How a transformer works, why it only works with alternating current, the turns ratio equation, and the power equation for an ideal transformer. Physics only content.
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1.What is a transformer made of?
Two coils of wire, the primary and the secondary, wound on an iron core.
2.Why is the core made of iron?
Iron is easily magnetised, so it carries the changing magnetic field efficiently from one coil to the other.
3.Explain how a transformer works.
An alternating current in the primary coil produces a continuously changing magnetic field in the core, which induces an alternating potential difference in the secondary coil.
4.Why does a transformer not work with direct current?
Direct current produces a steady magnetic field, and without a changing field nothing is induced in the secondary coil.
5.What is a step-up transformer?
One where the secondary coil has more turns than the primary, so the potential difference is increased.
6.What is a step-down transformer?
One where the secondary coil has fewer turns than the primary, so the potential difference is decreased.
7.State the transformer turns equation.
primary potential difference divided by secondary potential difference equals primary turns divided by secondary turns
8.A transformer has 100 primary turns and 500 secondary turns, with 20 V in. Find the output.
The ratio is 1 to 5, so the output is 5 x 20 = 100 V.
9.A transformer steps 230 V down to 23 V. If the primary has 1000 turns, how many has the secondary?
The ratio is 10 to 1, so the secondary has 100 turns.
10.What equation applies to an ideal transformer?
primary potential difference x primary current = secondary potential difference x secondary current
11.What does that equation assume?
That the transformer is 100 per cent efficient, so the power output equals the power input.
12.If a transformer steps the potential difference up, what happens to the current?
It steps down by the same factor, so the power stays the same.
13.A transformer has an input of 100 V and 4 A, and an output of 400 V. Find the output current.
Input power = 400 W. Output current = 400 divided by 400 = 1 A.
14.Why does the National Grid use step-up transformers?
To increase the potential difference and reduce the current for transmission.
15.Why is a lower current important for transmission?
Energy dissipated in the cables depends on the current squared, so a smaller current wastes far less energy.
16.Why are step-down transformers needed?
The very high transmission potential difference would be lethal and would damage appliances, so it must be reduced before use.
17.Why is the National Grid described as efficient?
Transmitting at high potential difference and low current means very little of the energy generated is wasted as heat in the cables.
18.What type of current must the grid use, and why?
Alternating current, because transformers only work with a changing magnetic field.
19.Where in the grid is a step-up transformer found?
Between the power station and the transmission cables.
20.Where is a step-down transformer found?
Between the transmission cables and homes or factories.
21.Why do real transformers get warm?
Some energy is dissipated in the resistance of the coils and in the iron core, so they are not perfectly efficient.
22.If the primary and secondary have the same number of turns, what happens?
The potential difference is unchanged, since the turns ratio is 1 to 1.
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