Physics Coursework

Plan

In this project, I am trying to assess the effect length on the resistance of brine soaked paper.

To do this, I am attempting to measure the current flowing through a circuit with the paper in.

In order to start the experiment, I will need the following apparatus:-

Here is a circuit diagram of the proposed experiment:

Here is a diagram of how the retort stand will be arranged:

Preliminary

Before starting my main experiment, I performed a preliminary experiment that helped me to determine what unforeseen obstacles or difficulties I would encounter:

1. I found that the paper becomes too fragile to handle when it is over 30cm long. It breaks when soaked in water.
2. I found that water stays evenly soaked onto the paper when it is clamped in a horizontal position
3. I found that the saline solution causes salty deposits to be left on the Bull-dog clips.
4. I found that high voltages are detrimental to the experiment as they decrease the readings to quickly due to electrolysis. The experiment has to be carried out quickly and efficiently or the too much of the brine will ionise.
5. I found that rust occurs on the bull-dog clips that has to be sanded off between tests, to keep the readings accurate and the electrode clean. – An extra material I will need is sand-paper.

0.84

0.00084

8333

8

1.00

0.00100

8000

8419

30

5

0.50

0.00050

10000

6

0.61

0.00061

9836

7

0.73

0.00073

9589

8

0.87

0.00087

9195

9655

The graph shows that length is directly proportional to resistance, which is what I had previously predicted. This means that as length increases, resistance also increases at the same rate.

Extension

I am going to extend the experiment by determining how paper width and concentration of brine solution effect resitance.

Width:

The experiment will be the same, except that the length will remain a constant, while the width will become the variable. I am using 1,2,3,4 and 5cm width measurements to test this.

Due to my previous work using copper wire to measure resistance, I found that the thicker the wire, the proportionally smaller the resistance is.

For this experiment - I predict that as the width increases the resistance will decrease inversely.

Here are the results I collected:

The graph shows that width decreases inversely to resistance, which is what I had previously predicted. This means that as width increases, resistance decreases at a faster rate.

Concentration:

The experiment will be the same, except that the width will remain a constant, while the concentration of the brine solution will become the variable. I am going to use 0.05, 0.1, 0.2, 0.3, 0.4, 0.5M solutions to test it.

These agree with my predictions, although I did not predict 1/width and 1/concentration.

From these results, I can safely assume that:

Width    X    Thickness    =    Area                        ➔ Resistance is directly proportional to 1/area.

Using a constant to simplify and using a combination of rules I can say that:-

Resistance    =   Constant    X    Length➔ R   =         C L

                            Area                                                      A

I after researching what I could about electricity, I found that this equation was similar to that of resistivity. Resistivity is measured in ohm meters and is a constant value for a given material; the resistance of a unit length of the material per unit cross-sectional area. I can re-arrange the above equation so I can calculate resistivity:

R A = p l        ➔ R A  = P                ➔ P = R A

                                         L                        L

Evaluation

Most of my points were on or very close to the line of best fit, and so my results were reliable and demonstrate the connection between the variables we were testing. We could have used lower voltages using more sensitive equipment – to minimise the electrolysis taking place and could have collected more results although this would not have been worthwhile due to the minor accuracy advantage.

We did our experiments over a period of days so a temperature change could have occurred, but the effect of this is negligible. The stock solution used for the brine may not have been the same and may have differed (also to a negligible effect) slightly due to evaporation.

For extension work, I would try and find an equation that would allow me to acquire readings without the practical experiment.