An Investigation into How well Wires Conduct Electricity

Scientific Background

Electricity is a form of energy; it is also very easy to use because it can flow along wires to wherever it is needed. It is a flow of negative electrons. We call this flow an electric current. The flow is measured in units called amperes or amps. 1 ampere is a flow of about six million, million, million electrons per second past each point. Voltmeters are used to measure the potential difference or “voltage” this is an electrical energy difference. Electricity can flow through a metal, such as copper, because the metal contains electrons that are free to move from one atom to another.

We will be able to measure how well a wire conducts electricity by measuring the resistance. Resistance is a measure of the opposition of a component to the flow of current through it i.e. how hard it is for current to flow. The current, however, tells you what is flowing when there is a given voltage, which is why it is better to use resistance. Also if the current flowing through the wire was constantly changing then it would be impossible to get an accurate idea on what the resistance was as there would be different amounts of electrons flowing through the wire and so resistance would change because there would be different numbers of electrons trying to get through the atoms and the true resistance could never be calculated.

The thin wire in a lamp tends to resist the movement of electrons in it. We say that the wire has a certain resistance to the current. The greater the resistance, the more voltage is needed to push a current through the wire. The resistance of a wire is calculated using this formula:

RESITANCE = POTENTIAL DIFFERENCE ÷ CURRENT

Factors affecting resistance:

Length

Length affects the resistance. Resistance is how hard it is overall for charge to flow. It depends on the resistivity of the wire, the length and the thickness. Resistance happens because the electrons are slowed down by collisions with atoms. If there is a long length of wire there will be more atoms to collide with so the electrons will be slowed down more. The longer the material, the more resistance there is. The longer the wire is, the more collisions an electron will have as it passes along the wire. This means it is harder for the current to flow i.e. the resistance is higher. For example in a corridor the longer it is the more people you have to avoid. As current flows through a wire some of it is lost internally as it passes through. It is dissipated; in fact it is changed to internal energy. The longer the wire, the more energy will be lost and therefore the greater the resistance.

Cross-sectional area

Thick wires work just the same as resistors in parallel - they let the electric current through more easily than thin wires.

The greater the area the lower the resistance. There is more space for the electrons to move and so there is less resistance. For example, in a corridor which is wider you have many more options of paths to take when there are a lot of people walking around than you would have in a narrower corridor with the same amount of people walking around.

Material the wire is made of

Copper is a good conductor and is used for connecting wires. Nichrome has more resistance and is used in the heating elements of electric fires. The material the wire is made from affects the resistance; this can be compared to walking through a corridor where there are sixth formers and year sevens. It would be much easier to walk through a corridor of year seven than it would be to walk through a corridor of sixth form; (because sixth form are much larger than year 7) this is like the electrons flowing through ...

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Material the wire is made of

Temperature

When electricity is passed along a wire, the wire will heat up a bit. As this happens some electrical energy is converted to heat energy and this is wasted. If the wire is shorter then there is less wire to get hot and so less energy will be wasted as heat. The energy is converted to heat because of the resistance of the wire. If you heat up the wire you increase the internal energy. The particles will be less static and so more resistant. Think of yourself trying to push through a crowd of people. If they are standing still they will be much easier to push past than if they are ‘heated up’ and more animated.

Type of wire

If the wire is of a different type each time we experiment then its atom structure will be different and so resistance will have already increased or decreased without even changing any of the other variables.

Which variable will be changed?

I will be investigating how the length of wire affects resistance because this is the easiest variable to change. I will keep the voltage fixed, alter the length of your piece of wire and take a reading of the current that flows through it. I will then calculate the resistance by dividing the voltage by the current to get a value for resistance in Ohms.

I will start by choosing a long length (100cm) and a short length (25cm) to show that one wire length has more resistance than the other. A problem I could encounter (especially with short lengths of wire) is that if the voltage was too high the wire would heat up much too quickly. If this happened then I would be introducing a second factor (temperature) and so the experiment wouldn’t be fair because when electricity is passed along a wire, the wire will heat up a bit. As this happens some electrical energy is converted to heat energy and this is wasted. If the wire is shorter then there is less wire to get hot and so less energy will be converted to heat energy.

The same wire must be used throughout the investigation because this could alter the results. The wire I will be using is: “constanton 32”

I will be measuring the resistance of 8 different lengths of wire (25cm, 35cm, 45cm, 55cm, 65cm, 75cm, 85cm, 95cm) this provides a wide range and so should give me reliable results. I will use 5 different voltages for each piece of wire and then work out the resistance and take the average resistance. This will mean that any mistakes made can be seen more clearly. The voltages I will be using are: 0.5v, 1.0v, 1.5v, 2.0v, 2.5v (or as close to these voltages as possible). I chose these voltages because they do not go too high, which might be dangerous, neither do they go too low. These are sensible values because you can see the difference easily.

e.g. 0.054A – rounded to 0.05A and 0.055 – rounded to 0.06A

There is only a very slight difference between these two readings whereas with 0.15 and 0.26 you can easily see the difference.

I will also complete a preliminary experiment to see how sensible the readings of currents and voltage are. This will allow me to see if my experiment will work. It will also allow me to try out different measurements so I know how short the lengths of wire can be without getting too hot.

