Basic Electrical Theory
MAGNETIC CIRCUITS
N
= number of turns in a coil
= rate at which the flux cuts across the conductor,
DF
Dt
Wb
s
Example 1:
Given: Flux = 4 Wb. The flux increases uniformly to 8 Wb in a period of 2
seconds. Find induced voltage in a coil that has 12 turns, if the coil is stationary
in the magnetic field.
Solution:
Vind =
N
DF
Dt
DF = 8Wb - 4Wb = 4Wb
Dt = 2s
then
DF
Dt
4Wb
2s
2Wb
s
Vind = -12 (2) = -24 volts
Example 2:
In Example 1, what is the induced voltage, if the flux remains 4 Wb after 2 s?
Solution:
Vind
12
0
2
0 Volts
No voltage is induced in Example 2. This confirms the principle that relative motion must exist
between the conductor and the flux in order to induce a voltage.
Lenz’s Law
Lenz’s Law determines the polarity of the induced voltage. Induced voltage has a polarity that
will oppose the change causing the induction. When current flows due to the induced voltage,
a magnetic field is set up around that conductor so that the conductor’s magnetic field reacts with
the external magnetic field. This produces the induced voltage to oppose the change in the
external magnetic field. The negative sign in equation (1-20) is an indication that the emf is in
such a direction as to produce a current whose flux, if added to the original flux, would reduce
the magnitude of the emf.
Rev. 0
Page 43
ES-01
