19.1. Electromagnetic Induction http://www.ck12.org
19.1 Electromagnetic Induction
Objectives
The student will:
- Understand magnetic flux.
- Understand Faraday’s law of induction.
- Understand Lenz’s law.
Vocabulary
- electromagnetic induction:Both a current and a voltage may be produced by changing a magnetic field.
- induced current:A current created by the changing magnetic field.
- magnetic flux:The product of the magnitude of the magnetic field vector~B, the magnitude of the area
vector~A, and the cosine of the angleθbetween them. - primary coil:The coil that is always attached to the battery and the iron ring in Faraday’s Experiment.
- secondary coil:The coil attached to a galvanometer and the iron ring in Faraday’s Experiment.
Introduction
We learned earlier that a current-carrying wire produces a magnetic field and that a magnetic field exerts a force
on a current-carrying wire. Physicists in the first half of the 19th century believed the converse might also hold,
and that a magnetic field should therefore produce an electric current. This was discovered simultaneously by two
scientists- the American physicist Joseph Henry (1797-1878) and the British physicist and chemist, Michael Faraday
(1791-1867).
Electromagnetic Induction
What experiment can show that the magnetic field creates electric current? It was already known that a static
(unchanging) magnetic field did not induce an electrical current within a stationary conducting wire. The thought,
then, was that perhaps a changing magnetic field might cause a current. Below, we describe two experiments
designed with that thought in mind.
InFigure19.1 andFigure19.2, a bar magnet is being moved toward a loop of wire and away from a loop of wire,
respectively. In each case, the loop of wire is connected to a galvanometer. (A galvanometer is a device that detects