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Full Answers to Review Questions: 13 Current–potential difference relationships
A similar argument in terms of current can be made. As the
resistance of the lamp is about the same as that of the resistor, the
circuit resistance is approximately doubled when they are connected
in series. This means the current in the lamp is only about half the
normal operating current and so the lamp will only glow dimly.
ii) The latter argument is better for considering the power, which
is given by I 2 R. In series, each will take the same current. As the
resistance of the lamp at a potential difference of less than 12 V is
less than that of the resistor (which remains at 32 Ω), the power
developed in the resistor is greater than that in the lamp.
3 a) i) Ohm’s law states that for a metallic conductor at a constant temperature
the current in the conductor is proportional to the potential
difference across the conductor.
ii) The resistance of any conductor is defined as the potential difference
across the conductor divided by the current in the conductor.
b) I = V
__
c)
d)
I/A
0.200
0.177
0.100
12 V
R =
______ 2.0 V
= 0.177 A = 177 mA
11.3 Ω
0 1.0 2.0 V/V � Figure A.3
Figure A.4 �
nichrome
ribbon
current
sensor
A
voltage
sensor
V
data
logger
computer
The current sensor and voltage sensor are connected to the data logger
(analogue to digital converter), the output of which is fed into a
computer. The potential divider is used to vary the potential difference
from zero to 2.0 V (note that a series variable resistor cannot do this –
see page 133) and the current and potential difference are recorded at
set intervals. This data is stored in the data logger and can then be used
to plot a graph via the computer.
The main advantage of using a data logger is that a large amount of data
can be collected and processed in a relatively short time, thus giving
better average values.
The only real disadvantage is the complexity of the set-up compared
with just using digital meters.
Tip
The conditions in italics must be
included in order to give a complete
definition and get full marks.
Tip
If you state the formula R = V __ , you
I
must define each term in the formula.
Edexcel Physics for AS © Hodder Education 2009
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