Find out how the thickness of a wire affects the resistance.

Title Electrical Resistance

The aim of this investigation is to find out how the thickness of a wire affects the resistance.

Background Knowledge

Structure of a metal:

A metal consists of a huge structure of atoms, which have released their electrons from their outer shell to form a ‘sea’ of electrons. Releasing the electrons causes the atoms to become positively charged ions. This leaves the metal atoms surrounded by a ‘cloud of electrons’. A metal item is like one big atom with many nucleii but only one electron cloud. Instead of the atoms having their own electrons, the outer electrons of each metal atom leave the individual atoms and create a sea of free electrons throughout the metal. This allows the outer electrons of each atom to move freely as they orbit among all the atoms as a whole. The sea of electrons is there merely to hold the ions together, however if a potential difference is applied to the metal, these electrons are free to move.

When these free electrons move around, an electric current is produced. In other words, an electric current in a metal is made up of electrons

supplied by metal atoms. A current is not ‘injected’ into the metal from outside. Instead electric fields caused by the power supply cause the wandering electrons to begin flowing. The power supply is not the source of the electrons. It is simply the electron pump. The electrical force form the power supply or battery keeps on trying to make the electrons flow along, but the collisions with other electrons keeps deflecting them, hence slowing down the overall flow.

So, the voltage is trying to push the current round, and the resistance is opposing this, so the size of the current will be determined by the size of the voltage and resistance. The balance is the higher the voltage, the bigger the current, and the more the resistance, the smaller the current.

Collisions between flowing electrons and other electrons are what make the metal go hot during electric current. Heat energy in metals are stored as wiggling electrons. So speeding up the electrons heats up the metal. This is because if you try to force the electrons to flow along, at first they will speed up. But then the will be deflected by the non f lowing electrons that are ‘orbiting’ the atoms that are not moving, and therefore the flowing electrons will slow down. This will cause the non f lowing electrons to speed up, but they do not move in the direction of the electric current. They actually move about in all directions with rio average drift. This shows that speeding up the electrons does not push them in the direction in which the current is travelling at, Instead, it makes the crowd of electrons wiggle faster, leading to the metal heating up, with no overall electric current.

That’s where Ohm’s Law comes from. Ohm’s Law says that electric current is directly proportional to the voltage difference, but there’s a simpler way to say it: the harder you push, the faster it flows.

Resistance is a measure of how hard it is to move the electrons through the wire. The resistance in a circuit is anything that slows down the flow of the current. The voltage (potential difference) is what pushes the current round, and the current is the flow of electrons round the circuit.

There is a link between all three; the voltage is the force which ...

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There is a link between all three; the voltage is the force which pushes the current around the circuit, the resistance is apposing this push, and the size of the current will be decided by the relative sizes of the voltage and resistance,

The factors that wilt affect the outcome of this investigation are:

U Length of wire

U Thickness of wire

U Temperature of wire

U Material of wire

U Potential bifference

U Volume of water

When experiments are done, in order to get clear and accurate conclusions, and to make sure it a fair test, only 1 variable should be changed each time, and the other variables should be controlled and kept the same. Changing 2 variables at once will give a very unclear conclusion, seeing as it will not be easy to figure out which variable caused the outcome to come as it has.

In my experiment, the factors that I plan to control are listed below with how I plan to control them:

Length of wire

Each time I do the experiment, I will cut the desired piece of wire of a certain SWG (Standard Width Gauge), using pliers, to 1.lm. In my experiments, the length of wire that the current will flow through is going to be im long. The reason that I will be cutting the wires to 1.lm is so I have an extra 10cm so I can place the crocodile clips Scm in from each side of the wire. I will coil up the wire in between the crocodile clips. This means the length of wire that the current will flow through is im seeing as the electrons will only flow between the crocodile clips.

The length of the wire affects the resistance. If the length of the wire is increased then the resistance will also increase, as the distance the electrons will have to travel will be longer. In other words there are more atoms in a longer wire for the electrons to pass through. This means more collisions will occur.

Collisions are when particles collide with one another. In this case, a longer wire means more atoms, and more atoms means more chance of the electrons colliding into them. Collisions slow down the rate at which the electrons are travelling in, and therefore cause a higher resistance. Due to this fact, the length increase should be proportional to the resistance increase.

If I changed the length each time I did the experiment as well as the other variable that I plan to change, then my results will come out pretty faulty, as it will be very difficult to work out which variable caused which outcome and so fourth. In order to make sure my results are not based on the length of the wire, I will keep the length the same throughout the whole experiment. This will ensure that the collisions will be moderately constant and the resistance will stay the same.

Temperature of wire

To keep the temperature of the wire the same each time I carry out the experiment, which is 24°C, I will place the wire in a tray of 2100m of cold water. I will be keeping the temperature at 24°C for the reason that the water out of the taps is usually of around that temperature. Because higher voltages gives out more heat, due to more current being pushed around, I will use small currents. The reason that I will be placing the wire in a tray of cold water is so that the wire does not heat up, and cause the experiment to be unfair. Using a thermometer, I will make sure the temperature is 24°C each time. If the temperature is too high, then I will leave it to cool, until it is the correct temperature. If the temperature is too low, then I will get rid of that water and this time I will get 2100m1 of warm water, and let it cool until it reaches the temperature I want, which is 24°C. I will then place the wire in the tray and start the experiment.

