Fair Test
To ensure that the experiment is a fair test I have to keep all factors constant except the factor that I am investigating i.e. Length. From preliminary work I have found out that temperature has an affect on resistance therefore to reduce this affect I have to control the temperature. However there are no means of controlling it so I will have to reduce the amount of time the current is switched on thus reducing the temperature rise. I will have to take the readings as soon as possible and then switch off the current immediately.
The power will always be kept the same as will the area of cross section of the wire. I will always use the same material of wire (nichrome) and only vary the length.
Prediction
I predict that as the length increases the resistance will also increase. It will increase proportionally i.e. if I double the length the resistance will also double. L ∝R
As the length increases there are more atoms which get in the way of the electron collisions. Heat will also increase the resistance because the heat makes the atoms vibrate more in random rapid motion so therefore the electrons will find it harder to move due to greater collisions.
Below is a graph of what I think/predict will happen.
Apparatus
Wire-nichrome
- Power supply
- Ammeter
- Voltmeter
- Rheostat
- Ruler
- Crocodile clips
Safety
This is not a very dangerous experiment but one must always handle electricity with care and dry hands. To ensure the experiments are carried out safely I will always check the circuit is safe, the power supply is set at a safe and appropriate voltage that will be kept constant. I will also obey all other laboratory rules.
Method
1.Set up the circuit as shown in the diagram.
2.From the positive side of the power pack connect a wire which is also connected in series to an ammeter. Also in series, connect the wire with crocodile clips. On the other end of the wire connect a rheostat in series which then connects to the negative side of the power supply. In parralel over the wire connect a voltmeter.
3. Measure the wire so it is 10cm long.
4. Decide on 3 different positions on the rheostat making sure that for every length the position is as accurate to the first ones as possible.
5. Switch on the voltage at 6V ensuring that every experiment is conducted at this voltage.
6. Quickly and accurately record the readings on the ammeter and voltmeter.
7. Repeat at the different positions on the rheostat and record the results in a table.
8. Repeat the experiment exactly the same but increase the length of wire by 10cm each time until a length of 60cm is reached.
9. In order to calculate the resistance you have to use ohms law = V=IR therefore by making resistance (R) the subject of the formula you get R=V
I
Accuracy
To keep this experiment as accurate as possible we need to make sure that the length of wire is measured precisely, from the same point every time i.e. at the end of the ruler. We need to ensure that the wire is straight when we measure and when the experiment is carried out. If it is not, short circuits may occur and bends in the wire may also effect the resistance. The reading from the voltmeter and the ammeter must be taken as quickly as possible. As soon as the power is switched on the wire will get hotter and I want to test the wire when the heat is effecting it the least, which is obviously at the beginning.
After carrying out the experiment I was able to obtain the following results :-
In order to gain the most accurate readings I took 3 readings and then calculated the average resistance for each length of wire. Using the average resistance I plotted a graph of length of wire against the average resistance values. From analysing the graph the main obvious trend is that as the length of wire increases the resistance also increases. The resistance is directly proportional to the length of wire this is shown in the graph by the straight line. When you double the length of wire you double the resistance for example when the wire was 10cm long the average resistance was 2.15(Ω) and when the wire was 20cm the average resistance was 4.2(Ω) . This is only 0.1 smaller than what it would have been if you doubled the resistance. The straight line also indicates that the increase of resistance is constant.
With electricity the property that transforms electrical energy into heat energy, in opposing electrical current, is resistance. A property of the atoms of all conductors is that they have free electrons in their outer shell. All metals are conductors and therefore have similar structures. As a result of the structure of all conductive atoms, the outer electrons are able to move about freely even in a solid. When there is a potential difference across a conductive material all of the free electrons arrange themselves in lines moving in the same direction. This is what forms an electrical current. Resistance occurs when the charged particles that make up the current collide with other fixed particles in the material. As the resistance of a material increases so to must the force required to drive the same amount of current. Therefore resistance (Ω) is equal to the electromotive force/potential difference (V) divided by the current (I) this is Ohm’s Law:-
R=V
I
As the length of the wire is increased the number of collisions of the current carrying charged particles make with fixed particles also increases and therefore the value for the resistance of the wire becomes higher. This is all shown and proved in my results.
Evaluation
I think that from my results I can confidently say that my prediction was correct. The resistance did change in proportion to the length. This is because as the length of the wire increased the electrons that made up the current, had to travel through more of the fixed particles in the wire causing more collisions and therefore a higher resistance. My predicted graph also looks extremely similar to the real graph with the correctly plotted points. The graph shows a straight line with no anomalous results. However most errors in my experiment were encountered when measuring the wire. This is because it was hard to measure the wire because it was not always straight this would effect the resistance of it. The crocodile clips were not always fixed securely to the wire with a good connection. This also meant that they were easy to move about on the wire therefore changing the length of it.
Errors did not occur when setting the current it was kept constant at 6V and recording the result readings on the ammeter and voltmeter was fairly simple and therefore accurate. I feel that taking more results would improve the accuracy because it would enable us to have more values to calculate a resistance. Errors could have been encountered when measuring the wire as it was not always pulled straight, the wire was not always kept at the same beginning point whilst measuring so each measurement could have been a few centimetres out.
It is also possible to extend the experiment of the factors affecting the resistance of a wire. We could use different materials of the wire to see how they behave because in this particular experiment I always used nichrome. One could also investigate different thickness’ of the wire i.e. increasing the cross sectional area as in the experiment already performed we used the same wire therefore the same cross sectional area. I would predict that an increase in the cross sectional area would also lead to an increase in resistance, due to the same reason length has on resistance i.e. the electrons that make up the current would have to travel through more fixed particles in the cross sectional area of the wire therefore causing more collisions and thus a higher resistance.
