The length and resistance Essay Example for Free

The length and resistance Essay The aim of this investigation is to find out how changing the length of a piece of wire will affect its resistance. Prediction I think that increasing the length of a wire will increase its resistance. This is because in a conductive metal, the electrons in the outer shell of each atom are free to move around. An electrical current is where all these electrons are caused to move in the same direction through the metal. Resistance is the property of a substance that restricts the flow of electricity through it, and is often associated with heat. As the electrons are passing through the metal, they are constantly colliding with the atoms of the metal, causing their course to be slowed down. The collisions cause changes of direction which dissipate energy as heat, which is why more resistant metals heat up more than metals which let electrons pass through more easily. It is easier for electrons to pass through metals in which the atoms are small and far apart, because the free electrons can pass through with less collision to slow their path. It is most important for the metal to contain a lot of free electrons. Fewer collisions mean that less energy is transferred to heat: this is low resistance. As the length of the wire is increased, there will be more fixed atoms for the free electrons to collide with, thus slowing their course. The length of the wire and the resistance of the wire will be directly proportional. If you double the length of the wire, the resistance will also double. This is because there will be double the amount of atoms in the wire for the electrons to collide with. The fact that it would take twice as long for the electrons to pass through in a metal twice the length is of almost irrelevant consequence because electrons move close to the speed of light, and so there is no point in taking this into consideration. If the resistance of the material is increasing, then it will need an increasingly large force to push it through: This is the voltage. The resistance (R) is how much voltage (V) is needed to drive a given current (I). R = V/I Based on my prediction, I would expect my graph to look like this: Resistance (? ) is also equal to the resistivity of the wire(?cm) multiplied by its length(cm), and then divided by its cross sectional area(cm2). Resistance (? ) = resistance of the metal(? cm) x length(cm)   cross sectional area (cm2) The cross sectional area of the wire is constant, and so is the material I am using. It is only the length that will be changing, so as you can see from the formula, the length and resistance must be directly proportional. Planning My experiment shall be set up as follows: I shall use the following apparatus:  connecting wires The power supply will be permanently set to 2volts, but it is important to keep the amperage below 1A so that the wire does not overheat. We will do this using the rheostat. The nichrome wire has a resistivity of 103 x 10-6, and a diameter of 0. 2285mm (0. 02285cm). We have chosen nichrome wire because its properties are suitable for this experiment. It is quite easy to keep the amperage low, but the experiment must still be started with the length of wire that is long enough so that the amps are not too high, as otherwise, this would result in an increase in temperature which is not related to increased resistance. We have chosen to calculate the resistance of the wire in intervals of 5 cm, starting at 5cm, and going up to 70cm. All our decisions are based on a variety of pre- tests. Fair testing The key variables in this experiment are:   temperature   diameter   type of wire length of wire In order for this experiment to be fair, it is important that these variables are carefully monitored. Temperature: It is important that any change in temperature is a result only of resistance. To do this, the room temperature must be kept the same, and the current running through the wire must be kept below 1amp. If it were to exceed this limit, it would result in an increase in the vibrations of the atoms in the metal. This would cause the free electrons to collide more often with the fixed atoms and would cause an apparent increase in resistance that is not due to the change in the length of the wire. In order to prevent this from happening, we have used a rheostat: using this, we can keep the current very low. If desired, we could keep it at a constant, but this is not necessary. Diameter: The diameter of the wire will be kept constant throughout the experiment. This is important because it affects the overall area of the wire. The resistance is inversely proportional to the cross-sectional area. The only way in which the area of the wire should be changed is in length. An increase change in diameter would affect the number of fixed atoms that the free electrons could collide with and would produce inaccurate results with regards to resistance. The smaller the diameter the better, as it will mean a smaller all-round area which will make it possible for more results to be taken, as the amperage would not have so great an effect on the wires temperature. Also, the larger the diameter, the easier it would be for the free electrons to flow through, just as water flows more easily through a wider pipe than a thinner one. Type of wire: All metals have different properties, and in order to make the test fair, the type of wire used must be kept the same. I have chosen to use nichrome as it has good properties which have been tested in the pre-test. It is also important to keep the wire straight, as this will make it easier to measure. Length of wire: The length of the wire is the variable that I shall be changing. I have chosen to do so in 5cm stages. It is very important that the wire is measured accurately, as otherwise the readings off the ammeter and voltmeter will not be as accurate as they could have been. To do this, we are using a ruler, but one problem that we are likely to encounter is the fact that is will be difficult to measure around the top of the rod on which the wire is fixed. So as not to get inaccurate readings, I am going to disregard the reading at 35cm, as that is the length that falls upon the end of the rod. Pre-test It is important to conduct a pre-test before doing the actually experiment. In this pre-test, I have tested 3 types of wire: nichrome, copper and Constantan, to which is most suitable for the experiment. I did the pre-tests on a computer program. Diameter I tested the three available metals using the computer program to find out how close to the maximum of 1 amp I could reach by adjusting the diameter of the wires. I want the amperage to be as close to 1 as possible because the amps will go down as I do the experiment, and I want to get a lot of results. The amps must not be above 1amp, because this would mean that the wire would get too hot, and the results would be inaccurate, as I have previously explained.