Electromagnetism, Properties Of Waves, Motion, Gases & The Kinetic Theory Worksheet With Answers Page 7
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36Using Induction - TRANSFORMERS
A transformer is a device that makes use of the fact that electricity can be created (induced)
by a changing magnetic field. Transformers are used to increase (step-up) or decrease (step-
down) the voltage.
Here’s a diagram of a transformer where two separate coils have been wound around two
sides of the same piece of solid iron ‘core’:
240 V
Secondary
Secondary coil
Primary voltage
voltage (output)
Primary coil
(input)
Iron core
Here’s a large transformer in the
National grid ……
…… and here’s a small transformer
– a phone charger
The explanation for how electricity is created in the secondary coil could be asked for in a
“QWC”-style examination question. Here’s an example of a well-structured answer :
The alternating current in the primary coil creates a changing magnetic field around it.
Iron is a magnetic material, and so easily transmits this magnetic field to the secondary coil.
The constantly changing magnetic field around the secondary coil induces a voltage in this
coil.
Additionally, whether this output voltage is greater or lesser than the primary voltage
depends on the amount of turns in the secondary coil as compared to the primary.
where
V
= voltage across the primary coil
1
V
= N
V
= voltage across the secondary coil
1
1
2
V
N
N
= number of turns on the primary coil
1
2
2
N
= number of turns on the secondary coil
2
Example :
The input (primary) voltage of a phone charger is 240V (mains). The output needs
to be 4.8 V. Calculate “N
” (the number of turns on the secondary coil) if N
= 2000.
2
1
N
= N
x V
= 2000 x 4.8 = 40 turns
2
1
2
V
240
1
7
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