Current represents the number of electrons flowing per second. If the resistance is high; the flow of electrons is slowed down and so the number of electrons per second is reduced (i.e. the current is reduced), increasing the resistance.
Method
- Decide upon the different lengths of wire you want to use for the experiment
- Attach two wires at the positive and negative on an Ohmmeter. At the other ends of these wires attach crocodile clips.
- On a meter ruler, sellotape the constantan wire down across it. Attach the positive crocodile clip to the constantan wire at o cm on the meter ruler and the negative crocodile clip is attached at the constantan wire at the different lengths chosen (e.g. across by 10 cm each time).
- Each time the crocodile clips are at the certain length, you read off of the ohmmeter the resistance of the wire at that length (the Ohmmeter is read off at a range of between 0 and 200 ohms).
- Each reading is taken three times, so an average can be taken.
Results table
Conclusion
The graph and results show me that the longer the wire, the more resistance it has which supports my prediction. In a longer piece of wire there would be more metal ions, and so more metal ions for electrons to collide with slowing their progress thus increasing resistance of the wire (resistance is a measure of how easy it is for the electrons to flow through the metal). Effectively, if you doubled the length of wire you would be doubling the resistance too. My results generally show this, but not as accurately. An example of the resistance being doubled when the length is doubled is when the length of wire was at 10.0 cm, the resistance of the wire was 0.5 Ohms. 10.0 cm doubled is 20.0 cm, and at 20.0 cm the resistance was 1.3 Ohms. 0.5 doubled is 1. It’s relatively accurate but not double exactly. I expected in my prediction that if you doubled the length of wire you would then double the resistance as it makes sense that if the wire were twice as long, the electrons would undergo twice as many collisions. If it’s twice as difficult to travel along the wire, we say that the longer wire has twice the resistance. My graph generally shows that length is proportional to the resistance, although it doesn’t clearly show this.
Evaluation
Overall, I think the experiment was done reasonably accurately. I tried to get accurate and reliable results by doing each resistance reading three times, so I could take an average. I also used constantan wire, as it doesn’t conduct heat as much as copper, this would stop resistance being effected as much. I also noticed in my results, that a wire with the length of 0.0 cm there was a resistance of 0.2 ohms. A wire 0.0 cm long should have had 0.0 ohms. To enable me with more accurate results I subtracted 0.2 ohms from the averages. I think there were some limiting factors for instance temperature, an increase in temperature would cause the metal ions to have more kinetic energy, in which causes the electrons to slow down when they collide with them. Also, I don’t think that the length of wire was measured as accurately as it could have been. It is quite difficult to get an accurate reading of length by eye and to keep the wire at an accurate length; it may not have been straight.
It was clear from my results and graph that the length wasn’t directly proportional to the resistance but I think this was due to inaccuracy. So I think if I did a second experiment, still using different lengths of wire to see how length effects resistance, but in this experiment use a voltmeter and an ammeter to record voltage and current. Using these recordings I can use the Ohms law to work out the resistance. This time though, I would get results to 2 decimal places, which means my results would be more accurate.
Experiment 2
Aim
Using different lengths of constantan wire, investigate how the length of wire effects its resistance. This time aiming to get more accurate results than experiment 1.
Prediction
I think that I will get the same sort of results I got in the first experiment but in this one the results will be more accurate. I still predict that the longer the wire, the more resistance it has and that if you doubled the length of wire, you would double the resistance of the wire. So, the length of the wire is directly proportional to the resistance.
Method
- In a circuit connect a power pack, Ammeter and variable resistor in series, then connect a voltmeter in parallel.
- Set up a meter ruler again, with the constantan wire along it. Connect the ammeter and voltmeter to the wire at each end to read the voltage and current (readings of a range between 0-200)
- Start with your first length of wire. (e.g. 10.0cm) then stopping along the wire at each of the lengths.
- Using the variable resistor, keep the ammeter reading the same each time. In doing this you will keep the current the same.
- Read off the voltmeter reading. Do this for each length three times so an average can be taken.
