1. Transformer The working principle of a transformer
The magnetic field produced by coil A varies at the same frequency with the a.c. source.
An a.c of the same frequency is induced in coil B.
coil A
coil B
a.c. source
galvanometer

2. The structure of a transformer
soft iron core
secondary coil
a.c. source
primary coil
output
Almost all the magnetic field lines produced by primary coil guide through the secondary coil.

3. Changing voltage
input voltage
number of turns in secondary coil
number of turns in primary coil
output voltage
The output voltage depends on the number of turns in the primary and secondary coils.

4. number of turns in the primary coil=
number of turns in the secondary coil 1
input voltage = output voltage
number of turns in the primary coil
<
number of turns in the secondary coil 2
input voltage < output voltage
It is called step-up transformer.

5. number of turns in the primary coil
>
number of turns in the secondary coil 3
input voltage > output voltage
It is called step-down transformer.

6. where Vp = input voltage Vs = output voltage
turn ratio
where Vp = input voltage
Vs = output voltage
Np = number of turns in the primary coil
Ns = number of turns in the secondary coil
Simulation: Transformer

7.  For ideal transformer, power input = power output
where Ip = input current
Is = output current 

8. Efficiency of transformers
Real transformers convert 90 % to 99 % of input power to the output power.
Efficiency = 90% to 99 %

9. Main energy loss in transformers
output a.c.
input a.c.
The coil is heated up when current flows through it.
Energy is required to magnetize and demagnetize the core
solid core
Eddy current heats up the coil.

10. Ways for improving the efficiency
output a.c.
input a.c.
Using thicker wire to reduce resistance of the coil
Using soft iron to reduce energy loss
Using a laminated core to reduce eddy current.
laminated core

11. That’s the end of Section 21.3
Key Ideas
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Section 21.4
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