Introduction and key concepts

Force between charges

The force exerted by non-moving (static) charges on each other is called the electrostatic force. The electrostatic force between:

• like charges is repulsive
• opposite (unlike) charges is attractive .

In other words, like charges repel each other while opposite charges attract each other. This is different to the gravitational force which is only attractive.

The closer together the charges are, the stronger the electrostatic force between them.

Experiment : electrostatic force

You can easily test that like charges repel and unlike charges attract each other by doing a verysimple experiment.

Take a glass rod and rub it with a piece of silk, then hang it from its middle with a piece string so that it is free to move. If you then bring another glass rod which you have also charged in the same way next to it, you will see the rodon the string turn away from the rod in your hand i.e. it is repelled . If, however, you take a plastic rod, rub it with a piece of fur and then bring it close to the rod on thestring, you will see the rod on the string turn towards the rod in your hand i.e. it is attracted .

This happens because when you rub the glass with silk, tiny amounts of negative charge are transferred from the glassonto the silk, which causes the glass to have less negative charge than positive charge, making it positively charged . When you rub the plastic rod with the fur, you transfer tiny amounts ofnegative charge onto the rod and so it has more negative charge than positive charge on it, making it negatively charged .

Two charged metal spheres hang from strings and are free to move as shown in the picture below. The right hand sphere is positively charged. The charge on the left hand sphere is unknown.

The left sphere is now brought close to the right sphere.

1. If the left hand sphere swings towards the right hand sphere, what can you say about the charge on the left sphere and why?
2. If the left hand sphere swings away from the right hand sphere, what can you say about the charge on the left sphere and why?

1. Identify what is known and what question you need to answer:

   In the first case, we have a sphere with positive charge which is attracting the left charged sphere. We need to find the charge on the left sphere.

2. What concept is being used?

   We are dealing with electrostatic forces between charged objects. Therefore, we know that like charges repel each other and opposite charges attract each other.

3. Use the concept to find the solution
   1. In the first case, the positively charged sphere is attracting the left sphere. Since an electrostatic force between unlike charges is attractive, the left sphere must be negatively charged.
   2. In the second case, the positively charged sphere repels the left sphere. Like charges repel each other. Therefore, the left sphere must now also be positively charged.

Polarisation

Unlike conductors, the electrons in insulators (non-conductors) are bound to the atoms of the insulator and cannot move around freely through the material. However, a charged object can stillexert a force on a neutral insulator due to a phenomenon called polarisation .

If a positively charged rod is brought close to a neutral insulator such as polystyrene, it can attract the bound electronsto move round to the side of the atoms which is closest to the rod and cause the positive nuclei to move slightlyto the opposite side of the atoms. This process is called polarisation . Although it is a very small (microscopic) effect, if there are many atoms and the polarised object islight (e.g. a small polystyrene ball), it can add up to enough force to cause the object to be attracted onto thecharged rod. Remember, that the polystyrene is only polarised, not charged. The polystyrene ball is still neutral since no charge was added or removed from it. The picture showsa not-to-scale view of the polarised atoms in the polystyrene ball:

Some materials are made up of molecules which are already polarised. These are molecules which havea more positive and a more negative side but are still neutral overall. Just as a polarised polystyrene ball can be attracted to a charged rod, these materialsare also affected if brought close to a charged object.

Water is an example of a substance which is made of polarised molecules. If a positively charged rod is brought close to a stream of water, the molecules can rotateso that the negative sides all line up towards the rod. The stream of water will then be attracted to the rod since opposite charges attract.

Conservation of charge

The principle of conservation of charge states that the net charge of an isolated system remains constant during any physical process, e.g. when two charges make contact and are separated again.