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A second set of questions on electromagnetic induction and transformers. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
Higher tier. A transformer has 400 turns on the primary coil and 40 turns on the secondary coil. The primary potential difference is 230 V. Calculate the secondary potential difference.
Answer: 23 V
Higher tier. A step-up transformer changes 20 V to 200 V. The primary current is 5.0 A. Assume it is 100% efficient. Calculate the secondary current.
Answer: 0.50 A
Higher tier. A transformer has 50 turns on the primary coil and 2000 turns on the secondary coil. The primary potential difference is 6.0 V. Calculate the secondary potential difference.
Answer: 240 V
Explain why a transformer does not work on direct current.
Answer: Direct current gives a steady field, so no potential difference is induced in the secondary coil
Short questions with options. Choose one before you open the answer.
What is electromagnetic induction?
Creating a potential difference across a wire or coil when the magnetic field through it changes.
What is another name for electromagnetic induction?
The generator effect.
When is a potential difference induced in a coil by a magnet?
Only while the magnet is moving, so the field through the coil is changing.
What is the induced potential difference when a magnet is held still in a coil?
Zero.
State three ways to increase an induced potential difference.
Move faster, use a stronger magnet, or use a coil with more turns.
What reverses the direction of an induced current?
Reversing the movement, or reversing the magnetic pole that moves in.
What does an alternator use to connect the spinning coil to the circuit?
Slip rings and brushes.
What does a dynamo use instead of slip rings?
A split-ring commutator, which gives direct current.
What are the two coils of a transformer called?
The primary coil and the secondary coil.
Why is the transformer core made of soft iron?
Soft iron is easily magnetised and demagnetised and carries the changing field to the secondary coil.
Why does a transformer need alternating current?
Alternating current makes a changing field. A steady field would induce nothing.
What does a step-up transformer do?
It increases the potential difference. It has more turns on the secondary coil than on the primary.
What does a step-down transformer do?
It decreases the potential difference. It has fewer turns on the secondary coil.
Higher tier: what is the turns ratio equation for a transformer?
Vₚ ÷ Vₛ = Nₚ ÷ Nₛ.
Higher tier: what is the power equation for an ideal transformer?
Vₚ × Iₚ = Vₛ × Iₛ, because the power in equals the power out.
Why does the National Grid transmit at a high potential difference?
For the same power the current is lower, so less energy is lost as heat in the cables.
What is the frequency and potential difference of UK mains?
50 hertz and 230 volts.
What happens to the maximum potential difference and frequency if a generator coil spins twice as fast?
Both double.
What is the generator effect the reverse of?
The motor effect, which turns a current in a field into movement.
Why do power stations use a turbine with a generator?
The turbine spins the coil or magnets so that a potential difference is induced.
What does a transformer do to the power in an ideal case?
Nothing. The power out equals the power in, so a higher potential difference means a lower current.
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