Experiment 2b
Class:
Name:
(
) Date:
2b To investigate the V–I relationship of a
light bulb
Objective
To investigate the relationship between the voltage across a light
bulb and the current flowing through it.
Background information
1
The resistance of a conductor is defined as:
voltage across conductor
Resistance =
current through conductor
or
R=
V
I
2
Ohm’s law states that the voltage across the ends of a conductor is
directly proportional to the current flowing through it, provided that
the temperature and other physical conditions are unchanged.
3
Rheostats are resistors with variable resistance. The resistance of a
rheostat can be changed by moving its sliding contact.
Apparatus
❏ 1 ‘12 V, 24 W’ light bulb
❏ 1 voltmeter
❏ 1 ammeter
❏ 1 power pack (0–12 V a.c./d.c.)
❏ 1 rheostat
❏ several connecting leads
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New Physics at Work (Second Edition)
© Oxford University Press 2007
Class:
Name:
(
Experiment 2b
) Date:
Procedure
✐ A rheostat with
resistance up to 10 Ω is
assumed to be used. If a
rheostat with resistance
up to 20 Ω is available,
set the output voltage of
the power pack to 3 V
d.c. Enough sets of data
can then be taken by
changing the resistance
of the rheostat only. This
reduces the complexity
of the experiment.
1
Set up the apparatus as shown in Figure 2b-1:
(a) Connect a ‘12 V, 24 W’ light bulb in series with a rheostat, a
power pack and an ammeter.
(b) Connect a voltmeter across the bulb.
(c) Move the sliding contact of the rheostat to the end to maximize
its resistance.
(d) Set the output voltage of the power pack to 1.5 V d.c.
power pack
sliding contact
ammeter
voltmeter
✐ Battery box can be
rheostat
used to replace the
power pack. By changing
the number of batteries,
the output voltage is
varied.
✐ The bulb no longer
obeys Ohm’s law when
the voltage across it is
about 0.4 V. Ts should
make sure that students
can take enough sets of
data within 0−0.4 V
and beyond 0.4 V,
otherwise students may
not be able to draw the
conclusion.
light bulb
1.5 V d.c.
+
rheostat
light bulb
A
+
Fig 2b-1
V
2
Take the ammeter and voltmeter readings. Record the results in
Table 2b-1.
It is suggested to take
the sets of data as
follows:
3
Take 5 sets of data within
0.1−0.5 V, 5 sets of data
within 0.7 V−1.5 V, and
3 sets of data within
2.0 V−3.0 V.
To take other sets of data, vary the current flowing through the bulb
by adjusting the output voltage of the power supply and by moving
the sliding contact of the rheostat. Record the results in Table 2b-1.
✎
Results:
Note
Voltage across bulb V / V
0.1
Current through bulb I / A
0.13 0.23 0.30 0.35 0.38 0.43 0.47 0.51 0.55 0.57 0.64 0.71 0.77
0.2
0.3
0.4
0.5
0.7
0.9
1.1
1.3
1.5
2.0
2.5
3.0
Table 2b-1
New Physics at Work (Second Edition)
© Oxford University Press 2007
21
Experiment 2b
Class:
Name:
4
(
) Date:
Plot a graph of the voltage V across bulb against the current I
through bulb in Figure 2b-2.
voltage across bulb V / V
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
current through bulb I / A
Fig 2b-2
✎
Describe the shape of the graph.
The graph is linear at the very beginning and then curves upwards.
Discussion
✎
Does the filament of the bulb obey Ohm’s law? Explain your answer.
The filament of the bulb obeys Ohm’s law only when the voltage across it is very
small. When the voltage is large, the filament is non-ohmic.
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New Physics at Work (Second Edition)
© Oxford University Press 2007
Class:
Name:
(
✎
Experiment 2b
) Date:
How does the resistance of the filament of the bulb change with the
voltage across it?
The resistance of the filament increases with the voltage across it.
✎
The temperature of the filament of the bulb increases with the
voltage across it. How does the resistance of the filament change with
its temperature?
The resistance of the filament increases with its temperature.
does not obey
The filament of a light bulb ________________________
Ohm’s law at
increases
high temperature and its resistance ________________________
with
temperature.
Further thinking
✎
Sketch the graph of voltage against current obtained if the light bulb
in this experiment is replaced by silicon, a semiconductor whose
resistance decreases when its temperature increases.
V
I
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© Oxford University Press 2007
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