Electromagnetic effects

A magnet’s mere presence is insufficient: holding it still relative to an unchanged coil does not continuously induce an e.m.f. Compare the rate of change rather than only the final field strength. In a practical investigation, keep the magnet path, coil and meter arrangement consistent while changing one variable. Increasing turns can also increase the coil’s resistance, so a change in meter current is not a pure measure of e.m.f. unless that effect is considered. Separate the induced voltage from the current it drives.

The cable resistance refers to the transmission path, while the desired transmitted power refers to the useful electrical transfer being delivered. Keep those quantities separate in the calculation. If current is halved and cable resistance remains constant, the heating loss becomes one quarter. Raising voltage brings its own insulation and design requirements, so the explanation is a reason for using high voltage rather than a claim that voltage can increase without limit. An ideal transformer preserves power; real equipment has additional losses.

The hardware should be matched to its purpose rather than memorised as two interchangeable rings. A motor’s coil rotates through positions where the same unaltered current direction would reverse its turning tendency, so commutation changes the coil current at the appropriate points. In the basic a.c. generator, rotation naturally reverses the induced e.m.f. every half-turn and the slip rings preserve that alternating output. Brushes provide contact with the rotating parts in both models. More complex machines exist, but the standard diagrams have these distinct functions.

Induction depends on changing magnetic linkage, not simply on current existing in the primary. At the instant a switch closes or opens, the current changes, so the field can change and induce a secondary signal. Once an idealised steady current is established, that mechanism ceases. Do not claim that only sinusoidal current can ever induce a voltage: other changing waveforms can do so too. The syllabus transformer explanation uses a.c. because it supplies repeated field changes while supporting the stated voltage-ratio model.

Describe one controlled change at a time. A north pole approaching a coil and the same north pole receding create opposite changes, so a centre-zero meter deflects in opposite directions. A stationary magnet produces no steady induced signal in this setup. Lenz’s law connects the direction to opposition to the change: the induced effect resists the process that produces it, consistent with needing mechanical work to generate electrical energy. The rule does not mean the induced field must always oppose the original field itself; it opposes the change.

For a conductor perpendicular to the field, Fleming’s left-hand rule links three mutually perpendicular directions. Electron drift in a metal is opposite to conventional current, so using the electron direction without correcting it reverses the predicted answer. If the wire is parallel to the field, the magnetic force from this arrangement is zero rather than a force along the wire. When a diagram has several arrows, label their meanings before applying the rule; a motion arrow is not automatically the current direction.