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Date November 2014 Marks available 2 Reference code 14N.2.HL.TZ0.6
Level Higher level Paper Paper 2 Time zone Time zone 0
Command term Determine Question number 6 Adapted from N/A

Question

This question is in two parts. Part 1 is about energy resources. Part 2 is about transformers.

Part 2 Transformers

Modern televisions (TVs) can be left in “standby” mode so that they are available for immediate use. The internal circuits are powered at low voltage using a step-down transformer connected to the mains power supply. To prevent the TV from using any energy, the transformer must be disconnected from the mains supply.

When in “standby” mode, a TV transformer supplies a current of 0.45 A at 9.0 V to the internal circuits.

Outline the features of an ideal step-down transformer.

[2]
f.

Real transformers are subject to energy loss. State and explain how two causes of these energy losses may be reduced by suitable features in these transformers.

 

1.

 

 

 

2.

[4]
g.

Calculate the power consumed by the internal circuits when the TV is in “standby” mode.

[1]
h.i.

The efficiency of the transformer is 0.95. Determine the current supplied by the 230 V mains supply.

[2]
h.ii.

The TV is on “standby” for 75% of the time. Calculate the energy wasted in one year by not switching off the TV.

\[{\text{1 year}} = 3.2 \times {10^7}{\text{ s}}\]

[1]
h.iii.

Markscheme

number of secondary coils < primary coils;

coils wound around an iron core;

ideally no flux leakage/zero resistive heating;

f.

solution;

with linked explanation;

another solution;

with linked explanation;

eg:

laminations in core;

prevents eddy currents in core;

thick/low resistance wires for primary and secondary turns;

reduces heating losses in wires;

g.

4.1 W;

h.i.

power required at primary \( = \frac{{4.1}}{{0.95}} = 4.3{\text{ W}}\); (allow ECF from (h)(i))

current \(\frac{{4.3}}{{230}} = 1.9 \times {10^{ - 2}}{\text{ A}}\);

h.ii.

approximately \(1.0 \times {10^8}{\text{ J}}\);

Allow use of either power value.

h.iii.

Examiners report

Many candidates described the purpose of a step-down transformer. The question asked for features, such as an iron core or more primary than secondary turns.

f.

Further features such as a laminated core or low resistance coils were not often given, with explanations, as ways of increasing the efficiency of a transformer.

g.

A very easy mark, 4.1W.

h.i.

To determine the primary current the efficiency needed to be applied correctly to the power in the two circuits. There were many correct answers, with a minority placing the efficiency on the wrong side of the equation.

h.ii.

TV power consumption in standby mode was an easy calculation for 1 mark.

h.iii.

Syllabus sections

Additional higher level (AHL) » Topic 11: Electromagnetic induction » 11.2 – Power generation and transmission
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