Changing Flux Induces an emf in Transformers
Transformers are used to increase or decrease an AC voltage. The rms voltage at the
secondary is related to the rms voltage on the primary by
(V ) = (V )
2 rms N 1 1 rms
and the currents are related by
(I ) = (I )
2 rms N 2 1 rms
A power pack for a portable stereo delivers 1.0 A at a voltage of 9.0 V under normal operation.
It has 400 turns of wire on the primary coil.
What is the power output?
P = IV = (1.0 A) (9.0 V) = 9.0 W
How many turns of wire are on the secondary?
9 V = 120
N 2 30 turns
What are the voltage and current in the primary?
V 1 120 V
I A = 1
I1= 0.075 A
Power should be the same in the primary and secondary
Sense of force opposes motion
The downward flux in one end is increasing The downward flux in the other end is decreasing
The change in flux in the ends induce eddy currents in the metal
The magnetic field exer