Electromotive Force EMF

1
Electromotive Force - EMF

• The potential difference when no charge flows to
  an external circuit is called the electromotive
  force or EMF.
• An induced EMF is produced by a changing magnetic
  field.

2
Magnetic Flux

• The EMF produced is proportional to the rate of
  change of the magnetic flux, ?B, passing through
  the loop of area A, which is defined as
• ?B B? A BA cos ?

B ? B cos ?
B - magnetic field strength
3
Faradays Law of Induction

• If the flux through N loops of wire changes by an
  amount ??B during a time ?t, the average induced
  EMF during this time is

4
Faradays Law of Induction

• The working equation for a moving conductor is
  given by
• EMF B l v
• where l is the length of the conductor
• and v is the velocity of the conductor.
• This equation is valid as long as B, l and v are
  mutually perpendicular.

5
Problem using Faradays Law

• An airplane travels 1000 km/hr in a region where
  the earths field is 5.0x10-5 T and is nearly
  vertical. What is the potential difference
  inducted between the wing tips that are 70m
  apart?

6
Problem using Faradays Law

• Since v 1000 km/hr 280 m/s
• ? EMF B l v 5.0 x 10-5 x 70 x 280
• ? EMF 1.0 V

7
Lenzs Law

• An induced EMF always gives rise to a current
  whose magnetic field opposes the original change
  in flux.
• This is the reason for the negative sign in
  Faradays Law of Induction.