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P3 Transformers. (revising P1 material)
In P1, the transformer was used to step up or step down VOLTAGE (or potential difference)
1. The transformer has 2 coils: primary and secondary
2. The core is made of ‘soft’ iron. It is easily magnetised by the effect of a current in a
coil around it but loses that magnetism immediately the current is switched off.
3. The primary coil is supplied by an ALTERNATING voltage so its magnetism changes
very rapidly.
4. The magnetism passes through the core to the other coil.
. The ALTERNATING MAGNETISM in the SECONDARY COIL INDUCES an ALTERNATING
VOLTAGE across it. ELECTROMAGNETIC INDUCTION
5. 5. The wires in each coil are insulated so that the current flows all around the cores
and magnetises them. Uninsulated wire would allow the current to short circuit its
route.
6. In the national grid, transmitting POWER over a long distance is made more efficient
by using a HIGH voltage and a LOW current. So at the power station end, a step up
transformer increases the voltage to as much as 400,000Volts whilst at the town or
city end, step down transformers reduce it to a safer value of 230volts.
Textbook: page 246-7, 248-9, 250-51
Using and making transformers
Electricity is transmitted around the country at a high voltage so
that less energy is wasted in the overhead power lines.
The voltage has been increased (stepped-up) at the power station by a transformer.
The high voltage would be too dangerous for homes or factories so it
must be stepped _ _ _ _ again by another t _ _ _ _ _ _ _ _ _ _ .
There is a C –core on each side of a transformer. A coil of wire is wound
on each side. The number of turns on each side decides if it is a step up
or step down transformer.
Examples: EVERY battery charger or mains adaptor is a STEP DOWN
transformer. It reduces the voltage from 230v to 3, 6, 9, or 12 volts D.C.
The transformer itself uses ALTERNATING CURRENT (A.C.)
Transformer details.
1. The PRIMARY coil is fed by high voltage AC from the mains.
2. This makes ALTERNATING MAGNETISM in the ‘soft’ iron cores.
3. The magnetism reaches the second half of the core.
4. An alternating VOLTAGE is INDUCED across the SECONDARY coil.
5. This voltage can then power another device with a new voltage by making an
alternating current flow in it.
In principle, the transformer is passing electrical POWER from one coil to the other.
To do it EFFICIENTLY, the coils need LOW resistance otherwise
_ _ _ _ is generated and power is wasted.
The changes that take place are that if the voltage is stepped UP, the current is
stepped DOWN by the same factor.
In the National Grid, stepping UP the voltage makes the CURRENT small so that the
wires don’t heat up and waste energy.
Transformer examples here:
Primary turns
Secondary turns
Step up or
down
Vprimary
Vsecondary
down
300
150
20
10
400
100
60
20
100
10
1000
3000
70
140
2000
500
60
120
80