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O-Level Electromagnetic Induction
What the O-Level syllabus expects for Electromagnetic Induction, and how to practise it.
What the syllabus expects
- Deduce from Faraday's electromagnetic induction experiments, or other suitable ones, that a varying magnetic field is able to induce an e.m.f. within a circuit, that this induced e.m.f. opposes whatever change produces it, and which factors set the size of the induced e.m.f.
- Describe a basic a.c. generator - either a rotating coil or a rotating magnet - and when slip rings are used.
- Sketch how the output voltage of a simple a.c. generator varies with time.
- Describe the structure and operating principle of a simple iron-cored transformer used to change voltages.
- Recall and use VP / VS = NP / NS and VPIP = VSIS in unfamiliar situations or problem solving, for an ideal transformer.
- Describe energy loss in cables and deduce why high-voltage transmission is advantageous.
How it's examined
Questions on this topic most often ask you to state. About 4% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (2 marks)
A small lamp is powered by an a.c. generator. (e) Give two modifications that could be made to the generator so that the lamp shines more brightly.
Show the worked answer
A larger induced e.m.f. produces a brighter lamp. Any two of: rotate the coil faster; increase the number of turns on the coil; use a stronger magnet; increase the area of the coil; wind the coil on a soft-iron core.
Example 2 (2 marks)
[Section B, Question 15 OR] An a.c. generator lights a small filament bulb, as in the figure. Give two changes that could be made to the generator so that the bulb glows more brightly.
Show the worked answer
A larger induced e.m.f. drives a larger current and makes the bulb brighter. This can be achieved by any of: increasing the rotation speed of the coil, increasing the number of turns on the coil, or using a stronger magnet.
Example 3 (2 marks)
State, with an explanation, how the amplitude of the current found in (c) would change if the coil's frequency of rotation were reduced to half its value. (This concerns the a.c. generator whose coil ABCD spins between the magnets, driving current through resistor R, from part (c).)
Show the worked answer
For an a.c. generator, the peak (amplitude of) induced e.m.f. equals the maximum rate of change of magnetic flux linkage, which is directly proportional to the frequency of rotation of the coil (peak e.m.f. = B A N (2*pi*f)). Since the current amplitude = peak e.m.f. / R, the current amplitude is also proportional to f. Halving the frequency of rotation halves the rate at which the coil cuts the field, so the peak e.m.f. halves, and hence the amplitude of the current halves. (The period of the a.c. would also double.)
More O-Level Pure Physics topics
Physical Quantities, Units and Measurement · Kinematics · Dynamics · Turning Effects of Forces · Pressure · Energy · all of O-Level Pure Physics