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

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

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.