Ohm's law states that a current flowing through a metal conductor is directly proportional to the voltage across its ends (provided all other conditions are constant). So I know that if we add a variable - in this case length - resistance will change. I expect that the longer the wire, the higher the resistance.
Potential difference is what "pushes" electrons around a circuit. When a wire has more electrons, for the same voltage it produces less current, meaning that there is more resistance. For example, say 6V are being put through a wire, and it produces a current of 3A, we can work out the resistance of this wire by using the formula V/I. 6 ?= 2Ω. Now, if we put the same voltage through the same wire, only shorter, it might produce 2A of current. 6 ?= 3Ω. So now the resistance would be more, because the current is less. Fewer electrons would be able to flow through the wire, and electron flow is the same as current.
Prediction
I predict that the longer the wire, the higher the resistance. This is because in a longer wire, there are more wire particles that electrons can bump into. Resistance is caused by electrons colliding with wire particles. If there are more wire particles, the chance of an electron colliding with a wire particle will be higher.
In a longer circuit, it is more of a struggle for electrons to get around the circuit without any collisions. There are many more wire particles (acting like obstacles) to avoid. Electrons cannot increase or decrease speed, but they can collide. They collide with the particles in the wire, therefore less electrons are able to flow than in a shorter length wire. As a result, the ammeter shows a lower current with the same voltage. In a short circuit are less particles of wire. This means less collisions and a lower resistance. I have illustrated this below.
I predict that with a wire length of 0.1m, the resistance will be lowest. I say this because in such a short wire there are not so many particles for passing electrons to collide with. You could compare it to a high street that you are walking down. In the time that you walk down a longer high street you would encounter more obstacles (in the form of people in this case) than you would in a shorter high street (this is assuming the streets are as busy as each other). It is the same with electrons. If they must travel further round the circuit, they will have to avoid a lot more wire particles. They will obviously collide more, producing less electron flow (otherwise known as current).
I predict that at 0.2m the resistance will be higher than that of 0.1m, because of reasons previously stated. There will be twice the amount of wire particles (as 0.2 is twice 0.1), which should technically mean that there would be twice the amount of collisions. Although we cannot measure the amount of collisions, we can measure the resistance and I would expect this to be higher than that of 0.1m.
I predict that the resistance will increase steadily as the length becomes greater (i.e. 0.3m's resistance will be more than in 0.2m, increasing in 10cm increments up to 1m, which I predict will have the highest resistance of all.
Safety
Whilst doing the investigation, it is important to keep safety into consideration. The scissors should only be used for cutting the wire to the appropriate length and for no other reason. Before using the power pack, the pointer should point at 0 volts. It is important to be careful while using the power supply. While handling live wires, it is essential to be careful. The voltage should be kept low because of the safety factor and the wires heating up.
Reliability
To make the experiment reliable, all apparatus must be checked to see if it is functioning properly and is giving a true reading. This will partly avoid systematic error. Another way to make the experiment reliable is to use two methods: to do the investigation in two different ways to measure the resistance when the diameters are changed. If one method contains systematic error or is very inaccurate, the other method will be used to recognise that.
Precision
I will take as many different results as possible so that there a wide range of results and that I am able to arrive at a good conclusion. To increase the accuracy of the experiment I will do repeats for all the experiments so when the mean is taken, an accurate table is drawn up and if one result is anonymous the other two results would contrast the anonymous result.
Fair Test
The investigation is to investigate the resistance when the diameter has changed. In order for the investigation to be a fair test, all other factors or variables should be kept constant. The investigation has to be done in a fair manor otherwise the results would be inaccurate and the conclusion would be incorrect. The variables, which must be kept constant, are:
1) Temperature When the temperature of a metal increases the resistance of that metal increases. This is because when the temperature increases the atoms of the metal vibrate more vigoursly because of the increase in energy. This means that the electrons have more difficulty getting through the wire as they collide with the atoms which are in their pathway. This increases the amount of collisions therefore there is more resistance. However it is hard to keep the temperature exactly the same as the room temperature might change from day to day. It is essential to use a low voltage because it means a low current that will not heat up the wires. If a high voltage is used the energy would be in form of heat which would make the experiment unfair. The investigation will be done at room temperature. The temperature cannot be investigated because it is hard to control the range of temperature needed without the correct apparatus.