The temperature of the wire affects the resistance greatly. This is because if the wire is heated up, the atoms in the wire will start to vibrate due to their increase in energy. This makes them move into the path of the electrons, and causes more collision between the electrons and the atoms. This increase in collisions means that there will be an increase in resistance.

If I used different temperatures each time I did the experiment, the test would be unfair since some of the wires are hotter than others, andtherefore would cause more collision, resulting is a higher resistance. This is why I will keep the temperature of the water at 24°C each time.

Material of wire

The material of the wire that I will be using is constantan. Each time I do the experiment, I will make sure the wire is of the constantan brand to keep it a fair test. The reason why I wilt not be using nichrome wire is because it has a higher resistance than constantan, as we found out in our preliminary run.

The material must be kept the same throughout the experiment, as changing it will have a great affect on the resistance of the wire, and will lead us to obtaining faulty results. bifferent materials of wire have different resistances, different average diameters (even though they are of the same SWG) and different heating points. The type of material will affect the amount of free electrons that are able to flow through the wire, The number of electrons depends on the amount of electrons in the outer energy shell of the atoms, so if there are more or larger atoms then there will 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 in the number of electrons. Also if the atoms in the material are closely packed then the electrons will have more frequent collisions causing the resistance to increase.

All these have an effect on the resistance and therefore the material of the wire should be kept the same.

Volume of water

The volume of the writer that I will have in the tray for placing the wire in will be 2lOOml, The reason I will be using this amount is because its not too less, and its not too much. Using a 300ml beaker, I will pour in2lOOml of water, by filling the beaker up to 300m1 7 times and pouring it into the tray. I will do this for every SWG I test.

A slight difference in the volume of water throughout the experiment will not greatly affect the results, however a major difference in the volume of water will. I will make sure that there is always 2100m1 of water. So I will be filling the tray 6 times, as there are 6 SWG

The reason that I will need the water is because if the wire heats up while I doing the experiment, which is more likely to happen whilst doing the bigger currents, the test will become unfair. This is because temperature does affect resistance, as mentioned earlier. So the tray of cool water will help prevent the wire, which will be in the water, from heating up.

Having more water in one test than another will be unfair seeing as the more water you use, the quicker the wire can cool, and so the results will come out inaccurate.

The factor that I plan to change is the thickness of the wire.

Thickness of wire

The thickness of the wire does affect the resistance. The thicker the wire, the lower the resistance, seeing as there will be an increase in space which means more routes for the electrons to flow through, as well as less collision happening to slow down the flow of the electrons, so the electrons will travel faster, and therefore the current will be bigger.

I will be testing out 6 different thicknesses. The different thicknesses that I will be measuring in are 20 SWG, 26 SWG, 28 SWG, 32 SWG, 36 SWG and 40 SWG. Using the variable resistor and the ammeter, I will get the currents that I will be needing to obtain the potential difference, which I will be using to work out the resistance. The currents that I will be measuring are 0.1 amps, 0.2 amps, 0.3 amps, 0.4 amps, 0.5 amps, 0.6 amps, 0.7 amps, 0.8 amps, 0.9 amps and 0.1 amps. I will be using small currents to prevent the wire from heating up. This is why I chose these values. I will only change the thickness out of all the variables.

I will have to change all my thicknesses into cross sectional area of the wire in order to get a readable graph from which I can get a decent conclusion from. The formuki that I will be using to convert the thickness into the cross sectional area is: Rc

I think as you increase the thickness of the wire, the resistance will decrease. This is due to the increase in the space for the electrons to travel through. The more routes there are for the electrons to travel to, the lower the resistance.

This is because of the increase in the space for the electrons to travel through. bue to this increased space between the atoms there should be less collisions. Less collisions means less resistance. This means that the thicker the wire, the more space for the electrons to travel through, meaning less collision.

ADparatus

® Micrometer-screw-gauge

® Variable resistor

® Ammeter

® Voltmeter

Tray

® Beaker

® Pen

® Connecting wires

® Scissors

® Pliers

® Meter ruler

® Power pack

® Thermomet

® Goggles

® Crocodile clips

® Calculator

How to measure

I will use the micrometer accurately, n use the meter stick in the correct way, to make sure all my measurements are accurate. My results will be verified due to me doing the test on each thickness of wire 10 times, with different current each time. I can be sure whether my results are precise by working out the resistance from the currents and the voltage.

Analysis

I did a graph of my results of the cross sectional area of the wire against the resistance. This gave me a reciprical graph, which did not give me a clear conclusion. I needed to redraw the graph with 1 over the cross sectional area. This gave me a straight line graph, and from that I could make out a very apparent result,

Evaluation

From the graphs, I can see that my prediction was correct. As you increase the thickness of the wire, the resistance decreases.