Magnetism & Electromagnetism

What a magnetic field is

• A magnetic field is the region around a magnet in which a force would act on another magnet, or on a piece of magnetic material placed within it
• Magnetic fields are visualised using magnetic field lines:
  • The direction of the field at any point is the direction the north pole of a small test compass would point if placed at that point
  • The strength of the field is shown by the spacing of the lines: close together where the field is strong, far apart where it is weak
• Two important drawing rules:
  • Field lines always run from N to S outside the magnet (and from S to N inside it, completing a closed loop)
  • Field lines never cross or touch each other; they only meet at the poles themselves

The field around a bar magnet

• The field is strongest at the poles, so the lines are packed close together there and spread out further from the magnet
• The pattern is symmetric: lines bow outward from the N pole, curve round, and re-enter the S pole

Uniform fields

• A uniform magnetic field has the same strength and the same direction at every point. On a diagram it is drawn as a set of equally spaced parallel field lines
• A uniform field can be made by placing two flat magnets with opposite poles facing each other, separated by a few centimetres. Between the poles, the field looks uniform to a good approximation; outside that gap it is curved like a normal two-magnet field