2.3.6Electricity
Use of Transformers — AQA GCSE Combined Science Trilogy 8464 - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 23 of 23
Section 1 of 7Structure of a transformer
Primary coil
- Primary coil Takes the alternating input p.d. and is wound on one side of the core.
Secondary coil
- Secondary coil Provides the output p.d. and is wound on the other side.
Iron core
- Iron core A soft magnetic material that links the two coils, so the changing magnetic field passes easily from primary to secondary.
Alternating input only
- Alternating input only A transformer does not work with d.c., because a steady current produces a steady field and no induction.
Section 2 of 7How a transformer works
1A.c. in the primary
- A.c. in the primary The alternating current produces a continually changing magnetic field.
2Field passes through the core
- Field passes through the core The soft iron concentrates the field and guides it through the secondary coil.
3Voltage is induced
- Voltage is induced A changing field through the secondary induces an alternating p.d. across it.
4Ratio of turns decides output
- Ratio of turns decides output More turns on the secondary gives a bigger p.d.;
- fewer turns gives a smaller p.d.
Section 3 of 7Transformer equations (HT only)
| Equation | Meaning |
|---|---|
| Vp ÷ Vs = Np ÷ Ns | Ratio of p.d.s equals ratio of turns on the coils |
| Vs × Is = Vp × Ip | For a 100% efficient transformer, output power equals input power |
| Step-up | Ns greater than Np, so Vs is greater than Vp and Is is less than Ip |
| Step-down | Ns less than Np, so Vs is less than Vp and Is is greater than Ip |
Section 4 of 7Example calculation (HT only)
1
A transformer has 200 turns on the primary and 50 turns on the secondary, with a 230 V input.
2
Rearrange: Vs = Vp × Ns ÷ Np = 230 × 50 ÷ 200.
3
The output is 57.5 V, so this is a step-down transformer.
4
If the secondary current is 2.0 A, the input current is Ip = (57.5 × 2.0) ÷ 230 = 0.50 A.
Section 5 of 7Uses
Where transformers are found
National Grid
- National Grid Step-up transformers lower the current for transmission;
- step-down transformers make the supply safe to use.
Chargers and adaptors
- Chargers and adaptors Reduce 230 V mains to a few volts for phones and laptops.
Efficiency
- Efficiency Real transformers are over 95% efficient, with small losses as heat in the coils and core.
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Primary coil | The input coil of a transformer. |
| Secondary coil | The output coil of a transformer. |
| Induced p.d. | A potential difference created by a changing magnetic field in a conductor. |
| Turns ratio | The number of turns on the primary compared with the secondary. |
| Soft iron core | An easily magnetised core that carries the magnetic field between the coils. |
Section 7 of 7Exam technique
1
Saying a step-up transformer increases the power; power is not increased, so the current falls as the p.d. rises.
2
Putting the turns ratio the wrong way round when using Vp ÷ Vs = Np ÷ Ns.
3
Suggesting transformers work with direct current from a battery.
4
Using a steel core instead of a soft iron core and not explaining that it must be easily magnetised and demagnetised.
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