Magnetism

A compass turns because its magnetic dipole responds to Earth’s magnetic field. The same field idea explains forces on moving charges, currents, and magnetic materials.

This lesson supports Chapter 18 of Physics for Beginners. Keep the physical picture in view, write the units, and check the direction of every vector before trusting a calculation.

Magnetic fields form closed loops

Outside a bar magnet, field lines run from the north pole toward the south pole, then continue through the magnet. Cutting a magnet creates two smaller dipoles. It does not isolate a single north or south pole in ordinary classroom experiments.

Magnetic force depends on motion and angle

A charge moving in a magnetic field experiences \[\vec F=q\vec v\times\vec B.\] The magnitude is \(F=|q|vB\sin\theta\). A stationary charge feels no magnetic force. A charge moving parallel to the field also feels none. The force is perpendicular to both velocity and field.

Currents create magnetic fields

A current-carrying wire produces a magnetic field around itself. A loop or solenoid concentrates the field and acts as an electromagnet. Magnetic materials respond through the alignment of microscopic magnetic domains, and heating or impacts can disturb that alignment.

Worked example

A positive charge moves east through a magnetic field directed north. The right-hand rule gives an upward magnetic force. A negative charge with the same velocity would feel a force downward.

Watch the idea in action

Khan Academy gives a focused explanation of the chapter’s central idea. Pause before each calculation and predict the next step on paper.

Practice questions

1. Do ordinary magnets have isolated poles?
2. What produces a magnetic field around a wire?
3. Write the magnetic force magnitude on a moving charge.
4. What force acts when velocity is parallel to the field?
5. Which way does magnetic force point relative to velocity?
6. What is a solenoid?

Answers

1. No. They have north-south dipoles.
2. Electric current.
3. \(F=|q|vB\sin\theta\).
4. Zero magnetic force.
5. Perpendicular to it.
6. A coil of wire that produces a concentrated magnetic field when current flows.

Keep studying

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