Electricity and Magnetism

• Memory anchor — N-to-S outside, never cross, only repulsion proves a magnet. Soft iron switches off; steel stays on. Extended adds: close spacing means a strong field, and the force itself is two fields interacting.
• Memory anchor — Charge is carried, current is charge flowing, e.m.f. gives energy per unit charge, p.d. takes it, resistance is volts per amp. Ammeter in series (low \(R\)); voltmeter in parallel (high \(R\)). \(P = IV\), \(E = IVt\), \(1\ \mathrm{kWh} = 3.6 \times 10^{6}\ \mathrm{J}\).
• Memory anchor — Series shares p.d.; parallel shares current. Series resistance adds up; parallel resistance is always smaller than the smallest branch. At constant current, more resistance means more p.d. across it.
• Memory anchor — Four hazards: bare wire, hot cable, damp, overload. The live wire holds the switch, the fuse and the trip switch. Earth turns a fault into a big current that blows the fuse. Casing: earthed, or double-insulated — and with no earth, the fuse still guards the circuit and the cabling.
• Memory anchor — Force from current in a field (motor, LEFT hand, split ring). E.m.f. from a changing field (generator and transformer, RIGHT hand, slip rings). aNticlockwise face = N pole. High-voltage transmission: same power, smaller current, and losses go as the current squared.

Why it matters: the generator, the motor and the transformer are the three devices most often confused with one another. Each has its own diagram, its own connector (slip rings or commutator) and its own governing principle, and it is those three differences that keep them apart.

E.m.f. and potential difference

E.m.f. is the electrical work done by a source in moving a unit charge around a complete circuit, while potential difference is the work done by a unit charge passing through a component; both are measured in volts.

Electric current

The charge passing a point in a circuit per unit time, I = Q/t, measured in amperes; one ampere is one coulomb per second.

Series and parallel circuit rules

The two conservation rules that govern every circuit: charge is conserved, so the current is the same everywhere in a series circuit and the source current in a parallel circuit is larger than any one branch current; energy is conserved, so the supply p.d. is shared between series components while every parallel branch has the same p.d. Combined resistance is R = R1 + R2 + R3 in series, and in parallel it is always less than either resistor alone.

Electrical power and energy

Electrical power is the energy transferred per unit time in a circuit, P = IV, and the energy transferred is E = IVt; a kilowatt-hour is the energy transferred by a 1 kW device running for 1 hour, equal to 3.6 million joules.

Electric field

A region in which an electric charge experiences a force; the direction of a field line at a point is the direction of the force on a positive charge placed there.

Magnetic field

A region in which a magnetic pole experiences a force; the field direction at a point is the direction of the force on a north pole placed there.

Resistance

Resistance is the potential difference across a component divided by the current in it, R = V/I, measured in ohms.

The mains circuit and its protective devices

A mains circuit consists of a live (line) wire at a high potential relative to earth, a neutral wire near earth potential, and an earth wire connected to any conducting casing; the switch, the fuse and the trip switch are all placed in the live wire so that operating one disconnects the appliance from the dangerous potential.

Electromagnetic induction

An e.m.f. is induced in a conductor when the conductor moves across a magnetic field, or when a changing magnetic field links with the conductor; the size of the induced e.m.f. depends on the rate of change and on the number of turns in a coil.

The transformer equation

For an ideal transformer the ratio of the primary to secondary potential difference equals the ratio of primary to secondary turns, Vp/Vs = Np/Ns; more turns on the secondary steps the p.d. up, fewer turns steps it down.