Question
A student uses a laptop computer. The student notices that the cable connecting the power
adapter for a laptop to the mains electricity supply is damaged as shown in Fig. 10.1.
(a) State the hazard of using mains equipment with damaged insulation.
(b) Describe how a fuse protects a mains electrical appliance.
(c) The laptop computer uses a transformer to change the voltage of the mains electricity supply.
The input (primary) voltage is 120 V.
The input (primary) coil has 2000 turns and the output (secondary) coil has 200 turns.
Calculate the output (secondary) voltage from the transformer.
output (secondary) voltage = ……………………………………………… V
(d) State the name of the material used in the core of the transformer.
Answer/Explanation
Answer:
(a) electrocution OR overheating / fire
(b) large current (in fuse)
(causes) fuse / it to melt
isolating appliance from supply OR prevents current in appliance OR breaks circuit
(c) \(V_s\) / \(V_p\) = \(N_s\) / \(N_p\)
\(V_s\) / 120 = 200 / 2000 OR \(V_s\) = 120 × 200 / 2000 OR \(V_s\) = 120 / 10
12 (V)
(d) (soft) iron
Question
(a) A teacher demonstrates electromagnetic induction using the apparatus shown in Fig. 10.1.
The teacher pulls the magnet down until one end is inside the coil. The teacher then releases the magnet. The magnet moves up and down repeatedly. As it moves, one end of the magnet enters and leaves the coil.
Describe and explain the readings on the centre-zero millivoltmeter as the magnet enters and leaves the coil.
(b) Fig. 10.2 shows a system for transmitting electricity from a power station.
State the name of the device used in X and the name of the device used in Y.
X ……………………………………………………………………………………………………………………………..
Y ……………………………………………………………………………………………………………………………..
Answer/Explanation
Answer:
(a) any four from:
needle moves (one way) as N pole / magnet moves downward / into coil
as coil cuts across magnetic field (of magnet)
needle moves in opposite direction as N pole / magnet moves upward / out of coil
field is cut in opposite direction
needle at zero when spring at max extension
magnet is stationary so no cutting of field lines
(b) (device) X is a step-up transformer
(device) Y is a step-down transformer