Brief: To investigate the factors affecting the resistance of a wire.
Some factors I came up with:
 Temperature
 Thickness
 Material
 Length
I have chosen to investigate how the length of a wire affects its resistance. I have rejected the other three
because:
 It is hard to measure the temperature of a wire.
 When measuring thickness, I have to use wires of the same length and thickness, and add an extra one
each time. I feel that this is not very accurate.
 Facilities do not allow for wires of different materials, and also the resistances will not be in proportion
at all.
To investigate how the length of a wire affects its resistance
Prediction: I predict that the longer the wire, the bigger the resistance. I also predict that when the length of
the wire doubles, the resistance will also double (direct proportion).
Reason: Metals consist of atoms containing electrons which are relatively free. When a voltage is placed on
either end of the metal, the electrons can move about (this is what causes current to flow). However, the
electrons have to ‘dodge’ the atoms and escape between the gaps, so if the length is doubled then the atoms have
to ‘dodge’ twice as many atoms.
Already we can see that, as an electron, it would be harder to negotiate the wire in the second diagram.
I will do a test run for this experiment. I will be measuring the resistance of a wire at different lengths. I will
do this by measuring the amps and volts, and dividing volts by amps to get the resistance. I will use an ammeter
to measure the amps and a voltmeter to measure the volts.
Diagram:
I performed the experiment at lengths 50 cm, 25 cm and 10cm, and these are my results:
Length
50 cm
25 cm
10 cm
A
2.2
3.4
10
V
4.2
4.1
3.8
R
1.9
1.2
0.38
I then decided that in my real experiment, I should use intervals of 10 cm and go from 0-50. I also decided to
use a rheostat to further improve accuracy. A rheostat gives different readings, so I will use it three times on
each length and average them together for the final result. I also decided to set the power supply on 4 volts,
and use the 1-10 scale on the ammeter and the 1-5 scale on the voltmeter.
Actual experiment!
Then, I performed my actual experiment.
Method:
First set the length of wire between the crocodile clips to 10 cms. Then turn the power pack on and set it to
4 volts. Slide the rheostat down to the bottom, and read the results from the ammeter and the voltmeter.
Write the results down and divide the voltmeter reading by the ammeter reading to give the resistance- write
this down too. Repeat, but move the rheostat to the middle. Then repeat again with the rheostat slider at the
top.
Then the entire experiment should be repeated at the lengths of 20 cms, 30 cms, 40 cms and 50 cms.
Exact equipment list
6 wires
1 power pack
2 crocodile clips
1 meter ruler
1 long piece of nichrome wire, attached to the ruler
1 ammeter
1 voltmeter
1 resistor
1 rheostat
Input variable
Length of wire.
Output variables
Resistance (volts and amps)
Controlled variables
Voltage (from power pack), temperature, thickness, wire material, strength of resistor, ammeter
Table
Length
50 cms
A
0.4
0.6
1.4
V
1.8
2.4
3.8
40 cms
0.4
0.6
1.8
1.6
2.1
3.6
30 cms
0.4
0.7
1.9
1.4
1.9
3.6
20 cms
0.5
0.8
2.6
1
1.2
2.8
10 cms
0.6
1.4
3.2
0.6
0.9
2.2
R
4.5
4
2.7
3.7
4
3.5
2
3.5
3.5
2.7
1.8
2.6
2
1.5
1
1.5
1
0.6
0.7
0.8
Discussing the graph
The graph clearly obeys Ohm’s Law as it is a straight line through the origin. It also follows the rule ‘when
the length is doubled, the resistance is doubled’, as on the graph a length of 25 cm gives a resistance of 1.85
Ω; when doubled, 50 cm should give a resistance of 3.7 Ω. If we look at the graph, we can see that this is true.
The graph shows that resistant is proportional to time, or inversely proportional to 1/A.
There is one obvious error on the graph- the reading for 30 cm is way off. I have ringed this. If I ever
repeated this experiment, these are the things I would change:
 I would be more accurate when taking the three rheostat readings. I would ensure that the rheostat
was exactly at the end, in the middle, and at the other end.
 I would try not to burn my wire by carelessly leaving the crocodile clips touching, as this could
adversely affect the results.
 I would use digital voltmeters/ammeters to give me the most precise reading possible.
Safety
For safety, our teacher Mr Yee checked all the circuits before he let us turn them on.
Evaluation
To obtain more evidence, the experiment should be repeated with smaller intervals. This would be better because
the more results land on the line, the more accurate the experiment.
I feel that this experiment was fairly accurate apart from the one anomaly.
by Ayesha Patel, 10H