Electric charge and electrostatic forces

Electric charge has two types: positive and negative. Charged objects exert forces on one another. These are called electrostatic forces.

The types of charge determine whether the force pushes or pulls:

Like charges repel: two positive charges or two negative charges push each other apart.
Opposite charges attract: a positive charge and a negative charge pull each other together.

Charging by friction

When two suitable solid materials are rubbed together, electrons can transfer from one material to the other. This leaves the two materials with opposite charges:

The material that gains electrons becomes negatively charged.
The material that loses electrons becomes positively charged.

Only negative charge is transferred between the solids. The positive charges do not move from one solid to the other. Rubbing does not create charge; it transfers electrons between the materials.

Detecting charge after friction

A plastic rod can be rubbed with a cloth and then brought close to small pieces of paper. If the rod attracts the paper, this indicates that it has become charged. The attraction alone does not show whether the rod is positively or negatively charged.

Charge can also be detected using an electroscope. When charge is transferred to the electroscope, its conducting stem and leaf acquire charges of the same type. Like charges repel, so the leaf moves away from the stem and rises.

Electrical conductors and insulators

An electrical conductor allows electric charge to move through it readily. An electrical insulator does not allow charge to move through it readily.

In a simple electron model, a conductor contains electrons that are free to move through the material. In an insulator, the electrons are not free to move through it. Metals are typical conductors, while plastic and glass are typical insulators.

Testing a material in a circuit

A material can be tested using a simple circuit containing a cell and a lamp. Leave a gap in the circuit, then place the material across the gap so that it connects both sides.

If the material conducts sufficiently, it completes the circuit and charge flows. The lamp lights. If the material is an insulator, charge does not flow readily through it, so the lamp remains unlit.

An unlit lamp does not by itself prove that the material is a perfect insulator: it may conduct, but not enough for the lamp to light.

Electric fields

An electric field is a region in which an electric charge experiences a force. A charged object produces an electric field around it; another charge placed in that field experiences a force.

The direction of the field at a point is the direction of the force on a positive charge placed there. A negative charge at the same point experiences a force in the opposite direction.

Electric charge, whether it produces or experiences a field, is measured in coulombs ( $\mathrm{C}$ ).

Electric fields around charges and spheres

Electric field lines show the direction of the field at each point. Around a point charge, the lines form a radial pattern:

For a positive point charge, the lines point outwards, away from the charge.
For a negative point charge, the lines point inwards, towards the charge.

Outside a charged conducting sphere, the field is also radial. It points outwards from a positively charged sphere and inwards towards a negatively charged sphere.

Electric field between parallel plates

Between oppositely charged parallel conducting plates, away from their edges, the field lines are straight, parallel and equally spaced. They point from the positive plate to the negative plate.

The field in this region is uniform: it has the same strength and direction at every point.