19.1. Electromagnetic Induction http://www.ck12.org
If the magnet is now moved away from the loop as inFigure19.2, the magnetic field through the loop changes once
again and the galvanometer needle deflects to the left. The induced current in the loop is directed opposite to the one
inFigure19.1.
If the magnet is stationary (that is, if there is no change in the magnetic field), the galvanometer measures no current.
The experiment would have the same set of results had the magnet been stationary and the loop of wire moved
toward and away from the magnet. In this way the magnetic field as experienced by the loop would still change. It
is relative motion between the magnet and loop of wire that matters.
Faraday’s Experiment
InFigure19.3, a coil of wire (theprimary coil) is wrapped around an iron ring. The iron ring increases the
magnetic field when a current flows through the wire. The coil’s ends are attached to a battery. The primary coil
is always attached to the battery. The other coil (thesecondary coil) is wrapped around the same iron ring, its ends
attached to a galvanometer.
FIGURE 19.3
Faraday’s Experiment.The switch is initially open and there is no current in the circuit. When the switch is closed, a current is established
in the primary coil, and a magnetic field is created around the coil. The galvanometer needle deflects for an instant
and quickly returns to a zero current reading, indicating that a current was established in the secondary coil for a
brief moment.
If the switch is opened, the current in the primary coil is turned off. As a result, the magnetic field drops to a zero.
The galvanometer briefly deflects, again, but this time in the opposite direction, and returns to a zero current reading.
The experiment is, essentially, the same as the one conducted with the magnet,Figure19.1 and 19.2. In this
experiment, however, the “magnet” was created by a current in the primary coil and the “magnet” was moved by
closing and opening the switch. By closing the switch, the secondary coil was exposed to a changing magnetic field
(the induced magnetic field lines pass through the secondary coil). The changing magnetic field produced a current
in the secondary coil. As soon as the current in the primary coil became constant, the magnetic field also became
constant, like a stationary magnet, and the current in the secondary coil ceased. When the switch was opened, the
magnetic field collapsed, and a current was induced in the secondary coil in the opposite direction.
We stated earlier that a current is produced when a potential difference (a voltage) exists between the ends of a
conductor. A potential difference was established by using a battery. Faraday, however, was able to show that a
voltage (and current) could be induced across a secondary coil by a changing magnetic field. No battery needed to
be connected directly to the secondary coil in order to produce a voltage or current in the coil. A voltage or current
created by the changing magnetic field are called, respectively, an induced voltage and an induced current.