Cambridge O Level Physics · Syllabus 5054 · Electricity and Magnetism
Magnetic field
What is Magnetic field?
A region in which a magnetic pole experiences a force; the field direction at a point is the direction of the force on a north pole placed there, and the field is stronger where the field lines are closer together.
This definition is part of the Electricity and Magnetism chapter in Cambridge O Level Physics.
Magnetic field in context
Like poles repel and unlike poles attract; a magnet also attracts an unmagnetised magnetic material because it induces magnetism in it. A magnetic field is a region in which a magnetic pole experiences a force. The field direction at a point is the direction of the force on an N pole placed there, so outside a bar magnet the lines run from N to S. Where the lines are closer together, the field is stronger.
This section is a two-way street with a strict division of labour. Current in a magnetic field produces a force — that is the motor effect, and its direction comes from Fleming's left-hand rule. Changing magnetic flux produces an e.m.f. — that is induction, and its direction comes from Lenz's law. Motors use the first; generators and transformers use the second. Keeping those two sentences apart prevents most of the errors in Topic 4.
Questions students ask about Magnetic field
How do you find the direction of the force on a current-carrying wire in a magnetic field?
This is the motor effect: a current in a magnetic field experiences a force. The relative directions of the force, the field and the current are found using Fleming's left-hand rule. Reversing either the current or the field reverses the direction of the force, which is exactly what the motor-effect experiment demonstrates.