This supports my hypothesis because, as the wire gets longer, there will be more atoms for the electrons to collide with. Therefore the electrons will get slower as the resistance increases, so the current decreases.
The resistance of a wire is calculated by using both the current, and the potential difference from within the circuit. The measurements are fed into this equation:
V = I R V = Voltage, I = Current, R = Resistance.
The equation can be arranged to:
R = V
I
Ohm's Law.
Ohm's Law, is also relevant to know. This states that, the current through a metallic conductor (IE a wire) at a constant temperature is proportional to the voltage. Therefore V is constant. This means that the resistance of a metallic conductor is constant providing that the temperature also remains constant. Also, the resistance of a metal increases as the temperature increases. This is because at higher temperatures, the particles in the conductor are moving around more quickly, so there is more chances of collisions with the free electrons.
Fair Test?
To make it a fair test, I must change only one variable; the length of the wire. All other variables must be kept constant at all times so that the length of the wire can be the only thing affecting the current.
I must also not let the wire get too hot, because increased temperature will increase the resistance, which could in turn alter the results. To do this, I will keep the jockey on the wire for the shortest time possible so that the current does not pass through the wire for too long. Also, I will allow the wire to cool inbetween readings; more-so in the shorter lengths of wire because they are more liable to heat up quickly in a short space of time.
Also, I will repeat the practical three times, and obtain an average current for each length of wire. This should ensure that my results are liable.
Apparatus.
- 3 Cells;
- Ammeter (analogue);
- Voltmeter (digital);
- Jockey;
- 100cm of exposed wire.
Preliminary Work.
After performing a rough trial, I observed that the current in the smaller lengths of wire was too strong as the needle was jumping off the scale on the ammeter. I added a rheostat into the circuit to reduce the current slightly so that the needle remained on the scale and I was able to take measurements.
Therefore, the circuit changed slightly to look like this:
Furthermore, as the wire was very thin, it heated up fairly rapidly, so i decided to allow the wire to cool between experiments; considerable heat was noticed at the shorter lengths. As I mentioned above, an increase in heat will increase resistance, so the temperature must be kept constant to ensure a fair test.
Method.
1. A basic circuit was assembled, including a 100cm length of wire (attatched to a metre rule), and a jockey.
2. An ammeter was added into the circuit, so as to measure the current emerging from the length of exposed wire.
3. A voltmeter was added into the circuit to measure the potential difference across the exposed wire.
4. The jockey was pressed down on the wire at 10cm intervals, each time completing the circuit. The positioning of the jockey should be vertical on top of the wire for all of the readings; this should ensure that all of the readings are taken in the same way and therefore not vary the results.
5. Between readings the circuit was broken to prevent the overheating of the wire.
6. Each time the circuit was completed, the readings from the voltmeter and ammeter were recorded. When using the ammeter, the recordings should be taken from directly above so that the eye is looking straight down at the needle; as the ammeter is analogue, reading the dial from this position would give a more accurate reading. The voltmeter however, is digital, and an accurate reading is given immediately.
Safety.
In order for the experiment to be performed safely, the wire must not be allowed to overheat, as it is exposed and could easily burn the skin. Lengths of wire under 10cm will not be attempted. When the current would pass through them, the wire would heat to a high temperature rapidly. Again this could result in burning.
Results Table.
(see seperate page).
From my results I calculate the average current for each length and plotted a graph to show its relationship with the length of wire. I have also constructed a graph by calculating the resistance for each length of my wire, and plotted it against wire length, to show their relationship.
Conclusion.
Having performed the investigation, I concluded the following:
- As i predicted, an increase in length resulted in an increase in resistance, and therefore a steady decrease in current. This is clearly illustrated on the two graphs showing the relationships between the length and the current, and the length and the resistance.
- The graph showing the relationship between wire length and resistance shows a strong, positive trend of a straight line. I can therefore say, that the resistance is directly proportional to the length of the wire. IE if the length of the wire doubles, then the resistance will also double. This is shown on the graph showing the relationship between the wire length and the resistance. (IE When the length of the wire is 40cm, the resistance, is 0.18. So when the length is doubled to 80cm, the resistance is also doubled to 0.36.)
- There is also a decrese in current as the wire increases, as predicted. This is due to the increase in resistance; the longer the wire becomes, the lower the current. The graph showing the relationship between the length and the current illustrates this. However the relationship is not directly proportional, this is shown by the markings on the graph; at 30cm, the current is 0.82. When that length is doubled to 60cm, 0.65. The second resistance of 0.65 is not half of 0.82, therefore it cannot be directly proportional.
Evaluation.
Overall, the results appeared to be extremely reliable, and there were no anomolous results whatsoever. I can say this because all the results were either lying on, or extremely close to the line of best fit. Also, the spread of the results for each length of results was extremely small, for example, for the length of 10cm:
0.01 x 100% = 0.01%
0.97
There is only 0.01% difference in the spread for ten centimetres. Similarly:
0.02 x 100% = 3.70%
0.54
There is only 3.70% spread in the length of ninety centimetres; this is the largest percentage of spread in the results. (percentages rounded to 2dp).
Some of the results, for example for the length of 100cm, there is no spread whatsoever.
The small percentages of the spreads prove the results to be very reliable; the smaller percentage of spread, the more reliable results are.
To improve the readings taken from the ammeter, I could've used a digital ammeter rather than the analogue one. This could have given the results to more decimal places, and would have been more accurate than me reading from the dial on the ammeter. AS the number of decimal places would have increased, the accuracy would have increased; this could have therefore reduced the spread of the results.
One thing I would change if I performed the same experiment again, is to be more accurate when reading my voltage measurements. As the voltmeter was digital, the measurements kept flickering by one or two decimal places; I took the measurement which stayed still for the longest, however, the results proved reliable as there were no anomolous results. In addition to this, rather than use a jockey to press down on the wire, I could clip the wire into the circuit each time using crocodile clips; this would prevent movement of the wire as may happen with the jockey.
Also, I could've pulled the wire more taut over the ruler, as it was fairly loosely attatched, and the readings I obtained could have been from longer lengths than the ones i thought i was reading.
Another experiment i could perform to support this investigation, is by changing a different variable, and keeping the length of the wire the same. For example I could change the diameter of the wire. However if I was to do so, I must ensure that I kept the temperature, length and metal the wire is made of, constant, and also not alter the potential difference passed through the wire. I would assume that if the length of the wire was kept the same, the amount of atoms in the wire would be kept the same. However, if the diameter increased, the number of atoms in the wire would increase. If the diameter was doubled, the amount of atoms in the wire would double. So, similar to the experiment I performed, as the diameter increases, the resistance would also increase and therefore, the current would decrease.
Kristy Vinton 11 L.