Induced Potential, Transformers & the National Grid (AQA GCSE Physics): Exam Questions

Higher Tier Only

A conducting wire is connected to a voltmeter and then moved through a magnetic field, as shown in Figure 1 below.

Figure 1

The following observations are made:

• When the wire is moved up and down, the needle on the voltmeter moves.

• When it is moved in other directions the needle moves much less.

• When it is moved from side to side it doesn’t move at all.

Explain the above observations.

Higher Tier Only

Figure 4 below shows a simple a.c. alternator consisting of a magnet that is able to rotate next to a coil of wire.

Figure 4

Define the term “alternating current” (a.c.)

Higher Tier Only

Explain why the alternator produces an alternating current when the magnet is rotated.

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The alternator is connected to a data logger and the magnet is spun at a steady rate.

Figure 5 below shows how the potential difference across the coil changes with time.

Figure 5

The coil is now spun at twice the speed.

Draw a line on the graph in Figure 5 showing the expected output.

Higher Tier Only

Other than changing the speed of rotation, describe two other changes that could be made to the alternator to increase the size of the potential difference produced by it.

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Figure 6 below shows a simple d.c. electric motor.

Figure 6

Motors such as this, can be used to produce an electric current, but unlike a.c. alternators, their output is a direct current.

Explain why.

Higher Tier Only

Sketch the expected output from the motor on the graph below.

The graph has been started for you.

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Describe three changes that could be made to motor to increase the voltage output.

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Describe how the output would change if the motor was rotated in the opposite direction.

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Figure 7 below shows a simple transformer.

Figure 7

Transformers can be used to change the voltage of an alternating power source.

Explain how a transformer works.

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Define the term “step-up transformer”.

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Identify whether the transformer shown in the diagram is a step-up or a step-down transformer, and justify your answer.

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A similar transformer has 8000 turns on its primary coil and an unknown number of turns on its secondary coil.

When it is connected to a 240 V a.c. power supply it is found to output 12 V a.c.

Using an appropriate equation, calculate the number of turns on the secondary coil of this transformer.

Higher Tier Only

Electricity is distributed around the country using a system of high voltage power lines called the National Grid.

Explain why it is more efficient to distribute electricity at a high voltage.

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A power station generates 100 MW of power.

This is passed through a transformer which increases the potential difference of the electricity to 250 kV, which is then transmitted through overhead electrical cables.

Calculate the total current in the overhead cables.

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A solenoid is connected to a datalogger which records the current in the solenoid several times a second, as shown in Figure 2 below.

The solenoid is then mounted vertically and a magnet dropped through it.

Figure 2

As the magnet enters the solenoid a positive current is recorded, which changes direction as the magnet leaves the solenoid, as shown in Figure 3 below.

Figure 3

Explain how the current is created and why it changes its direction.

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The graph in Figure 3 shows that when the magnet leaves the solenoid a larger current is produced than when it enters.

Explain why.

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It is noticed that when an identical magnet is dropped outside the solenoid, it falls much faster than when it is dropped inside it.

Explain why.

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The experiment is now repeated using a much stronger magnet.

Explain how this affects the current produced in the coil.

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The potential difference across the primary coil is varied for three different transformers, X, Y and Z. The recorded potential differences across the secondary coil are plotted in Figure 1.

Figure 1

Which transformer would be best suited to be positioned between power cables and consumers in the national grid?

Explain your answer.

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Transformer Y is 80% efficient.

Calculate the current in the secondary coil as a percentage of the current in the primary coil.

Use the Physics Equation Sheet.

   Secondary current as a percentage of primary current =......... %

The magnetic fields of transformer Y are shown in Figure 2.

Suggest why the transformer is only 80% efficient.

In a power station, water is heated by burning biofuels - the steam from this turns a turbine which in turn generates electricity using the equipment shown in Figure 1.

Figure 1

Figure 2 shows the an oscilloscope trace of the potential difference in the coil. The vertical axis represents potential difference and the horizontal axis represents time. 

Figure 2

Referring to the equipment in Figure 1, explain the shape of the potential difference graph in Figure 2.

Higher Tier Only

Sketch the trace on the oscilloscope in Figure 2 when the steam's flow rate slows down, so the turbine rotates at half the speed.