In a basic d.c. electric motor, what is the role of the split-ring commutator?
To periodically reverse the poles of the external permanent magnets
To reverse the direction of the current through the coil every half-turn, ensuring continuous rotation in one direction
To decrease the electrical resistance of the coil as its speed of rotation increases
To change the voltage of the external direct current power supply on every complete rotation
275 exam-style questions on CIE IGCSE Physics 4.5 Electromagnetic effects, covering 4.5.1a Electromagnetic induction, 4.5.1b Direction of an induced e.m.f., 4.5.2 The a.c. generator, 4.5.3a Magnetic effect of a current and its applications, 4.5.3b Strength and direction of the field around wires and solenoids, 4.5.4a Force on a current-carrying conductor, 4.5.4b Force on beams of charged particles, 4.5.5 The d.c. motor, 4.5.6a Construction and operation of a transformer, 4.5.6b Turns ratio and current in a transformer, and 4.5.6c High-voltage transmission of electricity. Each one has a worked solution and a mark scheme showing where the marks go.