Chapter 4
4 Magnetostatics
Magnetostatics is the physics of steady currents: from the Lorentz force and Biot–Savart's law this chapter builds the magnetic field of any current, its behaviour in magnetic materials, inductance and magnetic energy, and the magnetic circuit used to design transformers, motors and electromagnets.
Magnetostatics is the physics of steady currents. Where Chapter 3 started from the force between two charges at rest, this chapter starts from the force on a moving charge, the Lorentz force, and from the field that a current element produces, Biot–Savart's law. From these two experimental facts we build the whole theory: the vector potential, the two fundamental equations of magnetostatics with Ampère's law as the working tool, the behaviour of the field in magnetic materials, inductance and magnetic energy, Ohm's law and the Joule phenomenon, and finally the magnetic circuit, the engineer's shortcut for transformers, motors, recording heads and electromagnets. Each of the three sections ends with its own problems and solutions.
Work through the three sections in order, following the arrows of Figure 4.0.1 from the Lorentz force to the magnetic circuit. Each section ends with its own problems and solutions, in the same order as in Chapter 3: Problems, Home exercises, Further exercises and, where the section has them, Worked examples. Try each problem first and read the solution afterwards: the methods matter more than the answers.