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P4.5.6P4. Electricity and Magnetism

The transformer — Combined Science 0653 - Physics Revision Notes & Topical Questions

5 min read 10 parts Lesson 17 of 19 2 quizzes

📚 Combined Science 0653 - Physics Hub / Topical Revision
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Lesson summary

P4.5.6 The transformer — COMBINED SCIENCE notes: Describe how a transformer works, use the transformer equations and explain high-voltage power transmission.Transformer = a device that changes the size of an alternating potential difference or current using electromagnetic induct…

  • D.c. supply
  • Voltage ratio
  • What the symbols mean
  • Quantities
  • Ideal transformer
  • Conservation of energy

Section 1 of 10Structure

Structure
Transformer
Transformer a device that changes the size of an alternating potential difference or current using electromagnetic induction
Parts
  • Parts a primary coil, a secondary coil and a soft iron core
  • The primary coil is the input coil and the secondary coil is the output coil.
  • The coils are not electrically connected to each other.
  • Iron is used because it is easily magnetised and demagnetised, and it is laminated to reduce heating in the core.

Section 2 of 10Operation

1
An alternating current is supplied to the primary coil.
2
The primary coil produces a changing magnetic field.
3
The soft iron core carries this changing field through the secondary coil.
4
The changing field cuts the secondary coil and induces an alternating e.m.f.
5
The induced e.m.f. has the same frequency as the supply to the primary.
6
If the secondary coil is part of a complete circuit, an alternating current flows.
  • D.c. supply
    • gives a steady field that does not change, so no e.m.f. is induced except briefly at switch-on and switch-off

Section 3 of 10Step-up and step-down

TypeTurnsOutput p.d.
Step-upMore turns on the secondary (Ns > Np)Vs > Vp
Step-downFewer turns on the secondary (Ns < Np)Vs < Vp

Section 4 of 10Transformer equation

Voltage ratio
Voltage ratio the ratio of the voltages equals the ratio of the numbers of turns, Vp / Vs = Np / Ns
What the symbols mean: V = voltage (V) · N = number of turns · I = current (A) · p = primary coil · s = secondary coil
  • Rearranged for the output voltageVs = Vp × Ns / Np.
  • Keep the same letter (p or s) on the top of each fraction.
  • Put the unknown on the top of the fraction to reduce rearranging.
Quantities
  • Vp and Vs are the primary and secondary voltages in volts, and Np and Ns are the numbers of turns

Section 5 of 10Ideal transformer

Ideal transformer
Ideal transformer a transformer that is 100% efficient, so power out equals power in
  • EquationIp × Vp = Is × Vs.
  • Written as a ratioIs / Ip = Vp / Vs.
Conservation of energy
  • Conservation of energy a transformer can raise the voltage but it cannot increase the power
  • Stepping the voltage up therefore steps the current down by the same factor.
Real transformers
  • Real transformers very efficient but not 100%, as some energy is lost as heat in the coils and the core

Section 6 of 10High-voltage transmission

Reason
  • Reason a current in a wire heats it because of its resistance, so energy is wasted
  • Power is P = V × I, so a low current needs a high voltage for the same power.
  • A smaller current means less heating and a lower energy loss in the cables.
Power loss
  • P = I²R
  • Halving the current reduces the loss to a quarter.
Advantages
  • Reduced power loss makes the energy transfer more efficient.
  • Lower currents allow thinner and cheaper cables.

Section 7 of 10Transmission chain

1
The power station generator produces a.c.
2
A step-up transformer raises the voltage and lowers the current for transmission.
3
The power travels along the cables with small losses.
4
Step-down transformers reduce the voltage, because high voltage is dangerous in homes.
5
A final step-down transformer supplies homes at a safe voltage of about 230 V.
  • Why mains is a.c.
    • transformers change a.c. voltages easily

Section 8 of 10More from the specification

1Cutting magnetic flux field lines
  • Cutting magnetic flux field lines moving a conductor across magnetic field lines induces a potential difference across the conductor terminals.
2Transformer
  • Transformer two coils wound on a soft iron core;
    • an alternating current in the primary creates a changing magnetic field that induces a voltage in the secondary
3Turns ratio
  • Vp/Vs = Np/Ns, so more secondary turns gives a step-up transformer and fewer gives a step-down
4Power in an ideal transformer
  • Vp x Ip = Vs x Is, so stepping up voltage reduces current
5Why AC is needed
  • Why AC is needed a steady direct current gives no changing flux, so no voltage is induced in the secondary
6Lenz's law
  • Lenz's law the induced current opposes the change that causes it, and the direction of force on a conductor is given by Fleming's left-hand rule
7Transformer
  • Transformer two coils wound on a soft iron core;
    • an alternating current in the primary creates a changing magnetic field that induces a voltage in the secondary
8Turns ratio
  • Vp/Vs = Np/Ns, so more secondary turns gives a step-up transformer and fewer gives a step-down
9Power in an ideal transformer
  • Vp x Ip = Vs x Is, so stepping up voltage reduces current
10Why AC is needed
  • Why AC is needed a steady direct current gives no changing flux, so no voltage is induced in the secondary
11Lenz's law
  • Lenz's law the induced current opposes the change that causes it, and the direction of force on a conductor is given by Fleming's left-hand rule
12Field line
  • A line showing the direction of the force on a north pole.
13Permanent magnet
  • A magnet whose field is always present.
14Solenoid
  • A coil of wire that produces a magnetic field like a bar magnet when a current flows.
15Conductor
  • Material that lets current flow.
16Voltmeter
  • Meter that measures voltage, connected in parallel.

Section 9 of 10Key terms

TermMeaning
TransformerA device with a primary coil and secondary coil on an iron core that changes an alternating voltage.
Primary coilThe input coil of a transformer.
Secondary coilThe output coil in which the e.m.f. is induced.
Step-up transformerA transformer with more turns on the secondary coil than the primary, raising the voltage.
Step-down transformerA transformer with fewer turns on the secondary coil, lowering the voltage.
Ideal transformerA 100% efficient transformer for which Ip × Vp = Is × Vs.
Turns ratioThe ratio Np / Ns, which equals Vp / Vs.

Section 10 of 10Exam technique

1
Using the transformer equation upside down — Vp / Vs = Np / Ns, so the larger voltage goes with the coil with more turns
2
Thinking a transformer works on d.c. — it needs a changing field, which needs a.c.
3
Thinking a step-up transformer gives more power — power cannot increase, so the current falls
4
Thinking power lines use high voltage to increase the current — the aim is a lower current to cut I²R losses

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