Starter quiz
- Match the following terms, which are related to transformers, to the correct definitions.
- secondary⇔describes the coil across which p.d. is applied ✓
- step-up⇔describes a transformer that increases the p.d. ✓
- step-down⇔describes a transformer that decreases the p.d. ✓
- primary⇔describes the coil across which p.d. is induced ✓
- 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
- percentage of total energy transferred that is transferred usefully ✓
- difference between energy supplied and energy usefully transferred
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- Which of the following is the frequency of mains electricity?
- 0 Hz
- 25 Hz
- 50 Hz ✓
- 100 Hz
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- Match each of the following to the typical voltage(s) it supplies or requires.
- power station⇔25 kV ✓
- factory⇔33 kV, 11 kV or 450 V ✓
- home⇔230 V ✓
- long-distance transmission line⇔400 kV ✓
- Which of the following induce a potential difference across the ends of a coil of wire?
- moving a bar magnet into the coil ✓
- leaving a bar magnet inside the coil
- placing the coil in a changing magnetic field ✓
- moving the coil in an unchanging magnetic field ✓
- moving the coil and a bar magnet together at the same velocity
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- 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.
- '660' ✓
Exit quiz
- When is a p.d. (potential difference) induced across the secondary coil of a transformer?
- 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
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- Which of the following can a transformer increase?
- potential difference ✓
- current
- resistance
- power
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- 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.
- The ratio of turns equals the ratio of potential difference. ✓
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- 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.
- '120 V' ✓
- If the power input to a transformer is 1000 W and the power output is 950 W, what is the efficiency of the transformer?
- '95%' ✓
- 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.
- '65.2 mA' ✓
Worksheet
Presentation
Video
Lesson Details
Key learning points
- A potential difference that causes a current to flow in the primary coil magnetises the iron core.
- When the iron core is magnetised, it induces a potential difference across the secondary coil.
- An AC gives the changing magnetic field needed to induce a potential difference across the secondary coil.
- Transformers are almost 100% efficient so the power output is equal to the power input.
- 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