Peoples Physics Concepts

http://www.ck12.org Chapter 14. Magnetism

14.5 Generators and Motors

• Describe how a generator (and thus motor) works.

Students will learn how a generator (and thus motor) works. The lesson is on a conceptual basis and compliments

the lesson on electromagnetic induction.

Key Equations

em f=−

∆Φ

∆t

Faraday’s Law of Induction

Φ=N~B·~A Electromagnetic Flux

The 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).

Guidance

Changing magnetic fields passing through a loop of wire generate currents in that wire; this is how electric power

generators work. Likewise, a changing current in a wire will create a changing magnetic field; this is how speakers

and electric motors work.

Since only a changing flux can produce an induced potential difference, one or more of the variables in the equation

must be changing if anyemfis to be produced. 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.

Example 1

MEDIA

Click image to the left for use the URL below.

URL: http://www.ck12.org/flx/render/embeddedobject/333