These eight questions use original numbers. Sketch first, calculate second, then open the answer.
This set goes with SPM Physics electromagnetism. To practise against the clock, use the timed original practice session builder.
Questions
Q1. A wire carries current into the page. The field points from left to right. Which way is the force?
Answer
First finger points right (field). Second finger points into the page (current). The thumb points down the page.
If the current were out of the page, the force would be up.
Q2. A magnet is held still in a coil and the meter reads zero. Explain.
Answer
The flux linked with the coil is not changing, so no e.m.f. is induced. Induction needs a change in flux, not just the presence of a magnet.
Q3. State two ways to increase the e.m.f. induced when a magnet is pushed into a coil.
Answer
Push the magnet in faster, so the flux changes faster. Use a coil with more turns. Use a stronger magnet.
Q4. A north pole approaches a coil. State the pole formed at the nearer end of the coil and name the law.
Answer
A north pole forms at the nearer end. By Lenz’s law, the induced current opposes the increase in flux, so the coil repels the approaching north pole.
Q5. A transformer has 800 turns on the primary and 40 on the secondary. The primary voltage is 240 V. Find the secondary voltage.
Answer
Vₛ = 240 × (40 ÷ 800) = 240 × 0.05 = 12 V. It is a step-down transformer, so the answer is smaller than 240 V.
Q6. The secondary of the transformer in Q5 supplies 3.0 A. Find the primary current for an ideal transformer.
Answer
Output power = 12 × 3.0 = 36 W. Input power is also 36 W, so Iₚ = 36 ÷ 240 = 0.15 A.
Q7. 20 kW is sent at 2000 V through cables of resistance 5.0 Ω. Find the power lost.
Answer
I = 20 000 ÷ 2000 = 10 A. Power lost = I²R = 10² × 5.0 = 500 W, which is 2.5% of the power sent.
Q8. Why is a transformer supplied with a.c. and not d.c.?
Answer
It works by induction, which needs a changing flux. Alternating current gives a changing flux in the core. Direct current gives a steady flux, so no e.m.f. is induced in the secondary.
If you got these wrong
- Q1: determining magnetic-force direction.
- Q2, Q3 and Q4: explaining electromagnetic induction.
- Q5, Q6 and Q8: calculating transformer relationships.
- Q7: explaining transmission losses.
For machines, see comparing motors and generators. Log slips in the mistake log. If you want a teacher to go through new questions with you, see online one-to-one Physics tuition.