flowing in the counterclockwise direction, up the bar and then around the metal rail back
to the bottom of the bar. This is called an induced current.
The moving bar is a source of an electromotive force, called motional emf, since the
emf is generated by the motion of the bar.
The force is defined as:
The magnitude of the induced emf can be increased by increasing the strength of the
magnetic field, moving the bar faster, or using a longer bar.
EXAMPLE
A bar of length 10 cm slides along metal rails at a speed of 5 m/s in a magnetic field of 0.1
T. What is the motional emf induced in the bar and rails?Now that we’ve defined motional emf, solving this problem is simply a matter of plugging
numbers into the appropriate equation:
Faraday’s Law
Moving a conductor through a magnetic field is just one way of inducing an electric
current. A more common way of inducing current, which we will examine now, is by
changing the magnetic flux through a circuit.
Magnetic Flux
The magnetic flux, , through an area, A, is the product of the area and the magnetic
field perpendicular to it:
The A vector is perpendicular to the area, with a magnitude equal to the area in question.
If we imagine flux graphically, it is a measure of the number and length of flux lines
passing through a certain area.