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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.