2) Length of wire The larger the length of the wire, the larger the resistance. This is because there are more atoms from the metal so there is more chance that the electrons would collide with one of the atoms therefore there is more resistance. It is important to keep the length of the wire the same each time otherwise it could not be certain which variable is changing the resistance. The length of wire will be 50cm throughout the investigation. Electrons have a longer distance to travel so there are more collisions .The length of the wire will make a difference to the resistance. This is because when you have a long wire, the electrons have to squeeze together for longer to be able to pass through the wire than they do in order to be able to pass through a short wire. I predict that the longer the wire, the greater the resistance. If I had a 30 cm wire and a 60 cm wire, the 60 cm wire would have a resistance twice that of the 30 cm wire.
3) Type of material Different materials have different resistances because the materials' atomic structures are different so some metals have low resistances and some have high resistances. Therefore it is important to keep the material the same throughout the experiment unless a different material is used to check if the conclusion or theory works for all materials. If different materials are used throughout the investigation, it will affect the results. For example if sometimes copper is used and sometimes nichrome is used, the results where copper is used will be of a low resistance because of the material and not because of the diameter of the wire. Throughout the experiment Constantan and nichrome will be used. Constantan is used to make sure my conclusion works for all materials. The type of material will affect the amount of free electrons that are able to flow through the wire. The number of free electrons depends on the amount of electrons in the outer shell of the atoms, so if there are more or larger atoms then there must be more electrons available. If the material has a high number of atoms there will be high number of electrons causing a lower resistance because of the increase of the number of electrons. If the particles in the material are tightly packed together, the electrons will have more collisions and therefore more resistance.
All these factors must be kept constant to make the investigation fair. The same apparatus must be used throughout the investigation. It is also important to take three repeats and find the mean so if one result is very inaccurate, the others will average it out.
The Prelimernary Method
We did a prelimernary experiment so see how other factors affected the resistance of a wire, and also to see if we would have to alter the method for the final experiment. We had to find out which wire had the most resistance, eaisly done in this experiment.
Apparatus:
- Meter ruler: To measure the wire being tested to ensure a fair test.
- Selection of wires Different materials and widths but the same length.
- Crocodile clips: To connect the wire being investigated to the rest of the circuit.
- Voltmeter & Ammeter : To measure the voltage and current needed to calculate resistance
- Wires: To connect the above items and to complete the circuit.
Diagram of Circuit
Method
First of all, set all apparatus as shown in the diagram above. Measure the length of wire so we get a fair result- it seems logical to start at 10cm for this experiment. Attach crocodile clips to the wires and turn on the voltmeter and the ammeter and record the readings. Repeat twice at 10cm and twice at 90cm for each of the three wires and then work out the resistance using Ohms Law.
Results
Conclusion from Preliminary Experiment
Copper will not be used because the results are very random and because copper has a low resistance. This means copper is liable to be inaccurate and very imprecise. The smaller the resistance the larger the percentage error which can occur and since copper has a very low resistance, it will have a very large percentage error making the results inaccurate. The other two materials show a greater consistency and show there are patterns and trends. They also have high resistances, which not only are easier to work with but also have a small percentage error and it is easy to observe changes. This is because the other metals have a high resistance. Other metals could not be used because it is not widely available. I will continue to use a 100 cm wire, as it gives me more reliable results than other lengths of wire – it has higher resistances and so it is easier to calculate the effect upon it, and using any other length wire would mean it would be harder to use effectively. We will continue to use the same method, as it did work quite effectively, although the wire we are going to use is the Constantine 28 as it has more resistance differences which means it will be easier to calculate during the real experiment.
