Pieces of chromatography will be cut, and then soaked in salt water. Pieces of chromatography paper ranging from 10cm to 1cm in length (1cm steps, 10 readings in all), will be soaked in brine, the bulldog clips will hold these in place connecting the paper to the circuit. A reading from the ammeter and voltmeter will be taken, and the next piece of paper prepared. This will be repeated once for each reading to ensure accuracy. To ensure that this is a fair test the paper will always be of the same kind, 4cm in width, and 0.15mm in depth, the salt solution will also be of the same concentration throughout the experiment, (dense concentration would effect results because more sodium chloride molecules). Also two 1.5V batteries will be used although slightly different voltage will be taken into account when recording results (this will be noted in results table).
This method is repeated for analysing cross sectional area however; the chromatography paper is always 5cm in length, and pieces of width between 5cm and 0.5 will be used (steps of 0.5cm, 10 readings in all). The other constants to certify fairness remain the same.
This is generally not a dangerous experiment so little safety has to be taken into account, a low voltage and ensuring the task was carried out a good distance from the edge of the bench were the main factors to consider.
The resistance will be calculated by measuring the current and voltage through the chromatography paper, these readings will then be applied
to this formula;
Voltage = current x resistance
This can be rearranged to resistance = Voltage
Current
The fact that a milli-ammeter is being used because the current is so small, must also be taken into account. When calculating resistance the reading taken must be divided by 1000 to get the actual quantity of amps, before voltage is divided by this.
Results
Results for experiment in which length is varied
The amps in both tables were taken from to 2 decimal places on a milli-ammeter, I have converted them to amps (see planning) which is how they appear in the table.
Constants; width of paper is always 4cm
depth of paper is always 0.015cm
Results for experiment in which cross-sectional area was varied
Constants; length of paper is always 5cm
Depth of paper is always 0.015cm
Conclusion
My results show that as the length increases so does resistance which fully agrees with my prediction. I can tell this because the graph for varying lengths has positive correlation, without regard to anomalous results (circled) - the dependant variable (resistance) increases as the independent variable (length) does. At a length of 1cm, a mere 234.53 ohms of resistance were measured through the paper compared with the 2572.77ohms that was measured at 10cm. These 2 readings alone demonstrate the difference to resistance that length makes. The reason for this, as stated earlier in my prediction, is because as the length increases the current has more distance to travel raising chances of collision and hence the resistance.
My results also show that as cross-sectional surface area decreases resistance increases, this can be clearly seen by the negative correlation on the corresponding graph. Again this concurs with my prediction, the resistance increases because there is less space for the electrons to pass through and so they are more likely to collide with one another and molecules around them.
I have also discovered how paper soaked in Sodium Chloride solution is able to conduct electricity. The reason is because sodium chloride solution has free ions dissolved in water the compound will conduct electricity by the movement of ions. The bulldog clips act as electrodes, one positive, one negative. The water means that the ions are free to disassociate, the positive sodium ions travel to the negative electrode and the positive chloride ions to the negative electrode, causing an electrical current to flow.
Evaluation
During the experiment we encountered numerous problems, and weaknesses. Firstly, we found that as the experiment progressed, that each piece paper had absorbed different amounts of salt water. This effected results because a piece with more brine on, could conduct more electricity. This was due to an error in our method, we simply soaked each piece chromatography paper without regard to this, and without any attempt to make this fair. To overcome this, perhaps we could place the paper into a test tube of salt water for a specific amount of time, this way each piece of paper has the same time to absorb the brine making this fairer. Also the same quantity of brine could be used, to further ensure that the paper is always being prepared identically. Another problem we encountered, which seriously affected results was that were unable to carry out the experiment as one continuous effort. Each time we returned to this, we also returned to totally different equipment. Batteries varied in voltage and milli-ammeters in accuracy, it also meant the paper was soaked differently, the obvious way to solve this. This resulted in varying readings, compared with the point at which we picked up from particularly, . We also found that we took the reading from the milli-ammeter at a different point for each different length and cross sectional area. This was significant because the reading on the milli-ammeter seemed to drop gradually. We choose to take a reading when the current had seemingly stabilised, but readings often dropped or rose after this. To solve this we could set a specific amount of time, for example, 5 seconds, after which to record a reading from the milli-ammeter.
After studying my results I have noticed several strange and out of pattern readings. These results are circled on the graph to highlight them and actually decrease from a steady upwards line which they should follow. I can identify that they anomalous because they are not close to the line of best fit, and do not follow the line of the readings. The most likely reason for this is that at this point we had picked up the experiment from a previous time, and were using totally different equipment, this may have been less accurate.
If I were to extend this investigation I could use a wider variety of lengths and cross sectional areas, as too increase my database of readings for more certain and accurate conclusions. To extend this particular aspect, I could look into the reason for some currently unexplained patterns in results. For example, in both my original experiment, and repeat of the length varying practical I noticed that the reading for 4cm was considerably lower than for 5cm. The jump was by far the steepest from the set of results, and was seen no where else (except with anomalous results). Research into why this difference is seen here would be an interesting and relevant way to extend the investigation. I could also go on to investigate how heat affects the resistance or perhaps how the concentration of sodium chloride solution affects it.
Bibliography
During this investigation I used several books and web sites all listed below.
Collins GCSE sciences – Physics
CGP revision guides – double science physics
www.bitesize.co.uk
www.s-cool.co.uk