6) Once the recording of results has been done, you can now use these results to work out the resistance of the wire.
Using this formula, you can work out the resistance of the wire. R = V
I
V= Volts (v)
I= Current (Amps)
R= Resistance (Ohms)
Example: To work out the resistance of the wire with the length of 10.0 cm, use the average voltmeter result and the ammeter recording.
Resistance= Voltage
Current
R= 0.82
1.58
= 0.52
0.52 is the resistance of the wire.
Results table
Conclusion
From my results and graph I can see that resistance changes with length. As you increase the length of a piece of wire, you increase the resistance. Both experiments support this hypothesis; I expected this because if you increase the length, there would be more electrons. With more electrons you would expect more collisions with the metal ions. The electrons would have a longer distance to travel aswell and so more collisions will occur. Due to this the length increase should be proportional to the resistance increase. From my results, I can see that length is directly proportional to resistance.
Example: The result from 30.0 cm had a resistance of 1.44 Ohms, if you doubled this you would get 2.88 Ohms. 30.0 cm doubled is 60.0 cm, and the resistance for the length at 60.0 cm was 2.90. I wouldn’t expect it to be completely accurate but the results from experiment 2 were more accurate than the results in experiment 1. The results are extremely close to the expected ones, I can assume there was a slight inaccuracy e.g. temperature or an inaccuracy of the reading off of the meter ruler. I can therefore conclude that my prediction was correct, and that length is directly proportional to resistance. If I doubled the length of the wire, in doing so I would double the resistance of the wire. I can prove my prediction was correct as now I can use the graph to find what the resistance of the wire would be at a specific length.
Example: Using the line of best fit I can see what the resistance of the constantan wire would be at 25.0 cm. See where at 25.0 cm it hits the line of best fit. It hits the line at a resistance of 1.20 Ohms. I will now work out if I doubled the length of wire, I would double the resistance of it.
25.0 x2 = 50.0 cm, so I will read off from the graph what the resistance of the 50.0 cm wire is. The resistance of the 50.0 cm wire is 2.40 Ohms. The resistance I predicted for 25.0 cm was 1.20 Ohms, if I double this I get 1.20x2= 2.4. This proves that length is directly proportional to resistance, and that resistance does double with the length.
This is because the metal ions within the wire are all constant vibration. As the electrons travel through the wire, they are constantly colliding with the metal ions (and sometimes with each other); this causes their progress to slow down. In a longer piece of wire, there are going to be more metal ions. With more metal ions in the way, the flow of electrons will slow down as there are more electrons being deflected. Resistance is the opposing of the flow of an electrical current, so this means that in a longer piece of wire there is going to be more resistance (the slowing down of the flow of electrons in the circuit). The more the flow of current is deflected as it goes along, the more resistance there is.
Metals conduct electricity because the ions in them do not hold on to their electrons very well, and so create free electrons, carrying a negative charge to jump along the ions in a wire. Resistance is caused when these electrons flowing towards the positive terminal have to ‘jump’ ions. So if you double the length of wire, the number of ions in the wire doubles, so the number of ‘jumps’ double.
Evaluation
I found that I got much more accurate results in experiment 2 than I did in the first experiment. They were more reliable to make my overall conclusion. The results did prove my prediction. Although they aren’t completely accurate, as I think there is always some inaccuracy or error in results. In the second experiment a current was put through the wire, and this causes the temperature to rise, which will cause ions in the wire to vibrate, and so obstruct the flow of electrons, effectively causes the resistance to increase creating an error. However, when I was doing the experiment when taking the readings I turned the power pack off, so the temperature wouldn’t get to high.
There are other limiting factors which could have affected my results which I have mentioned, like inaccurate meter ruler readings, temperature change or a unique error etc. But also when I used the variable resistor to keep the ammeter reading constant, it wasn’t always exactly the same as I couldn’t always get it to be the same reading every time.
If I could do this experiment again I would use other variables aswell like temperature. How temperature might have an effect is that it provides metal ions with more kinetic energy, causing them to vibrate more, thus causing more electrons to collide with them. This causes the resistance to increase.