charging by friction

Teacher’s NotesThis sequence of slides is designed to introduce, and explain electrostatic charging by friction, as explained on page 242 in New Physics for You, 2006 & 2011 editions(page 248 in Physics for You, 2001 edition).

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Charging by frictionCharging by friction

New Physics for You, page 242

• How insulated objects can be charged by friction,

• That this is because some charges can move.

Learning Objectives

You should learn :

Charging by friction

The wool and polythene are each ‘uncharged’.What does this mean?

In this diagram:

Charging by friction

The wool is ‘uncharged’,because……it has equal amounts of positive and negative charge.Equal numbers of + and −On the wool

+ − +

+ −− + −

−+ +− −

+

+ −+−−+

On the polythene

Charging by friction

Equal numbers of + and − on the wool:(Count them!)7 + 7 −Equal numbers of + and − on the polythene:(Count them!)3 + 3 −

+ − +

+ −− + −

−+ +− −

+

+ −+−−+

Charging by friction

If you rub the wool on the polythene, some electrons (−) move from the wool to the polythene.There are now more + than − on the wool:(Count them!)

+ − +

+ − +

−+ +

− +

+ − −

+−−

− −+

So now the wool is charged positively,with a surplus of 3 +

Charging by friction

What has happened to the polythene?

There are now more − than + on the polythene:(Count them!)

+ − +

+ − +

−+ +

− +

+ − −

+−−

− −+

So now the polythene is charged negatively,with a surplus of 3 −

Charging by friction

Both objects are now equally charged,with opposite chargesbecause electrons − (only) have moved.

This is summed up in the diagram on page 242:

• Understand what it means when an object is ‘charged’ or ‘uncharged’,

• Understand how an insulated object becomes charged by rubbing,

• Understand why the objects have equal but opposite charges,

• Know that only electrons (−) can move.

Learning OutcomesYou should now:

For more details, see: New Physics for You, page 242

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