Course: National Grid C+S; how transformers work S only | Tier: Foundation + Higher (transformer equations HT) | Linked equations: Vp/Vs = np/ns, VpIp = VsIs | Lesson: Keep the Country Running
Notes
The National Grid (C+S)
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- The National Grid is a system of cables and transformers linking power stations to consumers.
- Step-up transformers increase the potential difference from the power station to the transmission cables.
- Step-down transformers decrease the potential difference to a safer level for homes (about 230 V).
- Transmitting at a high p.d. means a low current for the same power. A lower current means less heating of the cables, so less energy is wasted and the system is more efficient.
How a transformer works (S only, HT)
✏️ Copy into your book (S only, HT)
- A transformer has a primary coil and a secondary coil wound on an iron core. Iron is used because it is easily magnetised.
- An alternating p.d. across the primary coil causes an alternating current, which produces a changing magnetic field in the iron core.
- This changing field induces an alternating p.d. across the secondary coil.
- Step-up: more turns on the secondary coil. Step-down: fewer turns on the secondary coil.
- A transformer only works with a.c., because a d.c. would not produce a changing magnetic field.
Questions
Q1 (F) What is the National Grid?
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A system of cables and transformers that links power stations to consumers.
Q2 (F) What does a step-up transformer do?
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It increases the potential difference.
Q3 (F/H) Explain why electricity is transmitted across the country at a very high potential difference.
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A high p.d. means a low current for the same power. A lower current causes less heating in the cables, so less energy is wasted and the transmission is more efficient.
Q4 (F/H) Why is a step-down transformer needed before electricity reaches homes?
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The transmission p.d. is far too high to be safe in homes. The step-down transformer reduces it to about 230 V.
Q5 (S only, HT) Explain how a transformer produces a p.d. across its secondary coil.
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An alternating current in the primary coil produces a changing magnetic field in the iron core. This changing field passes through the secondary coil and induces an alternating p.d. across it.
Q6 (S only, HT) A transformer has 2000 turns on its primary coil and 100 turns on its secondary coil. The primary p.d. is 230 V. Calculate the secondary p.d.
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F: Vp ÷ Vs = np ÷ ns
I: 230 ÷ Vs = 2000 ÷ 100
F: Vs = 230 ÷ 20
A: 11.5 V
Q7 (S only, HT) A power station supplies 40 A at 25 000 V to a step-up transformer. The output p.d. is 400 000 V. Assuming the transformer is 100% efficient, calculate the output current.
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F: Vp × Ip = Vs × Is
I: 25 000 × 40 = 400 000 × Is
F: Is = 1 000 000 ÷ 400 000
A: 2.5 A
Q8 (S only, HT) Why won’t a transformer work with a d.c. supply?
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A direct current produces a constant magnetic field, not a changing one, so no p.d. is induced in the secondary coil.
