Magnetic field

Five quantities do almost all the work in Topic 4. Charge \((Q)\) is the property that is carried. Current \((I)\) is the flow of that charge. E.m.f. \((E)\) is the electrical work a source does on each unit of charge sent round a complete circuit. Potential difference \((V)\) is the work each unit of charge does passing through a component. Resistance \((R)\) is the p.d. across a component per unit current in it. Power and energy then follow. Magnetism adds fields and forces, and a changing magnetic field adds induced e.m.f.

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.

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. A changing magnetic field produces an e.m.f. — that is electromagnetic induction. Motors use the first; generators and transformers use the second. Keeping those two sentences apart prevents most of the errors in Topic 4.