Antonine Education
A2 Physics
Answers to Self-Test Electromagnetic Induction
Examination Time - 20 minutes
1
(a) Draw a diagram to show Fleming’s Right Hand Rule.
Field
(2)


Motion

Current
(b) When do we use this rule?
(1)

To work out direction of current in the generator effect.
2
(a) What is the difference between flux density and flux linkage?
(2)

Flux density: The flux per unit area at 90 o to the area.

Flux linkage: The product between flux and the number of turns.
(b) What is the unit for flux?
(1)

Weber
A single wire is passed through a magnetic field so that it cuts field lines at 90 o.
It is connected to a voltmeter to show the emf and the speed is recorded.
(a) Sketch the graph that shows how speed and emf are related.
(2)



EMF (V)
3
Gradient = Bl
Speed (m/s)
(b) On the graph above, show how the magnetic field strength can be worked out
from the graph. What else needs to be known?
(2)

Length of the wire in the magnetic field.
Turn over
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4
(a)
(i)
A2 Physics
A circular coil of diameter 24 mm has 40 turns. The coil is placed in a uniform
magnetic field of strength 85 mT
Calculate:
the area of the coil;
A = ( × (24 × 10-3)2) ÷ 4 = 4.52 × 10-4 m2
(2)


(ii) the flux linkage.
N = 85 × 10-3 × 4.52 × 10-4 × 40 = 1.54 × 10-3 Wb
(b) The coil was reversed in a time of 95 ms. Work out:
(i) change in flux linkage of the coil;
(2)

N= 1.54 × 10-3 Wb - - 1.54 × 10-3 Wb = 3.08 × 10-3 Wb

(ii) magnitude of the induced e.m.f.
E = 3.08 × 10-3 Wb ÷ 95 × 10-3 = 0.032 V (= 32 mV)
5
(a)
A step-down transformer reduces 240 V to 12 V to light a 12 V 36 watt bulb to
full brightness.
What is the turns ratio of the transformer?
(1)

240 : 12 = 20 : 1
(b) How much current would flow in the primary coil?
I sec = 36 ÷ 12 = 3 A
(1)

I prim = 3 ÷ 20 = 0.15 A
(c)
In fact the current was measured to be 0.20 A on an ammeter that is known to be
accurate. What is the efficiency of the transformer?
(3)

Input power = 240 × 0.20 = 48 W

Efficiency = (36 ÷ 48) × 100

= 75 %
(d) State one way in which losses are reduced in transformers.
(1)

Laminated soft iron core (to reduce eddy currents).
Total = 20 marks
End of examination. Now go back and check your work