Transformers and Power Transmission
A transformer changes the voltage of an alternating supply. One equation covers the calculations, and one idea — reducing the current — explains the entire National Grid.
1. How a transformer works
Construction: two coils — the primary and the secondary — wound on a soft iron core. They are not electrically connected.
The process:
- Alternating current in the primary coil produces a changing magnetic field
- The soft iron core carries this changing field to the secondary coil
- The changing field induces an alternating e.m.f. in the secondary coil
Transformers only work with a.c. A direct current produces a constant magnetic field, which does not change, so nothing is induced. A recorded error assumed a transformer would work with d.c. — it will not, and this is a favourite exam question.
Soft iron is used for the core because it magnetises and demagnetises easily, following the rapidly changing field.
2. The transformer equation
V_p / V_s = N_p / N_s
where V is voltage and N the number of turns; p = primary, s = secondary.
Rearranged: V_s = V_p × (N_s / N_p)
| Turns | Voltage | |
|---|---|---|
| Step-UP transformer | more turns on the secondary | voltage increases |
| Step-DOWN transformer | fewer turns on the secondary | voltage decreases |
The voltage ratio equals the turns ratio. More turns on the secondary means more voltage out.
Example: a transformer with 200 primary turns and 1000 secondary turns, supplied at 20 V.
- V_s = 20 × (1000/200) = 100 V — a step-up transformer
Example: finding the turns. A transformer steps 240 V down to 12 V with 2000 primary turns.
- N_s = N_p × (V_s/V_p) = 2000 × (12/240) = 100 turns
Keep primary and secondary consistent on both sides of the equation. Swapping them was recorded repeatedly, and it inverts the answer — a step-up becomes a step-down. Label which is which before you substitute.
Having found the ratio, use it. A recorded difficulty was calculating V_p/V_s and then not knowing what to do with it — that ratio is N_p/N_s, so multiply or divide the known turns by it.
3. Power in a transformer
For an ideal (100% efficient) transformer:
Power in = power out V_p I_p = V_s I_s
P = V I. Confusing this with P = I²R was recorded — both are valid power equations, but P = VI is the one for transformer calculations.
The consequence:
If the voltage goes UP, the current goes DOWN (and vice versa), because the power is unchanged.
Example: a transformer steps 240 V up to 2400 V. If the primary current is 5 A:
- P = 240 × 5 = 1200 W
- I_s = 1200 ÷ 2400 = 0.5 A
Current and voltage change in OPPOSITE directions. Uncertainty about how current changes in a transformer was recorded — remember that a transformer cannot create energy, so a ten-fold voltage increase means a ten-fold current decrease.
Real transformers are not perfectly efficient — some energy is wasted as thermal energy in the coils (resistance) and the core.
4. Power transmission — why high voltage
This is the most examined application in the topic.
Power lost in transmission cables:
P_lost = I² R
The loss depends on the square of the current, so reducing the current reduces losses dramatically.
The reasoning chain — give all of it:
- Power is transmitted as P = V I, so for a given power, a higher voltage means a lower current
- Power lost in the cables is I²R
- Halving the current cuts the losses to a quarter
- So transmitting at high voltage (and low current) minimises energy wasted as thermal energy in the cables
High-voltage transmission does NOT reduce the resistance of the cables. This exact misconception was recorded. The resistance is fixed by the cable’s material, length and thickness. What changes is the current, and since losses go as I²R, a smaller current means far smaller losses.
In the National Grid:
Step-UP transformers raise the voltage for transmission (to hundreds of kV). Step-DOWN transformers reduce it for safe use in homes (230 V).
Other ways to reduce losses: use thicker cables (lower resistance) or a material of lower resistivity — but high-voltage transmission is by far the most effective.
5. Method for transformer questions
- Label which coil is primary and which is secondary
- Note whether it is step-up or step-down — and sanity-check your answer against that
- Write the equation before substituting
- For current, use P = VI and the fact that power is conserved
Check the answer makes sense: a step-up transformer must give a larger output voltage and a smaller output current.
Don’t write “human error” as a source of error — tutors flagged this specifically. Give a specific reason, such as energy lost as thermal energy in the coils.
6. Mistakes that cost marks
Swapping primary and secondary in the equation.
Thinking a transformer works with d.c.
Confusing P = VI with P = I²R.
Saying current increases when voltage increases.
Saying high voltage reduces the cables’ resistance.
Omitting the I²R reasoning in a transmission explanation.
Not using the ratio once it has been found.
Using the wrong current in a power calculation.
Vague error sources like “human error”.
Frequently asked questions
How does a transformer work? An alternating current in the primary produces a changing magnetic field, carried by the iron core, which induces an alternating e.m.f. in the secondary.
Why won’t a transformer work with d.c.? A direct current gives a constant magnetic field, so nothing is induced.
What is the transformer equation? V_p / V_s = N_p / N_s.
What is a step-up transformer? One with more turns on the secondary, which increases the voltage.
What happens to the current if the voltage is stepped up? It decreases, because power is conserved (V_p I_p = V_s I_s).
Why is electricity transmitted at high voltage? High voltage means low current, and power loss is I²R, so the losses are much smaller.
Does high voltage reduce the cable’s resistance? No — the resistance is fixed. It reduces the current.
What is the core made of and why? Soft iron, because it magnetises and demagnetises easily.
Are transformers 100% efficient? No — some energy is wasted as thermal energy in the coils and core.
Quick revision checklist
- I can describe how a transformer works, step by step
- I know it needs a.c. and why
- I know why the core is soft iron
- I can use V_p/V_s = N_p/N_s
- I label primary and secondary before substituting
- I can identify step-up and step-down
- I know P = VI and that power is conserved
- I know voltage and current change in opposite directions
- I know power loss is I²R
- I can give the full reasoning for high-voltage transmission
- I know high voltage reduces current, not resistance
- I know where step-up and step-down transformers sit in the Grid
- I give specific sources of error
These notes cover transformers and power transmission in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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