Electricity and Magnetism

Five quantities do almost all the work in Topic 4. Charge \((Q)\) is the property that is carried. Current \((I)\) is the flow of that charge. E.m.f. \((E)\) is the electrical work a source does on each unit of charge sent round a complete circuit. Potential difference \((V)\) is the work each unit of charge does passing through a component. Resistance \((R)\) is the p.d. across a component per unit current in it. Power and energy then follow. Magnetism adds fields and forces, and a changing magnetic field adds induced e.m.f.

Like poles repel and unlike poles attract; a magnet also attracts an unmagnetised magnetic material because it induces magnetism in it. A magnetic field is a region in which a magnetic pole experiences a force. The field direction at a point is the direction of the force on an N pole placed there, so outside a bar magnet the lines run from N to S.

Charge is the property carried, and it is positive or negative. Current is the flow of that charge, measured in amperes with an ammeter in series. E.m.f. is the electrical work a source does moving unit charge round a complete circuit; potential difference is the work unit charge does passing through a component; both are measured in volts, read with a voltmeter in parallel. Resistance is the p.d. across a component per unit current in it, \(R = V/I\), measured in ohms. Power is the rate of energy transfer, \(P = IV\), and the energy transferred is \(E = IVt\).

A circuit obeys two conservation rules. Charge is conserved, which gives the current rules: the current is the same everywhere in a series circuit, and in a parallel circuit the source current is larger than the current in any one branch. Energy is conserved, which gives the p.d. rules: the supply p.d. is shared between components in series, and every branch of a parallel arrangement has the same p.d. across it. The current rules are Core; the p.d. rules are the background every candidate needs in order to make sense of Core 4.3.2.7, but as examinable statements they are Supplement 4.3.2.8.

Mains electricity is dangerous because a person can complete a path to earth. Safety rests on one idea: a fault must be disconnected quickly, and the disconnection must happen in the live wire. A metal casing is made safe by earthing it, so that a fault produces a large current that operates the protective device; a non-conducting casing is made safe by double insulation instead.

This section is a two-way street with a strict division of labour. Current in a magnetic field produces a force — that is the motor effect. A changing magnetic field produces an e.m.f. — that is electromagnetic induction. Motors use the first; generators and transformers use the second. Keeping those two sentences apart prevents most of the errors in Topic 4.