Resistance can also vary depending on the material the wire is made out of because the diameter of the wire could be different or there could be more or less atoms inside the wire. A wire made of Nichrome has more resistance than a copper wire of the same size. That’s why, in circuits the connecting wires are usually made of copper because it has a low resistance.
Previously, an experiment was carried out to show the relationship between voltage and current for a bulb. It showed that the bulb was non-ohmic and did not obey Ohm’s law. It explained that as more current flows, the metal filament in the bulb got hotter and so its resistance increased because its temperature rose. As the graph below shows:
A seconded practical was undertaken to determine the relationship of current and voltage with a wire, and the findings were quite different:
This experiment showed that the wire used had a certain relationship resistance to the current, because of this, the wire is said to be a ohmic conductor. Plus, the greater the resistance the more voltage is needed to push the current through. This is known as ‘Ohm’s Law’:
“The current flowing through a metal wire is proportional to the potential difference (voltage) across” (V=IxR).
By these two experiments, it will be easy to tell by the curve of the results graph whether the Nichrome is affected by temperature when a current is passed through it.
Each of the above experiments was carried out by using a variable resister. Resistors can be used to reduce the current in a circuit. They are also very useful for keeping the current the same when altering something else in a circuit (e.g. length of a wire).
Prediction
My prediction for this practical is:
‘As the length of the Nichrome 32SWG increases, the resistance will also increase’.
But, this prediction will only be true if the current flowing through the Nichrome is kept at a constant level (via the variable Resister). However, two experiments will be undertaken (see ‘Method’, page …):
- experiment one will be carried out at room temperature,
- and experiment two will be carried out when the Nichrome wire is at a constant temperature.
Bearing this in mind, the findings from the two experiments should be quite different. Below are two graphs’s, taken from ‘Physics for you’ – page259 to help explain this:
The graph to the left shows the resistance of a filament lamp. From this graph, it is clear that as more current flows through the filament, it gets hotter and so its resistance increases. This should be the case with experiment one because the Nichrome should get hotter when it is not kept at a constant temperature.
The second graph (left) shows the resistance of an ohmic conductor (e.g. copper). Because does not heat up when a current is passed through it, a straight line is produced, meaning that the current is proportional to the voltage. In experiment two the results should look something like this because the Nichrome wire will be kept at a constant temperature during the time when a current flows through it.
Method
Apparatus
The equipment that will be used for both experiments is listed below:
- Becker
- Voltmeter
- Power unit
- Variable resister
- Ammeter
- 100cm of Nichrome 31SWG
- 2 crocodile clips
- 6 wires
- 200ml of water
- thermometer
- 100cm ruler
- Voltmeter and ammeter stunts
For the experiments to be carried out correctly, the equipment listed above will have to be set out as below:
Experiment One
-
The Nichrome wire will be measured starting at 10cm and connected to the circuit via the crocodile chips,
- The power unit will be switched on,
- The variable resister will be set at the correct current by the reading on the ammeter,
- The voltage travelling across the wire will then be recorded, and the power unit will be switched off.
The same process will then be repeated for 20cm – 100cm
Experiment Two
- The Nichrome wire will be measured at 10cm and connected to the circuit via the crocodile clips, it will then be placed in the beaker of water,
- The power unit will be switched on,
- The variable resister will be set to the correct current by the reading on the ammeter,
- The voltage travelling across the wire will then be recorded and the power unit switched off.
In order for the experiments to be carried out safely the following safety precautions must be adhered to:
- The power unit must be turned off before touching any of the other pieces of equipment, and,
- Eye protection must be worn at all times.
Ten results will be taken with the wire increasing in length by 10cm each time; this will be repeated three times for best results and in the case of a result being anomalous.
