Magnetic Fields and Induction


In this learn page we will cover everything you will need to know for A level Physics, when it comes to Magnetism and electromagnetic induction. In my opinion, if I had to chose, this would be the most difficult part of A level physics conceptually- but even then it isn't too bad. We will start by defining magnetic fields and field strength, and go onto moving charges in fields and AC generators etc.

Magnetic Fields

A Magnetic field is a region around a magnetic material or electromagnet in which the force of magnetism acts. It is similar to gravitational fields where the field is the region where any particle with mass will have a force exerted on them.

As with charges in an electric field:

  • Like poles repel
  • Opposite poles attract

*Field lines always go from north to south (by convention):*

As with charges in an electric field:

  • Like poles repel
  • Opposite poles attract

These can be explained by looking at the field lines for an interacting pair of poles:

For the attracting opposite poles it is clear that the field lines line up, thus the net force on each pole is towards each other. However, for the opposite poles the field lines oppose, thus causing a repulsive force between the poles.

Magnetic Field Strength

The strength of a Magnetic field is called the Magnetic Flux Density (B) and is measured in Teslas (T) (equal to a Newton per meter per ampere).

Current Carrying Wires In Magnetic Fields

A current carrying wire in a magnetic field will experience a force. This is if the wire is a a non-zero angle to the magnetic field.

In the image below imagine the wire is coming out of the screen towards you, perpendicular to the field lines. Conventional current is flowing in the direction of the arrow.

The Electric Motor

Moving Charges in Magnetic Fields

Electromagnetic Induction

AC Generator

Alternating Current


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