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Electrical Systems · Part 3 of 4
9 exam-style questions with model answers, plus 12 quick multi-choice questions — every question on this part of the standard, grouped by the 3 pages of notes they come from.
Write a full answer before you reveal the model one. That comparison is where the learning happens.
A single loop of area m2 lies perpendicular to a magnetic field of T.
Calculate the magnetic flux through the loop.
A coil of turns and area m2 is in a magnetic field that falls uniformly from T to zero in s.
Calculate the induced emf, and explain why no emf would be induced if the field were held steady at T.
A strong bar magnet is dropped down a long vertical copper tube. It is observed to fall much more slowly than an identical unmagnetised bar dropped down the same tube, eventually reaching a constant slow speed.
Explain fully why this happens, referring to Faraday's law, Lenz's law and conservation of energy. Explain also why the magnet reaches a constant speed rather than continuing to slow down.
A coil of inductance H carries a current changing at A s−1.
Calculate the magnitude of the back emf induced.
A H inductor is connected in series with a Ω resistor across a V supply.
Calculate the time constant and the final steady current. Explain why the current does not reach its final value immediately.
A relay coil of inductance H and resistance Ω is operated from a V supply. When the switch is opened, the current falls to zero in about ms and a bright spark jumps across the switch contacts.
Calculate the steady current, the energy stored, and the emf generated as the current collapses. Explain fully why the spark occurs, and explain how a diode connected across the coil prevents it.
A transformer has turns on the primary and turns on the secondary. The primary voltage is V.
Calculate the secondary voltage and state whether this is a step-up or step-down transformer.
A transformer steps V down to V and is connected to a device drawing A. Assume it is ideal.
Calculate the primary current, and explain why a transformer will not work if connected to a DC supply.
A power station generates MW at kV. It is transmitted km along cables of total resistance Ω, first stepping the voltage up to kV and stepping it back down at the far end.
Calculate the transmission current and power loss with and without the step-up transformer. Explain fully why transformers make the grid viable, and explain why real transformers are built with laminated cores.