Resistance of wire ISA

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Resistance of a Wire Investigation
Wire
Conclusion
1. What is the pattern shown in your graph?
2. Choose 2 points from your best fit line and
describe how they support your answer to
Q.1
3. Why does the length of wire used affect
the resistance like this?
The resistance of a conductor is a measure of
how easy or hard it is for a current to flow
through it.
A wire will warm up when a current flows
through it. An increase in temperature
will increase the resistance of the wire.
So measurements have to be taken
quickly before the wire has a chance to
warm up.
In this investigation you measured the current in a
circuit containing a length of wire. You also
measured the voltage across the wire when a
current was flowing through it.
From these measurements you calculated the
resistance of the wire.
The formula for calculating resistance is
Resistance ( Ω) = Voltage (V)
Current (A)
Here are some sample results from a similar
experiment
Length of Test Test 2 Test 3 Mean
wire /cm 1 / Ω / Ω
/Ω
Resistance
/Ω
20
3.1
3.4
3.2
3.2
40
5.9
6.0
6.2
6.0
60
9.3
9.2
9.0
9.2
80
12.4
12.2
12.4
12.3
100
15.7
15.4
19.2
15.6
What was the Independent variable in this
investigation (The variable that we
deliberately changed)?
The length of the wire.
Here are the results again.
Length of Test Test 2 Test 3 Mean
wire /cm 1 / Ω / Ω
/Ω
Resistance
/Ω
20
3.1
3.4
3.2
3.2
40
5.9
6.0
6.2
6.0
60
9.3
9.2
9.0
9.2
80
12.4
12.2
12.4
12.3
100
15.7
15.4
19.2
15.6
What was the range of this variable?
20cm to 100cm
Was this a suitable range to choose?
Yes
Why was this?
This spread was big enough for us to see if
there was a pattern to the results.
Suggest one thing that could affect the
accuracy of your results?
Allowing the wire to warm up?
How could you prevent this happening?
Work quickly before the wire gets hot.
In this experiment we did not change the
output from the power supply. We also
kept all the same wires and connectors
during the experiment. Why do you think
we did this?
If we changed anything except the
independent variable it could have an
effect on our results and so it would not
be a fair test.
Here are the results again. Look at the result in
red
Length of Test Test 2 Test 3 Mean
wire /cm 1 / Ω / Ω
/Ω
Resistance
/Ω
20
3.1
3.4
3.2
3.2
40
5.9
6.0
6.2
6.0
60
9.3
9.2
9.0
9.2
80
12.4
12.2
12.4
12.3
100
15.7
15.4
19.2
15.6
What is wrong with the result in red?
It is Anomalous (it does not fit in with the
other two results).
What should you do if you get an
anomalous result in an investigation?
You can either ignore the result or you
can repeat the test.
Here is a graph of results from the investigation.
Resistance of a wire
resistance / Ohms
20
15
10
5
0
0
20
40
60
80
100
120
Length of wire / cm
What is the link between the Independent
and Dependent variable?.
As the length of the wire increases the
resistance also increases
How do you know this?.
The line of best fit shows a positive
correlation between the length of the wire
and its resistance.
Why did we use a line graph to display our
results?
Resistance of a wire
resistance / Ohms
20
15
10
5
0
0
20
40
60
80
100
Length of wire / cm
Because the length of wire is a
continuous variable.
120
Here are the results again.
Length of Test Test 2 Test 3 Mean
wire /cm 1 / Ω / Ω
/Ω
Resistance
/Ω
20
3.1
3.4
3.2
3.2
40
5.9
6.0
6.2
6.0
60
9.3
9.2
9.0
9.2
80
12.4
12.2
12.4
12.3
100
15.7
15.4
19.2
15.6
Resistance of a wire
In this graph we have
plotted the results from
the individual tests
instead of the mean
result.
25
Anomalous result
Resistance /Ohms
20
15
The results for each
length of wire show a
spread or scatter
10
5
0
0
20
40
60
Length of wire /cm
80
100
120
Can you spot the
anomalous result on
this sort of graph?
What can cause errors in our investigations?
1. Systematic errors. These are caused because either the
equipment has been set up incorrectly or is not being used
properly. This means the error will be the same each time
2. Zero Errors. These are caused because a piece of
measuring equipment is giving a reading that is too high or
too low because it has not been set at zero properly. This
means the error will be the same each time
3. Random errors. These are caused by things not under
our control such as the temperature of the room changing
while we carry out the experiment. These errors can change
each time and can make our results slightly different each
time.
To draw a line of best
fit on a graph like this
you try to take the line
through the centre of
each scatter of
results. You can
ignore anomalous
results.
Resistance of a wire
25
Resistance /Ohms
20
15
10
5
0
0
20
40
60
Length of wire /cm
80
100
120
Instead of length of wire imagine you wanted to find
out if the thickness of a wire affects the resistance.
What would be the Independent variable in this
investigation?
The thickness of the wires.
What would need to stay the same about the wires
you use?
The length would have to be the same each time.
The wires should be made from the same metal
What else would you need to keep the same in the
investigation?
The output of the power supply
Apart from the thickness of the wire what other
measurements would you have to take and what
equipment would you use to make them?
The current in the circuit using an ammeter and the
voltage across the wires using a voltmeter.
How many times would you test each thickness of
wire and why?
We should test each wire at least three times so
that we can spot any anomalous results.
How many different thicknesses of wire should you
use and why?
We should use at least 5 different thicknesses. This
is the minimum number of values needed to spot
any pattern in the results
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