Physics Coursework
Experiment: To investigate the factors that affect the resistance of a wire.
1) Scientific Knowledge
This experiment will involve the resistance of a wire, along with
current and voltage; we will think about Ohm’s law for this reason.
Ohm’s law states that the resistance of a wire is a constant and is
equal to the voltage divided by the current at constant temperature.
So…
R=V/I
V
I=V/R
V=IR
I
R
2) Fair Test
The factors that will affect how well the investigation will work
are: 1) Temperature
2) Length of wire
3) Length (time) of experiment
4) Thickness of wire
5) Voltage
These are all important factors because all of them would affect the
current, which would in turn affect the resistance.
To make sure this is a fair test, we will make sure to:
1) Be quick with turning the power pack on and off (so it doesn’t
overheat)
2) Use same thickness of wire (0.20mm)
3) Use same voltage power pack (6 volts)
We will not keep the length constant, as that is the factor that we
will use for this experiment. We will change the length of wire to see how
the resistance changes in comparison.
3) Method
i) Apparatus
For this experiment we will need the following apparatus:
1) Power pack (to provide the power)
2) Analogue Voltmeter (to measure voltage – unfortunately no digital
ones will be available)
3) Digital Ammeter (to measure current accurately)
4) Normal wires (to create the circuit)
5) Crocodile clips (to connect the wires to the rest of the circuit)
6) Constantan wire (55% Cu, 45% Ni Approx.)
7) Ruler (1 metre ruler…measures down to mm…great degree of
accuracy for cutting wire to set lengths)
8) Scissors (to cut the wire)
9) External micrometer (to measure the thickness of the wires accurately)
ii) Diagram
We will set up the apparatus as shown below:
Power pack
Normal wires
Ammeter
A
V
Voltmeter
Constantan wire
iii) Explanation
We will get all the apparatus from the list. We will first measure
out a certain thickness of constantan wire (0.20mm), using the external
micrometer. When the wire’s thickness has been confirmed as 0.20mm,
we will measure out the five different lengths (to nearest mm) and cut the
wires to these lengths using the scissors. We will then set up the
apparatus as in the diagram, making sure that the ammeter is in series and
the voltmeter is in parallel. We will repeat it several times, each time with
a different length. We will also do it with each length three times and find
an average to minimize errors.
v) Results and Range
We will take a total of 15 results using 5 different lengths (three
times each). The lengths we will use are: 20.0cm, 40.0cm, 60.0cm,
80.0cm, and 100.0cm.
vi) Prediction
I think that the longer the wire, the more resistance there will be in
the circuit. I think this because, as the wire is longer, there are more
atoms and molecules for the electrons to hit on the way through the wire,
and therefore, to be delayed by. More collisions would mean it would
take longer to go through the wire, which means greater resistance.
4) Results
i) Tables
These show the Length, Current, Voltage and Resistance. Lengths are to
the nearest millimetre as that is the greatest degree of accuracy we can get
with a ruler that has millimetres as its smallest measurement. Current and
Voltage are to 2d.p as this is a reasonable degree of accuracy; as these are
to 2d.p, the resistance must also be to 2d.p.
1st Time
Length (cm)
20.0
40.0
60.0
80.0
100.0
Current (A)
1.60
0.87
0.56
0.37
0.33
Voltage (V)
4.25
4.55
4.75
5.00
5.00
Resistance (Ω)
2.66
5.23
8.48
13.51
15.15
Current (A)
1.62
0.88
0.56
0.37
0.33
Voltage (V)
4.40
4.60
4.80
5.00
5.00
Resistance (Ω)
2.72
5.23
8.57
13.51
15.15
Current (A)
1.64
0.87
0.58
0.32
0.33
Voltage (V)
4.50
4.60
4.95
5.00
5.00
Resistance (Ω)
2.74
5.29
8.53
15.63
15.15
2nd time
Length (cm)
20.0
40.0
60.0
80.0
100.0
3rd Time
Length (cm)
20.0
40.0
60.0
80.0
100.0
Average
Length (cm)
20.0
40.0
60.0
80.0
100.0
Current (A)
1.62
0.87
0.57
0.35
0.33
Voltage (V)
4.38
4.58
4.83
5.00
5.00
Resistance (Ω)
2.71
5.25
8.53
14.22
15.15
ii) Graph
The following page is our results put into a graph, to better
illustrate the results. This graph can then be analysed, in my “Analysis”
section, in order for me to see whether it shows us anything that was not
clear from the tables.
5) Analysis
i) Prediction
My prediction was mostly correct. I predicted that the longer the
wire, the more resistance acting upon it. This was as, the longer the wire,
the more particles there were to be bombarded by electrons on the way
through. A greater number of collisions between electrons and other
particles would mean that it would take longer for the electrons to go
from one end of the wire to the other; hence the greater resistance.
The only parts of the graph that didn’t agree with my prediction
were when the length was 20.0cm and 80.0cm. Here there must have
been errors.
ii) Graph Analysis
The graph came out quite well in general, showing strong positive
correlation of the points. It suited my prediction, although the first and
fourth points were anomalies.
iii) What we have learnt
We have learnt that the greater the length of a wire, the greater the
resistance acting upon it.
iv) Errors
1)
2)
3)
4)
5)
6)
7)
8)
Some possible errors were:
Wrong measurement of wire
Wrong thickness wire
Wrong voltage set
Wrong set-up of apparatus
Wrong recording of results
Wire not being straight and smooth (having knots)
Leaving the power on for too long would lead to over-heating
(which would affect the current)
Wire going out of clips’ edges (wire being shortened would lead to
inaccurate results)
6) Evaluation
i) Method
I think my method was a good way of carrying out the
investigation as the results seemed to be accurate. It seemed to work
as, the theory behind it was correct and, thankfully, the practical work
was just as effective.
ii) Errors
There were errors at 20.0cm and 80.0cm. These errors may have
been caused by any number of things, so looking to identify the
precise cause would be time-consuming and impractical.
iii) Improvements
The experiment could be improved by being more accurate with
measurements, making sure the wire is straight, being more careful
with the set-up, not leaving it on too long, or making sure to use the
same voltage each time. Also, we could use a digital voltmeter, as well
as ammeter, to get more accurate readings. Finally, we could a greater
range of lengths, to obtain more accurate results.
iv) Further Work
To obtain further relevant information, we could carry out several
other experiments. I am interested in the idea of whether changing the
voltage or current would have any effect on the wire’s resistance. I
believe that the results obtained from such an experiment would be
just as valid as these; hence the experiment, in which these factors
change, would be a very good one to attempt.