Electrical Quantities

What these notes cover

These notes cover electric charge and electrical quantities for Cambridge O Level Physics (5054) and Cambridge IGCSE Physics (0625): positive and negative charge, attraction and repulsion, conductors and insulators, charging by friction, electric fields and field-line patterns, the gold-leaf electroscope, earthing, and electric current as the rate of flow of charge. They include past-paper questions on charge, current, resistance and power.

Key formulas and definitions

• There are two types of charge, positive and negative, measured in coulombs (C). The charge on an electron is −1.6 × 10⁻¹⁹ C and on a proton is +1.6 × 10⁻¹⁹ C.
• Like charges repel; unlike charges attract.
• Charging by friction: rubbing two insulators together transfers electrons from one to the other. The material that gains electrons becomes negatively charged, and the one that loses electrons becomes positively charged.
• Conductors (such as metals) allow charge to flow through them; insulators (such as plastic, rubber and glass) do not.
• An electric field is a region in which an electric charge experiences a force. The direction of the field at a point is the direction of the force on a positive charge placed there.
• Field lines point from positive to negative. Around a charged sphere they are radial; between two oppositely charged parallel plates they are parallel and equally spaced, showing a uniform field.
• Electric current is the rate of flow of charge: I = Q / t. Unit: ampere (A), where 1 A = 1 C/s.
• Earthing a charged conductor discharges it, as electrons flow between the conductor and the earth. In circuit questions, use V = IR and P = IV.