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Oak National Academy
Oak National Academy
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New
New
Year 11
OCR
Higher

Transformers

I can describe how a transformer works and solve problems using transformer equations.

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Share activities with pupils
New
New
Year 11
OCR
Higher

Transformers

I can describe how a transformer works and solve problems using transformer equations.

Download all resources
Share activities with pupils

Lesson details

Key learning points

  1. A potential difference that causes a current to flow in the primary coil magnetises the iron core.
  2. When the iron core is magnetised, it induces a potential difference across the secondary coil.
  3. An AC gives the changing magnetic field needed to induce a potential difference across the secondary coil.
  4. Transformers are almost 100% efficient so the power output is equal to the power input.
  5. The ratio of the number of turns on the coils is equal to the ratio of the potential differences across them.

Common misconception

The size of the current in the primary coil controls the size of current in the secondary coil.

Explain how the p.d. across and resistance of the secondary circuit determines the secondary p.d. and therefore current, and how the current adjusts to ensure that the power input is equal to the power output.

Keywords

  • Primary coil - a coil of insulated wire in a transformer, across which potential difference is applied

  • Secondary coil - a coil of insulated wire in a transformer, across which potential difference is induced

  • Soft iron core - pure iron inside the coil of an electromagnet that loses all of its magnetism when the electromagnet turns off

  • Induced p.d. - the p.d. across a conductor caused by a magnetic field moving relative to it

  • Efficiency - the proportion or percentage of energy supplied to a device that is transferred in a useful way

Pupil investigations using transformer kits often do not give good results, as even the tiniest gap between the C-cores can significantly reduce the p.d. induced across the secondary coil.
Teacher tip

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.
Match the following terms, which are related to transformers, to the correct definitions.
Correct Answer:secondary,describes the coil across which p.d. is applied

describes the coil across which p.d. is applied

Correct Answer:step-up,describes a transformer that increases the p.d.

describes a transformer that increases the p.d.

Correct Answer:step-down,describes a transformer that decreases the p.d.

describes a transformer that decreases the p.d.

Correct Answer:primary,describes the coil across which p.d. is induced

describes the coil across which p.d. is induced

Q2.
Which of the following is a definition of the efficiency of a device?
percentage of energy supplied that is wasted
difference between energy supplied and wasted energy
amount of energy supplied that is transferred in a useful way
Correct answer: percentage of total energy transferred that is transferred usefully
difference between energy supplied and energy usefully transferred
Q3.
Which of the following is the frequency of mains electricity?
0 Hz
25 Hz
Correct answer: 50 Hz
100 Hz
Q4.
Match each of the following to the typical voltage(s) it supplies or requires.
Correct Answer:power station,25 kV

25 kV

Correct Answer:factory,33 kV, 11 kV or 450 V

33 kV, 11 kV or 450 V

Correct Answer:home,230 V

230 V

Correct Answer:long-distance transmission line,400 kV

400 kV

Q5.
Which of the following induce a potential difference across the ends of a coil of wire?
Correct answer: moving a bar magnet into the coil
leaving a bar magnet inside the coil
Correct answer: placing the coil in a changing magnetic field
Correct answer: moving the coil in an unchanging magnetic field
moving the coil and a bar magnet together at the same velocity
Q6.
A transformer with 100% efficiency changes the voltage from 1000 V to 33 kV. If the current through the secondary coil is 20 A, the current through the primary coil is A.
Correct Answer: 660, 660 A, 660A

Exit quiz

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6 Questions

Q1.
When is a p.d. (potential difference) induced across the secondary coil of a transformer?
Correct answer: when there is a changing magnetic field in the primary coil
when the primary coil is connected to a direct current
when the secondary coil is connected to a battery
when the transformer is switched off
Q2.
Which of the following can a transformer increase?
Correct answer: potential difference
current
resistance
power
Q3.
How does the number of turns on the coils affect the potential difference in a transformer?
Fewer turns always mean higher potential difference.
More turns always mean higher potential difference.
The number of turns does not affect the potential difference.
Correct answer: The ratio of turns equals the ratio of potential difference.
Q4.
Calculate the output p.d. of a transformer if the primary p.d. is 240 V, the primary coil has 50 turns, and the secondary coil has 25 turns.
Correct Answer: 120 V, 120
Q5.
If the power input to a transformer is 1000 W and the power output is 950 W, what is the efficiency of the transformer?
Correct Answer: 95%, 95, 0.95
Q6.
What current is drawn from a 230 V power supply to a transformer if the output is 15.0 V and 1000 mA? Give your answer in mA to 3 s.f.
Correct Answer: 65.2 mA, 65.2