Fair Test
To make sure the two experiments are fairly undertaken, the following must be done:
-
When doing experiment one the room temperature must not be changed dramatically – because temperature effects the resistance of the wire,
-
The current must be kept the same – so that each length of wire has the same amount of amps passing through it because The same amount of amps will also be used for experiment two,
-
The power unit must be kept at the voltage (2 volts) – so that the current can be kept the same,
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The amount of water in the beaker must be kept the same for experiment two – so that each length of wire is put in the same amount of water,
-
The temperature of the water must be kept constant (21 °C) – so that each length of wire is tested at a constant temperature.
-
The Nichrome 32SWG must not touch itself during the experiments – because it will affect the resistance of the wire.
Results
Once the planning was completed the investigations was carried out. Here are the results for experiment one:
The resistance of each length of wire was calculated using the formula on page 4:
Resistance = voltage ÷ current
But before that calculation could take place, the average voltage had to be found out by all three results added together and divided by 3. By calculating the average voltage, two anomalous appear (shaded GREY). These results may also appear anomalous in the graphs.
Now the results for experiment two:
Analysis
From the results tables a number of clear patterns can be seen, but they are more apparent by studying the graphs over the page.
By looking at the graph showing the resistance both at room temperature and at a constant temperature we can see a clear difference between the two investigations. Firstly, the readings that were taken at a constant temperature create a much straighter line of best fit compared to the line for the readings collected at room temperature. This is because the results from experiment two are proportional to resistance and the length of wire because they were taken at a constant temperature rather than at room temperature. This shows that when Nichrome 32SWG is at a constant temperature is an ohmic conductor when kept at 21°C. In comparison to this the Nichrome gets hotter when at room temperature. This can be seen by the way the resistance increases and then gets flatter as the length of the wire is enlarged.
In conclusion, the results gained from the two investigations prove that the temperature of a wire affects the wires resistance. This is shown by the difference in *gradient between both result graphs:
*Gradient is calculated by dividing the resistance by the length of the wire over a certain range.
Experiment one has a line gradient of 0.17 Ω/cm which shows that the line of best fit in experiment one has a greater slope than the line of best fit in experiment two which has a line gradient of 0.47 Ω/cm.
The prediction on page 7 supports the conclusion very well. This is clear by comparing the example graphs of the filament lamp and the ohmic conductor and the three result graphs. A curve is visible in both the filament lamp graph and the graph for experiment one proving that the Nichrome wire increases in heat as well as a filament in a lamp. Also, a proportional straight line is present in the ohmic conductor graph and the result graph for experiment showing that the temperature is kept constant during a controlled temperature.
Evaluation
The method that was used when the experiment was carried out was good because it was not that difficult to obtain precise results. But more accurate results could have been recorded because:
- The coil that the wire had to be made into for experiment two may of been uneven and the surface area may of changed,
- The wire may have been touching itself during the experiment,
- The temperature may of changed dramatically,
- The water level for experiment two may have changed,
- The current may have varied during the investigations.
If the experiments were to be repeated the above details would be improved by:
- The Nichrome wire could be coiled in a more accurate way so the surface area was the same and the coil was even. This could be done by coiling the wire tightly round a even tube and then pressed to make the coil the same for each reading,
- The wire could be supported by clips to stop it touching itself,
- A temperature reading could be taken regularly and noted to see if temperature was approximately the same. If the temperature was not the same, you could postpone the experiment until it was,
- Measure to water level more precisely using a burette,
- A more sensitive resistor could be used to keep the current the same (e.g. rotary resister).
These more precise methods of carrying out the investigations would overcome the main problems to give a better table of results. Repeating the results for a 4th time could also help to improve the reliability and also dispose anomalous results.
There are also some other experiments and techniques that could be used to improve and extend the assignment:
- The results could be taken over a wider range by increasing the length of wire,
- A number of different experiments could be carried out were the other three factors that affect the resistance of wire were studied. Cross-sectional area, different materials, diameter.
- Also, to extend the coursework, instead of changing the length of the Nichrome wire the current could be changed.
References:
- EDEXCEL revision guide
- Class notes
- Coursework hand-outs