Electromagnetic Induction Notes

Edexcel GCSE Physics: Revision notes

Key facts

  • Electromagnetic induction: relative movement of a magnet and a conductor induces a potential difference (and a current in a closed circuit).
  • The induced p.d. is larger for faster movement, a stronger field and more turns on the coil.
  • Alternators give a.c.; dynamos give d.c.
  • Microphones turn sound into current; loudspeakers do the reverse.
  • Transformers (a.c. only): VpVs=NpNs\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}. The national grid uses high voltage to cut I2RI^2R losses.
  • TransformerVpVs=NpNs\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}
  • 100% efficientVp×Ip=Vs×IsV_p \times I_p = V_s \times I_s

Inducing a potential difference

Moving a magnet and a conductor relative to each other induces a potential difference; the faster, stronger and more turns, the bigger.

If a conductor moves relative to a magnetic field, or the field changes near it, a potential difference is induced, and a current flows if the circuit is closed. This can be shown with a magnet moved in and out of a coil, or on a large scale in power stations.

The induced field always opposes the change that caused it, which is why work must be done to induce a current.

0.511.522.533.544.552468101214xyinduced p.d.
Illustrative: the induced potential difference grows with the speed of movement, and more turns on the coil (slider) make it larger.

Larger induced p.d.

  • Faster relative movement
  • Stronger magnetic field
  • More turns on the coil

Direction depends on

  • Direction of movement
  • Orientation of the field

Which change would NOT increase the induced p.d. when a magnet moves into a coil?

Generators

A generator rotates a coil in a field (or a magnet in a coil) to induce a current; an alternator gives a.c. and a dynamo gives d.c.

An alternator rotates a coil in a magnetic field, or a magnet inside a coil, to give alternating current. A dynamo uses the same principle, but its split-ring commutator gives direct current.

24681012−1−0.50.51xyalternator (a.c.)dynamo (d.c.)
Output over two rotations: an alternator gives a.c. (solid); a dynamo's split-ring commutator gives d.c. (dashed).

Alternator

  • a.c.
  • Rotating coil or magnet

Dynamo

  • d.c.
  • Split-ring commutator

Which generator produces direct current?

Microphones and loudspeakers

A microphone turns sound into a varying current by induction; a loudspeaker does the reverse.

A microphone: sound waves vibrate a diaphragm attached to a coil or magnet, inducing a varying current that matches the sound.

A loudspeaker (or headphones) is the reverse: a varying current in a coil in a magnetic field makes a diaphragm vibrate and recreate the sound.

Microphone

  • Sound makes the diaphragm vibrate
  • Current is induced
  • Output: varying current

Loudspeaker

  • Varying current in a coil in a field
  • Diaphragm vibrates
  • Output: sound
  1. 1

    Sound wave

    Pressure variations hit the diaphragm.

  2. 2

    Diaphragm vibrates

    Moving the attached coil or magnet.

  3. 3

    Current induced

    A varying current matching the sound.

Microphone: sound to current

What does a loudspeaker convert?

Transformers

A transformer uses a changing field in an iron core to induce an a.c. voltage in a second coil, with the voltages in the ratio of the turns.

A transformer has a primary and a secondary coil wound on a shared iron core, with no electrical connection. An alternating current in the primary makes a changing field in the core, which induces an alternating p.d. in the secondary. It does not work with steady d.c.

Primary coilSecondary coilIron core
A transformer: two coils on one iron core with no electrical connection.
  • Turns ratioVpVs=NpNs\dfrac{V_p}{V_s} = \dfrac{N_p}{N_s}
  • 100% efficientVp×Ip=Vs×IsV_p \times I_p = V_s \times I_s

Worked example

A transformer has 1000 primary turns and 50 secondary turns. The primary p.d. is 230 V. Find the secondary p.d.

A 100% efficient transformer has VpV_p = 230 V, IpI_p = 0.5 A and VsV_s = 11.5 V. What is IsI_s?

The national grid

High-voltage transmission means a low current, so less energy is lost as heat in the cables.

The national grid transmits electrical energy at very high voltage. A step-up transformer raises the voltage (and lowers the current) for efficient long-distance transmission; a step-down transformer reduces the voltage again for safe local use.

  1. 1

    Step-up transformer

    Raises the voltage, so the current is low.

  2. 2

    Low current in cables

    For a given power transmitted.

  3. 3

    Less heating

    Losses depend on current: P=I2RP = I^2 R.

  4. 4

    Step-down transformer

    Reduces the voltage for safe use.

Why high voltage is used

Why is electricity transmitted at high voltage?

Try an exam question

Explain why electrical energy is transmitted through the national grid at a high voltage.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.