Download Slide 1

23-1 Induced Electromotive Force
Faraday’s experiment: closing the switch in the
primary circuit induces a current in the
secondary circuit, but only while the current in
the primary circuit is changing.
23-1 Induced Electromotive Force
• The current in the secondary circuit is zero as
long as the current in the primary circuit, and
therefore the magnetic field in the iron bar, is not
changing.
• Current flows in the secondary circuit while the
current in the primary is changing. It flows in
opposite directions depending on whether the
magnetic field is increasing or decreasing.
• The magnitude of the induced current is
proportional to the rate at which the magnetic
field is changing.
23-1 Induced Electromotive Force
Note the motion of the magnet in each image:
23-2 Magnetic Flux
Magnetic flux is used
in the calculation of the
induced emf.
23-3 Faraday’s Law of Induction
Faraday’s law: An emf is induced only when the
magnetic flux through a loop changes with time.
23-3 Faraday’s Law of Induction
There are many devices that operate on the
basis of Faraday’s law.
An electric guitar
pickup:
23-3 Faraday’s Law of Induction
Tape recorder:
23-4 Lenz’s Law
Lenz’s Law
An induced current always flows in a direction
that opposes the change that caused it.
Therefore, if the magnetic field is increasing, the
magnetic field created by the induced current will
be in the opposite direction; if decreasing, it will
be in the same direction.
23-4 Lenz’s Law
This conducting rod
completes the circuit.
As it falls, the magnetic
flux decreases, and a
current is induced.
23-4 Lenz’s Law
The force due to the
induced current is
upward, slowing the fall.
23-4 Lenz’s Law
Currents can also flow in
bulk conductors. These
induced currents, called eddy
currents, can be powerful
brakes.
23-5 Mechanical Work and Electrical
Energy
This diagram shows the variables we need to
calculate the induced emf.
23-5 Mechanical Work and Electrical
Energy
Change in flux:
Induced emf: