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Principles of electromagnetic induction

Electromagnetic induction is the production of a potential difference (more precisely, an electromotive force) due to a changing magnetic field.

If there is a complete circuit, this potential difference will cause a current to flow.

Moving a bar magnet in and out of a coil generates a potential difference across the coil because the magnetic field is changing

A stationary bar magnet inside a coil does not generate a potential difference because the magnetic field is constant.

A bar magnet is moved into a coil and a potential difference is induced. Top: A current flows because the circuit is complete. Bottom: No current flows because there is no circuit, but a p.d. is still induced.
A bar magnet is moved into a coil and a potential difference is induced. Top: A current flows because the circuit is complete. Bottom: No current flows because there is no circuit, but a p.d. is still induced.

A solenoid is connected in series to a galvanometer as shown below.

A galvanometer measures both the magnitude and the direction of a current.

A bar magnet is moved in and out of the solenoid.

This is the set-up of Faraday's experiment on electromagnetic induction.
This is the set-up of Faraday's experiment on electromagnetic induction.

Observations:

  • Current flows when the bar magnet is moving into the solenoid or out of the solenoid.
  • Current is zero when the magnet is stationary.

Conclusion:

  • A potential difference (electromotive force) is induced whenever the magnetic field passing through the solenoid changes. This causes a current to flow, which is detected by the galvanometer.

A solenoid is connected in series to a galvanometer as shown below. A bar magnet is moved in and out of the solenoid.

Observations:

  • Current flows anti-clockwise when the north pole is moving into the solenoid.
  • Current flows clockwise when the north pole is moving out of the solenoid.

The directions clockwise and anti-clockwise refer to the coils of the solenoid, not to the entire loop of the circuit.

This pattern is reversed if the south pole is moved in and out of the solenoid instead.

Conclusions:

  • The induced potential difference always produces a magnetic field that opposes the change.

    This is related to Newton's third law (forces occur in equal and opposite pairs) and the conservation of energy.

  • In this experiment, the north pole enters the solenoid from the right. Current flows anti-clockwise to create a north pole at the right hand side of the solenoid to repel the north pole and prevent it from entering.
  • When the north pole leaves the solenoid to the right. Current flows clockwise to create a south pole at the right hand side to attract the north pole and prevent it from leaving.

Several factors can change the size of the induced potential difference in a solenoid.

The current flowing through the circuit (and therefore also the induced potential difference) is larger when:

  • the number of turns in the solenoid is increased;
  • the magnetic field is strengthened (this can be done by using a stronger magnet or using several magnets at once); and
  • the magnet is moved in and out of the solenoid at a greater speed.