OCR B Electromagnetism

A bar magnet is allowed to fall vertically through a coil. Which graph shows the variation in induced emf. V across the ends of the coil with time t while the magnet passes through the coil?

• A

• B

• C

• D

The rate of rotation is doubled. Which of the following traces shows the new variation of the e.m.f.?

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• 
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A coil with side length 5.0 cm is free to rotate with its axis parallel to the face of two magnets. The current in the coil is 0.4 A. There is a uniform magnetic field of flux density 40 mT between the magnets. What is the magnitude of the force F acting on one side of the coil?

• 0.9 x 10^-4 N

• 1.0 x 10^-4 N

• 4.0 x 10^-4 N

• 8.0 x 10^-4 N

Three equally spaced identical long straight wires carry different currents. In which direction will the middle wire try to move when the currents are switched on?

• to the left

• to the right

• out of the page, towards the viewer

• Into the page, away from the viewer

The diagram shows a wire, carrying a current I, placed between the poles of a magnet. In which direction does the force on the wire act?

• down

• up

• towards the N pole of the magnet

• towards the S pole of the magnet

The diagram shows an iron-cored transformer assumed to be 100% efficient. The ratio of the secondary turns to the primary turns is 1:20. What is the current in the primary coil?

• 0.10 A

• 0.14 A

• 2.0 A

• 40 A

A wire of length 3.0 cm is placed at right angles to a magnetic field of flux density 0.040 T. The wire carries a current of 5.0 A. What is the magnitude of the force which the field exerts on the wire?

• less than 0.006 N

• 0.0060 N

• Greater than 0.0060 N but less than 0.60 N

• 0.60 N

The relationship flux = permeance x current-turns for a magnetic circuit is analogous to the relationship between potential difference, current and conductance in an electrical circuit. Which of the following combinations correctly matches the magnetic and electrical circuit terminology?

• potential difference: permeance, current: current-turns, conductance: flux

• potential difference: flux, current: current-turns, conductance: permeance

• potential difference: current-turns, current: flux, conductance: permeance

• potential difference: current-turns, current: permeance, conductance: flux

A transformer with a 4000-turn primary and 1000-turn secondary transfers 200 kW from the grid to the local supply. Which of these combinations is possible for this transformer?

• A

• B

• C

• D

Transformers may be constructed using insulated steel laminations. Here are three true statements about such a transformer construction. 1. Steel laminations have a low permeance 2. The insulation reduces the effects of eddy currents 3. Steel laminations have a high conductivity Which combination of these factor(s) is important in designing an efficient transformer?

• Statement 1

• Statement 1 and 2

• Statements 2 and 3

• All three statements