Investigating the factors affecting the size of current flowing through a length of resistivity putty.
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Introduction
04/02/97
GCSE PHYSICS SC1
INVESTIGATION
1996/7
INVESTIGATING THE FACTORS
AFFECTING THE SIZE OF CURRENT
FLOWING THROUGH A LENGTH OF
RESISTIVITY PUTTY.
CANDIDATE NUMBER 6247
CENTRE NUMBER 15131
PLANNING
AIMS
The main aim of this investigation will be to investigate the factors
that have an effect on the size of the current passing through a length of
resistivity putty. I will also try to explain my results using a number of
models formulae and theories including Ohm's law, the formula for
resistivity and a water model.
VARIABLES
Below is a list of the variables which are likely to have a major
effect on the size of the current passing through a length of resistivity,
putty:
1. The length of putty used.
2. The cross-sectional area of the putty. (Including the amount
of contact between the plates and the putty).
3. The temperature of the putty.
4. The voltage across the putty.
5. The resistance of the wires etc. and the contact resistance
between the plates and the putty.
Other factors, for example the density of the carbon inside the
putty, or the pressure at which the experiment is carried out, may also
have an effect on the size of the current, however these factors would
either be difficult to control in a school physics laboratory or are not
likely to have a significant effect compared to the inaccuracies of the
experiment.
The two factors which I have chosen to investigate are:
1) How the length of putty used affects the size of the current.
2) How the cross-sectional area of the putty affects the size of
the current.
I have chosen these factors as they are not too difficult to set up or
test. They are also factors which should give meaningful results which
can be used to make good conclusions.
BACKGROUND INFORMATION
The resistivity putty that I used in the experiments is supposed to
have had a resistivity of about 4x10-2 Ω m. For this experiment I will refer
Middle
The apparatus was set up as shown in the diagram below:
The circuit was set up as shown on the previous page. The putty
was rolled out into pieces with the correct length and cross-sectional area
by placing a blob of the putty onto a plastic board and then rolling a
wooden board over the top. Once the putty was nearing the correct shape,
metal rollers with a diameter of 9mm were placed on the board so that the
putty could not be rolled out thinner than the required diameter. The putty
was then measured with the ruler and cut to the appropriate length with a
spare copper plate.
In each experiment the rheostat was adjusted so that the voltage
across the putty was 3.0V + or - 0.03V. The putty was then connected to
the circuit via copper plates connected to crocodile clips which were then
connected to the circuit. I tried to make sure that the connection between
the putty and the copper plate was as good as possible by doing the
following:
1. The copper plates were cleaned with emery paper before
they were placed against the putty.
2. I inserted the plates at right angles to the putty and gently
pushed the two pieces of putty together to try to ensure that
good contact was made without changing the shape of the
putty.
3. The plates were inserted in the same way for each
experiment to ensure the results were fair.
Depending on the size of the current flowing through the putty I
changed the shunts on the ammeter. If the current was below 100mA I
used a 100mA shunt so that the ammeter had a full scale deflection of
100mA. If the current was larger, I used a 1A shunt. I designed the
experiments so that there was never a current of more than about 0.5A
Conclusion
some large experimental errors in my experiments. Below is a list of the
factors I consider to have been responsible for the largest errors:
1. Inaccurate measuring equipment such as ammeters.
2. Inaccurate use of measuring instruments such as rulers
which lead to inaccuracies in the results.
3. The fact that the resistance of the pieces of putty I was using
seemed to change due to perhaps uneven distribution of the
carbon inside the putty or changes in the temperature of the
putty. The changes could be due to large currents flowing
through the putty or the putty being heated up by my
hands, friction when it was being rolled out or changes in
the temperature of the room.
4. Differences in the uniformity of the putty.
5. Contact resistance between the plates and the putty.
LIMITATIONS
This experiment was limited by a number of factors. Firstly it was
limited by the amount of time available. Although we, as a class, had
enough time to carry out enough experiments to be able to produce
accurate conclusions, we did not have enough time to carry out more
experiments on pieces of putty with particularly large cross-sectional
areas or long lengths of putty. This means that range of results is limited.
However I still believe that the data I collected was accurate enough and
covered a large enough range of cross-sections and lengths for me to have
been able to draw accurate conclusions.
FINAL CONCLUSIONS
A summary of my final conclusions for this investigation is as
follows:
1. The length of a piece of resistivity putty is inversely proportional to
the size of the current flowing through it.
2. The cross-sectional area of a length of resistivity putty is directly
proportional to the size of the current.
FUTURE EXPERIMENTS
Based on this investigation I have found that both the length and
cross-sectional area of a piece resistivity putty affects the size of the
current flowing through it. However, if I was to continue the investigation
it would be interesting to see if this is the same for all conducting
materials.
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