15.3. Effects of Magnetic Fields http://www.ck12.org
will equal 4 cosθBπr^2. Think of the magnetic flux as the part of the “bundle” of magnetic field lines “held” by the
loop that points along the area vector.
If the magnetic flux through a loop or loops changes, electrons in the wire will feel a force, and this will generate a
current. Theinduced voltage(also calledelectromotive force, or emf) that they feel is equal to the change in flux
4 Φdivided by the amount of time 4 tthat change took. This relationship is called Faraday’s Law of Induction:
em f=−∆Φ
∆t
[6] Faraday’s Law of InductionThe direction of the induced current is determined as follows: the current will flow so as to generate a magnetic
field thatopposesthe change in flux. This is called Lenz’s Law. Note that the electromotive force described above
is not actually a force, since it is measured in Volts and acts like an induced potential difference. It was originally
called that since it caused charged particles to move — henceelectromotive— and the name stuck (it’s somewhat
analogous to calling an increase in a particle’s gravitational potential energy difference a gravitomotive force).
Since only a changing flux can produce an induced potential difference, one or more of the variables in equation [5]
must be changing if the ammeter in the picture above is to register any current. Specifically, the following can all
induce a current in the loops of wire:
- Changing the direction or magnitude of the magnetic field.
- Changing the loops’ orientation or area.
- Moving the loops out of the region with the magnetic field.