The Magnetic Field: Induction, Ampere and Lorentz Forces
How electric currents create magnetism, how a field pushes on currents and moving charges, and why iron is special
Big Idea: A magnetic field is produced by moving electric charges (currents) and by magnetised matter, and it acts back only on moving charges. Its strength and direction are captured by one vector, the magnetic induction B. From this single vector follow the two central forces of this topic: the Ampere force on a current-carrying wire and the Lorentz force on a single moving charge.
By the end you will be able to:
- explain how a current makes a magnetic field and find the field direction with the gimlet (right-hand) rule;
- define the magnetic induction B and state its unit, the tesla;
- calculate the Ampere force F = BIL sin α and find its direction with the left-hand rule;
- calculate the Lorentz force F = qvB sin α and describe the circular motion of a charge in a field;
- distinguish diamagnetic, paramagnetic and ferromagnetic materials and explain magnetic recording.