An electronics student has an 18 V a.c. power supply. She wishes to use it to power a 6.0 V, 4.0 Ω lamp at its normal operating brightness, using either a transformer or a resistor.
State and explain what would happen to the lamp if she connected it directly to the 18 V power supply.
She decides to use an ideal transformer to step down the voltage to the required level.
Describe how the power supply and the lamp should be connected to the primary and secondary coils of the transformer.
Calculate the value of the ratio:
number of turns on secondary coilnumber of turns on primary coil \frac{\text{number of turns on secondary coil}}{\text{number of turns on primary coil}} number of turns on primary coilnumber of turns on secondary coilneeded for the lamp to operate at its normal brightness.
Alternatively, she considers using a single resistor connected in series with the lamp across the 18 V supply.
Calculate the resistance of the resistor required for the lamp to light at normal brightness.
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.