Applications of Electromagnetism

Electromagnetic Waves

A changing magnetic field generates an electric field.

B E

A changing electric field generates a magnetic field.

E B

changing E

ELECTROMAGNETIC WAVE

speed = c (speed of light; 3.00 x 108 m/s)

changing B

changing Echanging B

EM Wave

E ┴ B ┴ v

v

EM waves produced by accelerated electric charges.

To produce an EM wave:

Electromagnetic Spectrum

v = λ f

Transformers

A changing magnetic field in the primary coil induces a current in the secondary coil.

Use an alternating current to produce a constantlychanging magnetic field.

EMF in a coil depends on the number of turns of wire in that coil.

Step-up Transformer

EMF α N

Step-Down Transformer

Secondary coil has fewer turns (N) than the primary.

EMFs Ns

---------- = --------

EMFp Np

Power in a Transformer

If there are no losses (ideal transformer),

Powerprimary = Powersecondary

Power = voltage x current

EMFp x Ip = EMFs x Is

since voltage = EMF

Step-up transformer: increases EMF, decreases current

Step-down transformer: decreases EMF, increases current

EMF α 1/I

Electric Power Transmission

Power loss in wires occurs when current is high.

Use step-up transformer to transmit power at high emf and low current (minimize power loss).

Electric Motor

Motor: Converts electrical energy into mechanical energy.

Commutators – switch the current direction in the armature coil every half turn

wire wrappings which turn

field poles

In an A.C. motor, input electricity continually switches direction; commutators not needed.

Maximum induced current whencutting through lines of magnetism

With each ½ turn, wire in the armature changes its direction of motion in magnetic field.

Induced Alternating Current

A.C. Generator

D.C. Generator

To produce an output of D.C., use

commutators to switch current direction.

Motor vs. Generator

Motor: converts electrical energy to mechanical energy

Generator: converts mechanical energy to electrical energy

When a motor is spinning, it also acts as a generator and generates an EMF oppositethe supplied EMF.

This is called the back EMF.