Page 144 - Science Coursebook

10.2 Positive and negative charge

When people first began to study electricity scientifically,

over 300 years ago, they had very little understanding
of what was going on. They discovered different ways

of charging things, and they suggested different uses for
electricity.
The picture shows a German physicist, Otto van
Guericke, working 350 years ago. When he turned
the yellow ball of sulfur and rubbed it with a pad,
he saw sparks.

Explaining electric forces
It took many years for scientists to develop a good
explanation of why charged objects sometimes attract
each other and sometimes repel each other.

Here is the theory they came up with.
There are two types of electric charge, which we call
positive (+) and negative (−).
• Positive and negative charges attract one another.
• Positive charges repel one another.
• Negative charges repel one another.
You can remember this by remembering that ‘opposites
attract’. (This is the same as for magnetic poles – a north
pole and a south pole attract one another.)

The diagram shows what happens when two charged The green arrows show the forces
balls are hung close together. between the electric charges.

Questions
1 Look at the picture of the charged balls. What
symbols are used to represent positive and
negative charge?
2 Draw a similar diagram to show what happens
when two negatively charged balls are hung next
to each other.

Finding the sign of an electric charge
You can use a digital meter called a coulombmeter

to find out whether a charged object has positive or
negative charge.
The photograph shows a polythene rod which has been
charged by rubbing. The reading on the meter has a Finding the sign of an electric charge
minus sign, showing that the rod has a negative charge. using a coulombmeter.

142 10 Electricity

A+E