Two kinds of charge
There are two types of electric charge: positive and negative. Protons carry positive charge and electrons carry negative charge. The law of electrostatics states that like charges repel and unlike charges attract. Charge is measured in coulombs (C).
Key idea
Charge, current and time are linked by Q = I t (charge = current × time). One coulomb is the charge that flows when a current of 1 A flows for 1 s.
Charging by friction
When two insulators are rubbed together, electrons are transferred from one to the other. The object that gains electrons becomes negatively charged, and the object that loses electrons becomes positively charged. Only electrons move — the positive protons stay fixed in the nucleus. For example, rubbing a polythene rod with a cloth makes the rod gain electrons and become negative.
Conductors, insulators and fields
- Conductors (metals) let charge flow freely; insulators (plastic, rubber) do not.
- An electric field surrounds a charge; field lines point from positive to negative.
- Connecting a charged conductor to earth lets charge flow away, removing it safely.
Worked example
A current of 2 A flows for 5 s. The charge transferred is Q = I t = 2 × 5 = 10 C.
Remember
- Like charges repel; unlike charges attract.
- Only electrons transfer during charging by friction.
- Gained electrons → negative; lost electrons → positive.