Factors Affecting the Resistance of a Wire.

the equation ??=?RA/L Where: R:- Is the resistance of the wire in ?öohmsô and can be determined using the equation R=V/I where öVô is voltage in volts and öIô is current in amperes. L:- Is the length of the nichrome wire used in metres. A:- Is the cross-sectional area of the wire in metres square and can be determined using the equation A= ??d2 where ödô is the diameter of the wire. If I plot a graph of length on the x-axis against resistance on the y-axis. From the relation R = ? L /A which corresponds to the st. line equation ? y=mx the graph should be a st. line passing through the origin where ömô is the gradient of the st. line graph and corresponds to ?/A. Since the cross-sectional area of the wire can be found by knowing itÆs diameter. Therefore the resistivity of nichrome can be calculated. ? Diagram of the circuit used in this experiment List of the apparatus used: - 1- Power pack supply of 4V 2-A variable resistor 3-A full scale deflection ammeter with a measuring range of 0-1 A 4-A digital voltmeter with a measuring range of 0-5 V 5-P, Q represents terminal blocks. 6-Circuit wires 7-PQ=Nichrome wire 8-A meter ruler 9-Michrometer screw-gauge 10-Sellotape Detailed method: - I set up the circuit as shown in the diagram that I have drawn. I started the experiment by taping a meter ruler between the terminal blocks P, Q so that I could measure 100cm of nichrome wire.I made sure that the wire was carefully tightened at both terminals to try to minimise the kinks or twists in the wire. I then switched on the power pack supply and adjusted the variable resistor until a constant current of 0.2A was flowing through the circuit. I then recorded the corresponding voltage reading that was displayed on the digital voltmeter. I repeated this procedure using different lengths ranging from 30-100cm and adjusting the variable resistor until 0.2A was flowing through the circuit. After recording the corresponding voltage readings for each length and tabulating them I decided to repeat the whole experiment again another 2 times so that I could take the average voltmeter reading for each length. Using a micrometer screw-gauge I measured the diameter of the wire at 3 different positions along the wire and then calculated its average diameter from the 3 values. I then plotted a graph of the length of the wire against average resistance and used it to calculate the resistivity of nichrome as mentioned in my plan where the average resistance can be calculated using the relation??? R=??V . I Variables in the experiment: - *In this experiment I varied the length of the wire each time using a range of lengths from 30-100cm. *I kept the current flowing through the circuit constant using a variable resistor which I kept varying for each length of wire so that the ammeter would always read 0.2 A. I then recorded the voltage reading from essaybank.co.uk s from the voltmeter which corresponded to the length of the wire being used. *The temperature of the wires in the circuit needed to be kept constant to prevent the whole circuit from overheating. I managed to do this by quickly switching off the power pack supply every time I had recorded my set of readings. I then left the power pack to rest for a small interval of time before switching it on again to record the next set of readings. * I also kept the diameter of the wire constant by using the same piece of wire throughout the whole experiment. Justification of the equipment range: - Ammeter: - I used a full scale deflection ammeter of current range from 0-1A because I only wanted to measure small currents passing through the circuit and not large currents however all the digital ammeters that were available had a current range of 0-20A and there werenÆt any available digital ammeters that had a current range from 0-1A. I chose to keep the current passing through the circuit constant by passing a small current of 0.2A throughout the whole experiment. This maintained the temperature of the wires in the circuit because large currents would heat up the wires thus damaging the circuit and affecting the resistivity of nichrome. Voltmeter: - I chose to use a digital voltmeter of range 0-5V instead of a full scale deflection voltmeter to avoid parallax error when recording the voltage readings. It would have been ideal for me to have used a digital voltmeter of range 0-2V but this type of voltmeter was unavailable. The only digital voltmeters that were available for me to use had either a voltage range from 0-1V or from 0-5V or from 0-20V.When I did a preliminary experiment I found that the voltage readings that I recorded ranged from 0-1.9V and so the digital voltmeter with a range of 0-5V was the most ideal one to use. I recorded 3 sets of voltage readings and decided to take an average voltage reading for each length to ensure a greater accuracy in the obtained results and in the plotted graph. Length of the wire: - At the beginning when I did a preliminary experiment I was using crocodile clips instead of terminal blocks to connect the wire to the circuit. Although the crocodile clips made it easier for me to measure the length of the wire however I found it very difficult to keep the wires in these clips since it kept slipping out so the wire wouldnÆt get connected successfully to the circuit. I therefore decided to use terminal blocks because even though these blocks make it more difficult for the wire to be measured accurately using a meter ruler, this can be overcome by using large wire lengths so that the inaccuracy in measuring its length will become very tiny and so can be neglected. I decided to chose 8 different lengths between 30-100cm because when I did a preliminary experiment I found that these lengths gave accurate results and that 8 readings was sufficient enough for me to plot a st. line graph and draw a good line of best fit through the points. Micrometer screw-gauge: - I decided to measure the diameter of the wire at 3 different positions on the wire. I then calculated the average diameter from the 3 measurements taken to enable me to measure the diameter of the wire as accurately as possible.