25: Inductance
- Page ID
- 165910
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Try to stop a current suddenly, by pulling a plug from its socket while a vacuum cleaner is running, and you may see a spark jump between the prongs. A current that has built up in a coil does not want to stop, much as a moving car does not want to stop. The coil fights the change with an induced emf, large enough, if the current is interrupted quickly, to drive a spark across an air gap. This electrical “inertia” is inductance.
Faraday’s law says that a changing current in one circuit induces an emf in any circuit whose flux it changes, and that includes the circuit itself. This chapter follows that idea. It introduces inductors, the circuit elements designed to have inductance, and the mutual inductance that couples one circuit to another. It shows that an inductor stores energy in its magnetic field, just as a capacitor stores energy in its electric field. It follows the rise and fall of current in circuits with inductors and resistors, a close cousin of the RC circuits of Chapter 21. And it ends with the transformer, the device that makes it practical to send electrical power across a continent.
Learning Objectives
- Define self-inductance and mutual inductance, and calculate them for solenoids.
- Find the emf across an inductor from the rate of change of its current.
- Calculate the energy stored in an inductor and the energy density of a magnetic field.
- Analyze the growth and decay of current in RL circuits, using the time constant \(L/R\).
- Explain how transformers change voltages and currents, and why power is transmitted at high voltage.