My Prediction

I think that the length of a wire does affect its resistance. Resistance is caused by the internal structure of the metal getting in the way of moving electrons. The more difficult it is for the electrons to move the higher the resistance. Therefore the longer the wire is, the more atoms there will be to collide with so the electrons will be slowed down more, meaning that it is harder for the current to flow (i.e. the resistance is higher).

Method:

Set up the apparatus as show in the diagram above. Make sure that when the wire is in the circuit it does not coil round and cross over itself as this could decrease the length of wire that the current has to flow through and in doing so will make the results inaccurate.

Using your first selected length of wire (in this case 25cm), set the voltage to 0.5V (or as close as possible) then record the reading shown on the ammeter (this is the current) you then use the formula (resistance = potential difference ÷ current) to fill in the resistance column. It is important to record all the readings you take.

To enable you to repeat the experiment so you can get a sensible average, you should set the voltage to 1.0V (or as close as possible) and record the new reading on the ammeter and again work out the resistance. These two results for resistance should be about the same. However even if this is the case it is always best to get at least three results for each length so you can be sure that you are getting sensible readings.

After you have collected a minimum of three results for the first length of wire, move the crocodile clips along so that the length of the wire is now 35cm. Use the same voltages as before and record the current readings. Then you will be able to compare the two average resistances.

Repeat this process until you have an average resistance result for each of the 8 different lengths.

Results of preliminary experiment:

To back up my evidence I carried out a preliminary test which resulted in gaining another set of results. These results agree with what I have said in my prediction; that as the length of a wire increases so too does its resistance; so I feel that my prediction will be correct.

Results of actual experiment

Average Resistance

The experiment was carried out safely. It was important that the wire did not become too hot because this would mean another factor was being introduced, therefore stopping it being a fair test. It was also important to use the equipment safely so as to obtain accurate results. For each different length, I measured the resistance 5 times as I felt this would give me a more reliable average. My data also has quite a large range, going from 25cm to 95cm. This makes the evidence much more reliable because it shows that there is a general relationship between the length of the wire and its resistance. We carried out the amount of tests and got the amount of results we were planning to get and we carried them out in a systematic and sensible way. We took the results using the same equipment and in the same period of time so all experiments were subjected to the same factors.

All the results have been recorded in a clear, neat table that shows all measured factors clearly and is easy to read and extract information from. We used all our equipment as accurately as we could, making sure we did each experiment in the same conditions as the previous ones. This minimalised the risk of there being any major differences between experiments and so gave us a much more accurate results. We kept to our original guidelines and we made sure all the other factors remained constant.

My graph clearly shows that as the length of the wire increases, so does its resistance. This proves that my prediction is correct. It also provides sufficient evidence to back up my statement that the length of a wire affects its resistance. This is because the longer the wire is, the more atoms there will be to collide with, so the electrons will be slowed don more, meaning that it is harder for the current to flow (the resistance is high).

The best fit line shown in red on the graph above goes through the origin (0,0). This shows that my results are good because at (0,0) – when there is no voltage, there will be no resistance – however if my best fit line did not go through the origin it would show that another factor was having an effect on my results (for example, using very short lengths of wire means that it heats up, thus bring temperature into my experiment). Slight “blips” (for example the circled result) in the way the results are shown on the graph indicate that other factors are having an effect on the resistance.

Therefore I conclude that when you increase the length of a piece of wire you also increase that wires resistance. I say this because as we know the piece of wire itself consists of billions and billions of atoms which are all slowly vibrating, as they would in any solid object. The electrons that are flowing around the circuit have to pass through all these atoms and so the greater the distance they travel the more atoms they will encounter, and this will mean they will encounter greater resistance when trying to pass through this part of the circuit. To further back this up you can see on the graph itself that when we increased the length of wire we see a greater resistance. This is shown by the line of best fit going up (showing that the resistance is increasing, not decreasing, if it was decreasing the line would be heading down). There results agree with my prediction, in which I stated that for the resistance to increase the length must also increase, and they also correspond with the results from my preliminary tests.

Evaluation

Whilst carrying out my experiment I encountered no abnormal results, all complying with one another and with my prediction. I think that if I were to carry out my experiment again I wouldn’t change the way I actually carried out the experiment because I found that the way I did it anyway was quite successful, instead I would just try to make my experiment even more accurate. I would do this by monitoring all the factors much more closely. I would use instruments/equipment that would allow me to make sure that every other factor did stay absolutely constant, such as a temperature sensor to record even the slightest change in the wire temperature and small measuring device to make sure that the wire is the same thickness all the way through, so as not to make the test unfair by having the wire different thickness, which could make it easier for the electrons to pass through if it was thicker and so not give an accurate resistance result. I think that even though we didn’t have any equipment to notice slight changes in wire temperature, thickness, etc. I think that overall our experiments were accurate enough to make an certain conclusion, as all our experiments gave very similar results as you can see in the results table and there weren’t any glaringly obvious errors or differences between them. I think that with the equipment we had we managed to carry out a very accurate experiment and produce an accurate conclusion and evaluation. We could do more work to take the investigation further, such as increasing the length of wire we use or shortening the intervals between recording, such as taking results every 5cms instead of every 10cms. We could also use several ammeters to see if any of the current is lost as it passes through the wire or several voltmeters to see if the potential difference changes at different points on the wire. This, as well as adding “extra-sensitive equipment” would all add to making the results more accurate and we would be able to give a definite, firm conclusion due to the sheer volume of results we would have and how accurate all these would be.

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