Electromagnetic effects

An a.c. generator uses slip rings — two continuous rings that keep the coil on the same brushes throughout, so the naturally alternating e.m.f. reaches the circuit unchanged. A d.c. motor uses a split-ring commutator, whose gap swaps the connections every half-turn to keep the coil turning one way. Pairing to learn: generator → slip rings, motor → commutator.

The size of an induced e.m.f. depends on the speed of the movement, the strength of the field and the number of turns — never on the conductor's resistance. Resistance would change the induced current in a complete circuit, but the e.m.f. itself is set only by how fast field lines are cut.

Around a straight wire the field is concentric circles, in planes at right angles to the wire — not straight lines. Their direction comes from the right-hand grip rule and reverses when the current reverses, so arrow them. A solenoid instead gives a bar-magnet field, uniform inside with N and S ends; more current, or a soft-iron core, strengthens it.

An electromagnet or transformer core is soft iron, because it magnetises and demagnetises easily. Copper is the wrong answer — copper is the coil winding, not the core. Steel is wrong too: steel keeps its magnetism, so it makes a permanent magnet rather than a core that must follow a changing current.

The motor-effect force is greatest when the current is perpendicular to the field and zero when the current runs parallel to it — a wire lying along the field lines cuts none and feels nothing. Reversing the current reverses the force; reversing the field reverses it as well; reversing both leaves the force unchanged.

Transmitting at high voltage does not send more power, and it does not lower the cable resistance — that is fixed by the cable. The real chain: for a fixed power, $P = IV$, so a high voltage means a small current; the cable loss is $P = I^2R$, which depends on the square of the current, so a small current wastes far less.

e.m.f. is the work done per unit charge by the source — the energy a generator or cell supplies to each coulomb. p.d. is the energy transferred from each coulomb by a component. Both are measured in volts, but they describe opposite directions of energy transfer, and questions ask you to distinguish them.