Page 145 - Science Coursebook

10.2 Positive and negative charge

Charging by friction
In an experiment to investigate static
electricity, you might start with a polythene polythene
rod and a woollen cloth. Neither has an
electric charge. We say that they are neutral.

When the rod is rubbed using the cloth, the
rod gains a negative electric charge. It is the
force of friction that gives charge to the rod.
wool
At the same time, the cloth gains a positive
electric charge.
If you use a rod made of acrylic instead of

polythene, you will find that the rod has a
positive electric charge. acrylic

The charges that appear when two different
materials are rubbed together depend which
materials are used. One material gains a cotton
positive charge, the other a negative charge.

Questions

3 What force causes an object to become charged when it is rubbed?
A+I 4 What charge will the cloth have when it has been used to rub an
acrylic rod?

Activity 10.2
Testing ideas about charge

SE When a plastic rod and a cloth are rubbed together, they gain opposite electric
charges. Your task is to test this idea in two ways:

• Use a coulombmeter to find out about the charges on the rod and the cloth.
• Show that the charged rod and the charged cloth attract each other.
Write a step-by-step description of how you will go about each of these tasks.
Check with your teacher before you carry out your plans.

Summary
• There are two types of electric charge, positive (+) and
negative (−).
• Opposite charges attract, like charges repel.
• A digital meter (coulombmeter) can be used to show whether an object
has positive or negative charge.

10 Electricity 143