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

Exam Questions

1 hour11 questions
1a4 marks

Higher Only

Figure 1 shows a labelled transformer supplying homes in a village with electricity from a power cable.

Figure 1

7-3-e-1a-transformer-question

Complete the sentences, identifying each component of the transformer. 

Choose answers from the box. 

Each answer can be used once, more than once or not at all.

cobalt direct input
secondary output primary
nickel iron alternating

 

     

Part W labels the input .............................. current.

  

Part X is the .............................. coil.

   

Part Y is the .............................. coil.

   

Part Z is made from .............................. .

1b2 marks

Higher Only

State the type of transformer shown in Figure 1.

Explain how the diagram shows this.

1c1 mark

Higher Only

Compare the voltage in each coil of the transformer in Figure 1.

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2a1 mark

Higher Only

The national grid allows power to be efficiently delivered across the UK.

Describe where a step-up transformer is used as part of the National Grid.

2b5 marks

Higher Only

A teacher writes a note about a transformer but intentionally misses out some words.

Use the correct words from the box to complete the spaces.

  

coil core current
ends magnetic wire

 

     

A transformer works because an alternating .............................. in the primary .............................. produces a changing .............................. field in the ..............................  and then in the secondary coil.

    

This induces an alternating potential difference across the .............................. of the secondary coil.

2c2 marks

Higher Only

Explain why it is beneficial for current to be small in power cables.

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3a3 marks

Higher Only

A magnet is passed through a coil of wire connected to an ammeter, as shown in Figure 1.

Figure 1

7-3-e-3a-generator-effect--

Explain why the ammeter shows a non-zero reading.

3b1 mark

Higher Only

A student moves the magnet towards the coil, from right to left. 

Describe the force they experience while moving the magnet.

3c3 marks

Higher Only

The student performs different actions and records the different ammeter readings they make for each action. 

Describe the ammeter reading for each action, using answers from the box. The first has been completed for you. 

Needle doesn't move Needle moves to 0.5 A Needle moves to −0.2 A

   

Action of student Reading on ammeter
Hold magnet still and move coil slowly towards magnet Needle moves to 0.2 A
Hold magnet still inside the coil
   
 
Move the magnet and coil together very quickly
   
 
Flip the magnet around and move slowly towards the coil  

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13 marks

Higher Only

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

Figure 1

fig-1-7-3-medium-aqa-gcse-physics

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.

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2a1 mark

Higher 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

fig-4-7-3-medium-aqa-gcse-physics

Explain what is meant by the term “alternating current” (a.c.)

2b3 marks

Higher Only

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

2c3 marks

Higher Only

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

fig-5-7-3-medium-aqa-gcse-physics

The coil is now spun at twice the speed.

Add a new line to the graph in Figure 5 showing the expected output.

2d2 marks

Higher 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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3a2 marks

Higher Only

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

Figure 6

fig-6-7-3-medium-aqa-gcse-physics

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.

3b2 marks

Higher Only

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

The graph has been started for you.

q4-7-3-medium-aqa-gcse-physics

3c3 marks

Higher Only

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

3d1 mark

Higher Only

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

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4a4 marks

Higher Only

Figure 7 below shows a simple transformer.

Figure 7

q3-7-3-medium-aqa-gcse-physics

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

Explain how a transformer works.

4b1 mark

Higher Only

Explain what is meant by the term “step-up transformer”.

4c2 marks

Higher Only

State whether the transformer shown in the diagram is a step-up or a step-down transformer, and explain how you can tell.

4d3 marks

Higher Only

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.

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5a2 marks

Higher 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.

5b4 marks

Higher Only

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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1a4 marks

Higher Only

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

fig-2-7-3-medium-aqa-gcse-physics

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

fig-3-7-3-medium-aqa-gcse-physics

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

1b1 mark

Higher Only

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

Explain why.

1c2 marks

Higher Only

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.

1d2 marks

Higher Only

The experiment is now repeated using a much stronger magnet.

Explain how this affects the current produced in the coil.

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2a3 marks

Higher Only

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

Figure 1

7-3-h-2a-transformer-comparison

State which transformer would be best suited to be positioned between power cables and consumers in the national grid.

Explain your answer.

2b4 marks

Higher Only

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 = .................................... %
2c1 mark

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

7-3-h-2c-flux-leakage

Suggest why the transformer is only 80% efficient.

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3a3 marks

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

7-3-h-3a-steam-generator

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

7-3-h-3a-p-d--trace

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

3b2 marks

Higher 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.

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