charge.

Normally, atoms have equal numbers of electrons and protons, so the net (overall) charge on a materiał is zero. Ho\vever, when iwo materials are rubbed together, electrons may be transferred from one to the other. One materiał ends up with morę electrons than normal and the other with less.

So one has a net negative charge, while the other is left with a ner posirive charge. Rubbing materials together does not make electric charge. It just separates charges that are already there.

A When polythene is rubbed with a woollen cloth, the polythene pulls electrons from the wool,

A When Perspex is rubbed with a woollen cloth, the wool pulls electrons from the Perspex,

fewer electrons than normal: net p05itive charge

morę electrons than normal; net negatiye charge

Conductors and insulators

When some materials gain charge, they lose it almost immediatdy. This is because electrons flow through them or the surrounding materia! until the balancc of ncgative and positire charge is rcstored.

Conductors are materials that let electrons pass through them. Metals are the best electrica] conductors, Some of their electrons are so loosely held to their atoms that they can pass freely betwecn them. These free electrons also make metab. good thermal conductors.

Most non-mctals conduct charge poorly or not at all, although carbon is an exception.

Conductors

Good    Poor

metals

especially:

siher

copper

aluminium

water

human body earth

carbon

Semlconductors

Insulators are materials that hardly conduct at all. Their electrons are tightly held to atoms and are not free to move - although they can be transferred by rubbing. Insulators are casy to charge by rubbing because any electrons that get transferred tend to stay where they are.

Semiconductors are ‘in-bctwcen’ materials. They are poor conductors when cold, but much better conductors when warm.

Silicon    germanium

Insulators

plastics    glass

e.g:    rubber

PVC    dry air

polythene Perspex

insulator <PVCJ

< The ‘electricity¹ in a cable is a fbw of electrons. Most cables have copper conducting wires with PVC plastic around them as insulation.

rO--

1    Say whether tire following auraci or repel:

a)    two negative charges

b)    a ncgative charge and a posttive charge

c)    two posttive charges.

2    In an atom, what kind of charge is carricd by a) protons b) electrons c) neutrons?

3    What makes copper a better elcctrical conductor than polythene?

4    Why is it casy to charge polythene by rubbing, but not copper?

5    Name one non-mctal that is a good conductor.

6    When somcone pulls a piastic comb through their h air, the comb be co mes negativcly charged,

a)    Which ends up with morę electrons than normal, the coinb or the hatr?

b)    Why does the hair become positivcly charged?