- Faraday's law of electromagnetic induction states that an electromotive force (emf) is induced whenever there is a change in the magnetic flux linked with a circuit. The magnitude of the induced emf depends on the rate of change of flux, the number of turns in the coil, and the cross-sectional area of the coil. - Lenz's law states that the direction of the induced emf opposes the change that produced it. - In a transformer, a change in the primary coil's current induces a current in the secondary coil through mutual inductance. The ratio of voltages between the primary and secondary coils equals the ratio of their turns.

1. PHYSICS
ELECTROMAGNETIC INDUCTION
Short revision series

2.  EMF can be generated if the magnetic flux
through a coil is altered.
 Emf can be generated by either moving the
coil or source of flux relative to each other or
changing the magnitude of the source of flux
in some way

3. Faraday’s law of em induction
 E = - dФ/dt
 Whenever there is change in magnetic flux
linked with a circuit an electromotive force E
is induced the strenght of which is
proportional to the rate of change of the flux
Ф linked with the circuit.
 The magnitude of induced emf will depend
on
 The rate of change of flux
 The no. of turns in the coil
 The cross-sectional area of the coil

4. Lenz’s law of em induction
 The direction of the induced emf is such that
it tends to oppose the change that produced
it.
 In an AC Generator
E = NdФ/dt
N = number of turns of the coil

5. EMF generated in a rotating coil
 Assuming a coil of N turns and area A being
rotated at a constant angular velocity ω in a
magnetic field of flux density B with its axis
perpendiculaar to the field.
 When the normal to the coil is at an angle θ
to the field, the flux is
 B A N cos θ = B A N cos (ωt)
 since θ = ωt
 Emf = -d (B A N cos ωt)
dt
E = B A N ω sin t

6. Emf during self inductance
 Self inductance occurs when current
through a coil is altered, the flux through
that coil will also change thereby inducing
the emf.
 E = LdI
dt
 dI = change in current
 L = self inductance of the coil

7. Energy stored in an inductor
 = ½ L I2
 L – Inductance
 I - Current

8. The transformer

9. The transformer cont.
 A transformer is a setup that uses the
property of mutual inductance to change the
voltage of an alternating supply
 Step up transformers raises the voltage.
Output > than input. Used in power stations
to to produce high voltage
 Step down transformers output < input.
Used in homes.

10.  FOR AN IDEAL TRANSFROMER
 VS/VP = NS/NP = IP/IS
 VS IS = VP IP

11. EG
 A transformer has an efficiency of 60%. It
can produce 10V from a 200V supply. What
is the current in the primary coil if the
current in the secondary coil is 10A?

12. solution
 Efficiency = 60/100
 Efficiency = Vs Is/Vp Ip
 60/100 = 10 x 10/200 x Ip
 3/5 = 100/200 Ip
IP = 5 x 100/3 x 200
= 5/6 = 0.8A

13. end