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Cambridge IGCSE Physics · 0625

Chapter 4: Electricity and magnetism — Part 16

Topic 4.5.6 · The transformer

Construction and the turns equation

A simple transformer has a soft-iron core with a primary coil and a secondary coil. It is used to change a.c. voltages.

A step-up transformer has more secondary turns than primary turns (Vs > Vp). A step-down transformer has fewer secondary turns.

Vp / Vs = Np / Ns

An alternating current in the primary produces a changing magnetic field in the core, which induces an e.m.f. in the secondary.

A transformer has 500 primary turns and 2000 secondary turns. The primary p.d. is 12 V. Calculate the secondary p.d.

Worked example: secondary voltage is 48 volts for a 1 to 4 turns ratio
Vp / Vs = Np / Ns → Vs = 48 V (step-up).
Simple transformer: laminated soft-iron core, primary coil and secondary coil, labelled step-up (more secondary turns) and step-down (fewer secondary turns).
Diagram 1: Simple transformer: laminated soft-iron core, primary coil and secondary coil, labelled step-up (more secondary turns) and step-down (fewer secondary turns).

Transmission and efficiency

Transformers are used in high-voltage transmission of electricity. High voltage means a smaller current for the same power, so heating losses in the cables are smaller.

Advantages of high-voltage transmission: lower current, thinner (or cheaper) cables possible, less energy wasted as thermal energy in the cables.

For 100% efficiency: IpVp = IsVs

Power loss in a cable: P = I2R. Larger I means much larger heating for the same cable resistance.

A 100% efficient transformer has Vp = 240 V, Ip = 0.50 A and Vs = 12 V. Calculate the secondary current.

Worked example: secondary current is 10 amperes from Ip Vp equals Is Vs
IpVp = IsVs → Is = 10 A.

A transmission cable has resistance 0.40 Ω and carries 20 A. Calculate the power lost as heat in the cable.

Worked example: power loss I squared R is 160 watts
P = I2R = 160 W. Doubling I quadruples the heating.
High-voltage transmission: step-up at the power station, cables, step-down near users; lower current in the cables means smaller heating losses I²R.
Diagram 2: High-voltage transmission: step-up at the power station, cables, step-down near users; lower current in the cables means smaller heating losses I²R.

Exam Traps

  • A transformer needs a changing current (a.c., or a changing d.c.). A steady d.c. primary current does not induce a secondary e.m.f.

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