Experiment 6c
Class:
Name:
(
) Date:
6c Simple transformer
Objective
To construct a simple transformer by winding wires around two
C-cores and study how the transformer works.
Background information
1
An e.m.f. can be induced in a conductor when the magnetic field
through it is varying, or when the conductor is cutting through
magnetic field lines.
2
Lenz’s law states that the direction of the induced e.m.f. tends to
oppose the change causing it.
3
Transformers make use of the effect that an e.m.f. is induced across
a coil if the current through a nearby coil is changing, even though
there is no movement involved.
Apparatus
❏ 1 pair of double C-cores with a clip
❏ 1 battery box (6 V)
❏ 1 centre-zero galvanometer
❏ 1 switch
❏ 1 power pack (0–12 V a.c./d.c.)
❏ 2 lamps (2.5 V)
❏ 1 demountable transformer with a U-core and a coil of 2400 turns
❏ several PVC-coated copper wires
❏ several connecting leads
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Class:
Name:
(
) Date:
Experiment 6c
Procedure
1
Wind 10 turns of wire round a C-core and 25 turns of wire round
another C-core. Clip the two C-cores together.
2
(a) Connect the coil of 10 turns (the primary coil) to a switch and
a battery box, and the coil of 25 turns (the secondary coil) to a
centre-zero galvanometer (Fig 6c-1).
(b) Close the switch and then open it. Note for any deflection of the
pointer of the galvanometer.
battery box
primary coil (10 turns)
switch
C-cores
secondary coil
(25 turns)
centre-zero
galvanometer
Fig 6c-1
✎
When the switch is closed, the pointer deflects to one side, then returns to zero
position.
✎
When the switch is opened, the
pointer deflects to the other side, then returns
to zero position.
✎
induced current
An ___________________________________
is produced in the secondary
coil when the current in the primary coil is switched on or off.
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123
Experiment 6c
Class:
Note
The switching action of
the battery is replaced
by a 1 V a.c. power
supply which reverses
the direction of current
in the primary coil at a
certain frequency
(50 Hz).
Name:
(
) Date:
3
(a) Connect the primary coil to an a.c. power supply, and the
secondary coil to a 2.5 V lamp (Fig 6c-2).
(b) Connect another 2.5 V lamp to the power supply.
(c) Switch on the power supply. Compare the brightness of the two
lamps.
(d) Repeat with a secondary coil of 50 turns. Note the change in the
brightness of the lamp in the secondary coil.
power pack
2.5 V lamp
C-cores
2.5 V lamp
primary coil (10 turns)
secondary coil (25 turns)
Fig 6c-2
✎
brighter
The lamp in the secondary coil is __________________
(brighter/dimmer)
than the lamp in the primary coil. This shows that the voltage in
higher
(higher/lower) than the
the secondary coil is ________________________
voltage in the primary coil.
✎
The brightness of the lamp and the voltage in the secondary coil
increases
________________________
(increases/decreases/does not change) when
the number of turns of the secondary coil is increased.
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Class:
Name:
✐ Here the 220 V a.c.
mains supply has to be
stepped down to 2.5 V
a.c. (voltage ratio being
220 : 2.5), so a turns
ratio of 2400 : 28 is
required. At least 28
turns have to be wound
before the lamp is lit up
to its normal brightness.
(
Experiment 6c
) Date:
4
(a) Put a coil of 2400 turns round one arm of the U-core of a
demountable transformer and connect it to an a.c. power supply
(Fig 6c-3).
(b) Connect a 2 m wire to a 2.5 V lamp.
(c) Switch on the power supply. Wind the long wire turn by turn
for at least 28 turns around the other arm of the U-core. Note
the brightness of the lamp.
To produce a dramatic
effect, get the class to
count, in chorus, the
number of turns wound
on the core. Also,
partially black out the
laboratory to enable the
class to see the initially
dimly lit lamp.
coil of 2400 turns
to a.c. power supply
demountable
transformer
2.5 V lamp
Fig 6c-3
✎
When more turns are wound around the arm of the U-core,
the lamp is lit, first dimly, then more brightly.
Discussion
✎
What is the function of the double C-cores?
The double C-cores enable the field lines produced by the primary coil to go into the
secondary coil.
induced
1 A changing current in one coil can produce an _________________
current
_______________
in another coil.
ratio of the voltages
2 The ___________________________________
in the primary and
secondary coils of a transformer depends on the number of
turns in the coils.
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© Oxford University Press 2007
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