This is the formula that expresses Ohms law
V / I = R
Potential difference / Current = Resistance
The formula can also be rearranged so that the current that is fed in to the circuit can be found.
I = V / R
Current = Potential difference / Resistance
If you rearrange it once again the voltage or the P.D can be found with this formula
V = I x R
Potential difference = Current x Resistance
There are a few factors that can affect the resistance across a wire, they are the length of the wire, temperature The cross Section Area and the type of material the wire is made of
Length of the Wire
The resistance of a metal becomes higher when the length of the metal increases. This happens because there would be more atoms in the wire and the chances of the electrons hitting the atoms would increase. (See below)
Key
Electron
Atom
A wire that is 10cm.
A wire that is twice as long, there is more resistance in this wire because it is longer and contains more atoms.
Temperature
As the temperature of the metal increases, the resistance increases as well. This happens because all atoms vibrate even when the atom is in freezing temperatures. An atom will stop vibrating at -273°C. So when the temperature increases all that extra energy makes the atoms vibrate more than usual. As the atoms vibrate the electrons find it even harder to go past the atoms. That is why the resistance increases.
Cross Section Area
If the wire is thick and the cross section area is large there will be less resistance and if the wire is thin and the cross section area is small then there will be more resistance, this is because, the electrons will find it harder to move through the wire if it is thin. All the atoms will be will be close together, which will mean the chances of the electrons hitting atoms will be more.
Type of Material
Different types of metals have different resistance because of the structure of that metal. If a metal is densely packed with atoms then the resistance will be high but if it isn’t that packed then the resistance will be low.
I could have chosen to do anyone of the options above as my experiment, and I have chosen to investigate how the length of the wire affects the resistance because I feel it will give me accurate results and also this is the easiest factor to control. Keeping the temperature constant would have been very hard.
Hypothesis
My hypothesis is that as the wire becomes longer, there will be more resistance. As the length becomes longer there will be more atoms in the wire so the electrons will find it harder to move around.
Predictions
I predict that when the wire becomes twice as long as it started, the resistance will double.
Preliminary Investigation
The reason that I am going to do a preliminary investigation is that I will know what I have to do in the main investigation without making any mistakes. This is where all the mistakes happen. If a mistake happens I can learn from it.
Method for Preliminary Investigation
- Set up the apparatus
- Work out a suitable length of wire to start of experiment.
- Set up power pack (D.C)
- Workout what voltage will be suitable for the experiment.
- To keep these factors constant during the experiment.
- Temperature
- Cross area Section
- Type of wire
- Potential Difference
After doing the preliminary investigation I have collected the correct information on how to make this experiment as safe as possible and how to get the most accurate results.
These are the following-
- The correct equipment that I will have to use.
- The fact that a variable resistor will be needed or the wire will burn.
- The correct amount of voltage I will need to use.
- The Length of wire I will need to start with.
- How big the interval should be on the wire.
- How many times the experiment will have to be done to get more accurate readings.
The Apparatus
This is the list of the apparatus that I will use in the experiment-
- Variable Resistor
- Power Pack
- Voltmeter
- Ammeter
- Nichrome wire
- Clips (4)
- Normal wire
- Ruler
I have used a voltmeter so I can record down the voltage and an ammeter to record down the current. With this knowledge I can use the formula that I showed at the beginning to work out the resistance. I have to use a variable resistor because if I don’t the wire heats up and burns.
This is a diagram of the experiment.
Method
- Set up the apparatus as above, according to the diagram.
- Set the power pack to D.C
- Change the length of wire at an interval of 10 cm.
- Do the experiment twice and work out the average, this will make the results more accurate.
This is a table of the results.
Analysis
From the readings that I took I constructed a graph. From the graph you can see that the current is directly proportional to the P.D ( see Graph 1). You can see that the current is directly proportional to the P.D by observing the graph that I have drawn. In the graph you can see that if I were to draw a line through all the points on the graph then I would end up with a straight line. The straight line means that the current is proportional to the P.D (See diagram below). I also made another graph, this one compares the length of the wire and the resistance (See graph 2). As you can see on the graph the resistance increases as the wire becomes longer, the reason the resistance increases is that as the wire becomes longer there are more atoms for the electrons to collide with. This causes resistance. As you can see my prediction and hypothesis were right.
Evaluation
The results were quite accurate because I had done the experiment twice and got an average from the two results.
There are quite a few factors that could affect the results of the experiment. Some of these are variables that were mentioned earlier and could not be controlled, or they were variables that were not initially considered.
While performing the experiment, the temperature could have changed and that would have made the results really useless. Even a change of 1°C could effect the results, I am not entirely sure if that happened or not but the results seem pretty fine.
Also a combination of human error and faulty equipment could render the experiment a failure. Sometimes people rush things and takes down a wrong reading and also faulty equipment can sometimes give wrong readings.
The method of the experiment could probably also be improved to obtain more reliable results.
Due to the nature and convenience of the experiment, it could be easily modified to investigate another factor. I investigated the length when I could have investigated the cross width or the temperature, by investigating them I could have got better results but controlling the temperature would have been really difficult, putting in all that effort might not have been worth it.
