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Topic p14 TEST!
• P14 Electromagnetic Effects
• Monday 14th December
Last lesson - Electromagnetic
induction
If a magnet is
moved inside a
coil an electric
current is
induced
(produced)
Generator/dynamo
A generator works
by rotating a coil in
a magnetic field (or
rotating a magnet in
a coil). A slip-ring
commutator
produces
alternating current
(a.c.)
Today’s lesson - Transformers
Transformers –
Revenge of the Fallen
Transformers
A transformer consists of 3 main parts
(5)
…induces a
changing (a.c.)
voltage here.
(2)
Produces a changing
magnetic field here…
(1)
Changing a.c.
current here…
(3)
The changing
magnetic field is
carried by the iron
core
(4)
A changing
magnetic field
here….
Transformers
A changing current in the primary coil
produces a changing magnetic field in the
core. This changing magnetic field induces
a changing current in the secondary coil.
Changing voltages
A transformer where the voltage rises is
called a step-up transformer
Changing voltages
A transformer where the voltage falls is
called a step-down transformer
Changing voltages
The ratio of the number of turns on the
coils equals the ratio of the voltages
# of turns = Np
# of turns = Ns
Vp/Vs = Np/Ns
To know how an electric current
can be generated in a wire.
Example 1
A transformer has 1000 turns on its primary
coil and 3000 turns on its secondary. 30 V
is applied across the primary coil. What is
its output?
VP N P

VS N S
30
 0.33
VS
30 1000

VS 3000
VS
3
30
VS  3 30
VS
1

30 0.33
VS  90 V
To know how an electric current
can be generated in a wire.
Example 2
A transformer has 3800 turns on its primary and
100 turns on its secondary coil. Its output
voltage is 6 V.
a)
Is it a step up or step down transformer?
b)
Calculate the input voltage.
c)
Explain why this could be suitable for
charging a mobile phone.
a) It is a step down
transformer.
b) V p N p

Vs N s
Vp
3800

6
100
Vp
 38
6
VP  38 6
VP  228 V
c) Mains voltage is
230V so this
transformer is suitable
for stepping down the
voltage to a safe level
for charging a mobile
phone.
No energy for nothing
However, no new energy is made
No energy for nothing
Power in = power out
VpIp = VsIs
a.c. only
Because transformers rely on induction
(changing fields), they only work with
alternating current (a.c.)
Can you
copy this
please?
Why do we need transformers?
Why do we need transformers?
Current flowing through wires causes them
to get hot and energy is lost.
Why do we need transformers?
Keeping the current
low means
electricity can be
transported long
distances without
losing too much
energy.
Why do we need transformers?
However, in order to send a large quantity
of energy, high voltages are needed.
Transmission of Electricity
Step-up
Step-down
The effect of lower current
When energy is transferred at a low current, it uses a higher voltage, which
results in less energy being lost as heat in the cables.
The National Grid has to transmit electricity over large distances.
Step-uptransformer
If the National Grid was to
transmit at 25,000V from the
power station, about 40% of
the electrical energy would be
lost as heat – the electricity
would be expensive as a result.
But at 400kV, only 1% of the energy will be lost as heat.
This would also mean that the tall pylons could carry thinner, lighter wires, so
they would not have to be as strong.
Some questions!
Vs
Vp
=
Ns
Np
Primary
turns Np
Secondary
turns Ns
Primary
voltage Vp
Secondary
voltage Vs
100
20
400
10,000
10
50
240
12
5,000
33,000
11,000
3
Step up
or step
down
Vs
Vp
=
Ns
Np
Primary
turns Np
Secondary
turns Ns
Primary
voltage Vp
Secondary
voltage Vs
100
20
400
10,000
10
50
240
12
5,000
33,000
11,000
3
Step up
or step
down
Vs
Vp
=
Ns
Np
Primary
turns Np
Secondary
turns Ns
Primary
voltage Vp
Secondary
voltage Vn
Step up
or step
down
100
20
15
3
down
400
10,000
10
250
up
1,000
50
240
12
down
15,000
5,000
33,000
11,000
down
Primary coil
secondary coil
a.c. input
a.c. output
Iron core
There are 4 turns of coil in the primary coil and 32
turns in the secondary coil. The output voltage is
4000 Volts.
a. Is this a stepping up or stepping down transformer?
b.Calculate the input voltage.
c. Is the output voltage a.c or d.c.?
Transformers.
• http://www.youtube.com/watch?v=VPxdl1z
pcC8
Do now!
Can you stick the
sheets in please?
Let’s try some questions!