I predict that if the length is doubled then the current will halve, because the resistance of the wire is directly proportional to the length of the wire. As the length of the wire is increased, the amount of energy lost through heat increases. The additional energy loss subtracts from the energy being transferred through the conductor, which results in a decrease in current flow for a given constant voltage.
Voltage across the wire accelerates the electrons; the electrons lose the kinetic energy in collision with metal ions. So they have to be accelerated again – and the process continues. When the wire’s length increases there is more chance of collisions with metal ions, because the electrons have further to travel and have to therefore pass by more metal ions. So the voltage will have to re-accelerate the electrons, which increases the resistance of the wire.
Apparatus:
100cm ruler, nichrome wire, variable voltage power supply, ammeter, voltmeter and various wires with crocodile clip and pointer connectors.
Firstly I will do a preliminary test, which will be to find a constant voltage for the main investigation for the main experiment. The voltage has to be large enough to obtain a good enough result at both 10cm and 100cm, so that I will be able to plot a graph easier when it comes to the main investigation. The voltage also has to be low enough to ensure that the wire does not overheat at neither 10cm nor 100cm.
The results of the preliminary test showed that the best voltage to use would be 0.8V, which gave a reading of 0.54A at 10cm and 0.05 at 100cm on the ammeter. I chose this voltage because when using the 10cm length, at 0.9V the wire started to overheat. When using 100cm length, the wire did not overheat until 1.7V, which proves my prediction’s point - that as the length increases the heat/energy loss through heat also increases. So it would take more energy for the wire to heat up, if a lot of the heat is lost.
For the main experiment I will set up the apparatus again as listed on the previous page and fasten the nichrome wire to the 100cm ruler with sellotape. Then I will record the temperature, before and after the whole experiment, because temperature is one of the factors that can affect the size of a current. I will connect the 2 wires/cables w/cc at a distance of 10cm apart (measure with ruler). Then I will switch on the power supply and carefully adjust the voltage to 0.8V. To ensure fair testing I will keep 0.8V as the constant voltage throughout the experiment, because voltage is a factor that affects the size of current in a wire. I will then record the reading from the ammeter. To ensure safety I will switch off the power supply first, before reconnecting the wires to the next length. I will repeat the above with 20cm, 30cm, 40cm, 50cm, 60cm, 70cm, 80cm, 90cm, 100cm. Each time I restart the experiment I will check that the reading on the voltmeter shows 0.8V. Afterwards I will repeat the whole experiment again, but unlike the first time, I will measure as the length is decreased (starting with 100cm moving down to 10cm). This is to ensure that the readings are accurate. If both readings are the same or very close then I will know that they are accurate. But if the second reading is not close, I will have to repeat the experiment again to find which of the two readings are accurate.
Results
Temp. start: 21°C Temp. at end:21.5°C
The table shows that the quality of results is very good, because the second reading is always the same as the 1st reading apart from the 10cm reading.
I kept all the following constant in the experiment: voltage, temperature (could not control), cross-sectional area and material.
From the experiment I have found out that, as in my prediction, as the length increases, the size of the current decreases. Because as the length of the wire increased, the amount of energy lost through heat increases. The additional energy loss subtracts from the energy being transferred through the conductor, which results in a decrease in current flow for a given constant voltage. And voltage across the wire accelerates the electrons; the electrons lose the kinetic energy in collision with metal ions. So they have to be accelerated again – and the process continues. When the wire’s length increases there is more chance of collisions with metal ions, because the electrons have further to travel and have to therefore pass by more metal ions. So the voltage will have to re-accelerate the electrons, which increases the resistance of the wire.
If I take two points on the x-axis of the graph – 20 and 40 (the length has doubled). I can then find that the average current for those two points are 0.29A and 0.14A. In my prediction, I predicted that if the length doubled the current would halve, and 0.14A is roughly half of 0.29A.
To obtain a straight line graph I used 1/Average current on the y-axis. This graph’s formula is y=0.1947x – 0.6002. The line of best fit in this graph shows that there are a few slight anomalies. But this could be due to resistance in the wires causing slight errors, or the slight change in temperature.
I am confident that my results are accurate, because both the first and second current readings are the same or 0.01A off, and all the points lay on or close to the line of best fit in the graphs. Therefore there is no need to repeat the experiment, and I believe that at greater lengths the size of the current will continue to decrease.
Although the conclusions above are correct for this experiment, there are factors which may limit its validity in other situations, such as equal cross-sectional area, uniformity of material and constant temperature.
The graph showing 1/Average current across Length of wire, shows that there are several anomalies (at 70cm, 80cm and 100cm). These anomalies could be due to:
- In the experiment I found that it was quite hard to keep the voltage reading at 0.8V because it kept on flickering between 0.8V and a closer reading. I also had a similar problem with the current because if the voltmeter’s digits were flickering then the ammeter’s digits were most probably flickering.
- Another thing I could not be sure of was the kinks in the wire – knowing the exact length of the nichrome wire was uncertain. In order for this not to happen, wire with a larger cross-sectional area could be used, so that it does not ‘kink’ or bend as easily.
- For some of the results the quality of the connections might have been bad, causing extra resistance at the connections. To prevent this I could experiment with different types of connections.
- The final factor that could have affected the results was the slight change in temperature. There was nothing I could have done about this, unless the experiment was carried out in a special room where temperature could be controlled and kept constant.
But overall the results are reliable and accurate, because all points are on the or very close to the line of best fit on both graphs and in the table of results, the 1st and 2nd readings are both same (apart from at 10cm length).
If I was to investigate how the cross-sectional area of a wire affects it’s current. Then I would use the following apparatus:
Wires with different thicknesses, ruler, voltmeter, variable power supply, various wires with either pointer or crocodile connectors, ammeter, voltmeter.
Firstly I will as in the previous investigation, carry out a preliminary investigation to find a constant voltage for the main investigation.
Then once this is found I will keep this constant throughout the experiment. The temperature will be recorded before and after the investigation. I will set up the apparatus, and firstly take a wire of 0.5mm thickness into the apparatus. After switching on the power supply, I will record the result. Then switch off the power supply before connecting the next wire, to ensure safety. I will repeat the same method for wires that have a cross-sectional area of 1mm, 1.5mm, 2mm, 2.5mm and 3mm. Then I will repeat the whole experiment again, but unlike the first time, I will measure as the cross-sectional area is decreased. If both readings are the same or very close then I will know that they are accurate. But if the second reading is not close, I will have to repeat the experiment again to find which of the two readings are accurate.
I don’t understand this at all from the way you’ve written it.
Delete this phrase it is cobfusing.