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Date May 2021 Marks available 4 Reference code 21M.2.HL.TZ2.7
Level Higher level Paper Paper 2 Time zone 2
Command term Explain Question number 7 Adapted from N/A

Question

The primary coil of a transformer is connected to a 110 V alternating current (ac) supply. The secondary coil of the transformer is connected to a 15 V garden lighting system that consists of 8 lamps connected in parallel. Each lamp is rated at 35 W when working at its normal brightness. Root mean square (rms) values are used throughout this question.

The primary coil has 3300 turns. Calculate the number of turns on the secondary coil.

[1]
a.i.

Determine the total resistance of the lamps when they are working normally.

[2]
a.ii.

Calculate the current in the primary of the transformer assuming that it is ideal.

[2]
a.iii.

Flux leakage is one reason why a transformer may not be ideal. Explain the effect of flux leakage on the transformer.

[2]
a.iv.

A pendulum with a metal bob comes to rest after 200 swings. The same pendulum, released from the same position, now swings at 90° to the direction of a strong magnetic field and comes to rest after 20 swings.

 

Explain why the pendulum comes to rest after a smaller number of swings.

[4]
b.

Markscheme

«15110×3300=» 450 «turns»

 

a.i.

ALTERNATIVE 1

calculates total current =3515×8«=18.7 A»

resistance =«1518.7=» 0.80«Ω»


ALTERNATIVE 2

calculates total power = 35×8 « = 280 W» ✓

resistance =«152280=» 0.80«Ω»


ALTERNATIVE 3

calculates individual resistance =15235«=6.43Ω»

resistance =«6.438»=0.80«Ω»

a.ii.

total power required = 280 «W»
OR
uses factor 3300450
OR
total current = 18.7 « A»

current = 2.5 OR 2.6 «A» 


Award [2] marks for a bald correct answer.

Allow ECF from (a)(ii).

 

a.iii.

the secondary coil does not enclose all flux «lines from core»

induced emf in secondary
OR
power transferred to the secondary
OR
efficiency is less than expected


Award [0] for references to eddy currents/heating of the core as the reason.

Award MP2 if no reason stated.

 

a.iv.

bob cuts mag field lines
OR
there is a change in flux linkage

induced emf across bob

leading to eddy/induced current in bob

eddy/induced current produces a magnetic field that opposes «direction of» motion

force due to the induced magnetic field decelerates bob

damping of pendulum increases/there is additional «magnetic» damping 


MP4
and MP5 can be expressed in terms of energy transfer from kinetic energy of bob to electrical/thermal energy in bob

b.

Examiners report

[N/A]
a.i.
[N/A]
a.ii.
[N/A]
a.iii.
[N/A]
a.iv.
[N/A]
b.

Syllabus sections

Additional higher level (AHL) » Topic 11: Electromagnetic induction » 11.1 – Electromagnetic induction
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Additional higher level (AHL) » Topic 11: Electromagnetic induction
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