18d
E
18d
Class:
Name:
(
)
Date:
Simple transformer
Apparatus
You will make a simple transformer by winding wires round two C-cores.
You will then study how a transformer works.
• 1 galvanometer
• 1 pair of double
C-cores with
a clip
• 1 battery box
• 1 switch
Procedure
• 1 low voltage
power supply
1
• 1 demountable
transformer,
complete with
a U-core and
a 2400-turn coil
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 10-turn coil (the primary coil) to a switch and
a battery box, and the 25-turn coil (the secondary coil) to a
centre-zero galvanometer (Fig 18d-1).
• 2 2.5 V lamps
• several PVCcoated wires
(b) Close the switch and then open it.
(c) Observe the effect on the galvanometer.
primary coil (10 turns)
switch
secondary coil
(25 turns)
galvanometer
Fig 18d-1
As we close the switch, the pointer
deflects to one side, then returns to
zero position.
As we open the switch, the pointer
deflects to the other side, then
returns to zero position.
induced current
An
is produced in the secondary coil when
the primary current is switched on or off.
42
New Physics at Work (Second Edition)
© Oxford University Press 2006
18c
Class:
Name:
Note: The switching
action of the battery is
replaced by a 1 V a.c.
supply which reverses
the direction of current
in the primary coil at a
certain frequency (50 Hz).
(
3
)
Date:
18d
(a) Connect the primary coil to an a.c. power supply, and the
secondary coil to a 2.5 V lamp (Fig 18d-2).
(b) Connect another 2.5 V lamp to the a.c. supply.
(c) Switch on the a.c. supply and compare the brightness of the two
lamps.
a.c. supply
primary coil (10 turns)
secondary coil (25 turns)
Fig 18d-2
brighter
The lamp in the secondary coil is
(brighter/dimmer)
than the lamp in the primary coil. This means that the secondary
voltage is
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) and so a turns ratio of
2400 : 28 is required. At
least 28 turns have to be
wound before the lamp is
lighted up to its normal
brightness.
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.
4
higher
(higher/lower) than the primary voltage.
(a) Place a 2400-turn coil in one arm of the U-core of a
demountable transformer and connect it to the a.c. mains
(Fig 18d-3).
(b) Connect a wire of 2 m long to a 2.5 V lamp.
(c) Switch on the mains and wind the long wire turn by turn round
the other arm of the U-core.
(d) Wind at least 28 turns.
2400-turn coil
to mains supply
demountable
transformer
Fig 18d-3
As more turns are wound, the lamp
© Oxford University Press 2006
is lit, first dimly, then more brightly.
New Physics at Work (Second Edition)
43
18d
Class:
Name:
(
)
Date:
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.
1 A current is induced in a coil when current through a neighbouring
coil is
changing
mutual inductance
. This effect is called
.
2 The voltage in the primary and secondary coils of a transformer
depends on
44
New Physics at Work (Second Edition)
turns ratio
in the coils.
© Oxford University Press 2006