# Electricity and Magnetism - Sample Essay

Aims of Experiment The aim of this experiment is to see if there is a relationship between resistance and length for Nickel-chrome wire of a diameter 0. 19mm. By measuring the voltage and current I plan to calculate the resistance of the wire at various lengths. R=V/I is then used to calculate resistance. R=Pl/A is then used in the form of a Y=mx+c (where R=y , P/A=m, 1=x and c is 0. )

Apparatus Instrument Accuracy/Range Ammeter 0-100mA (? 1mA) Voltmeter 0-5v (? 0. 01V) Wire with crocodile clips Battery (? 2) 3. 0V Micrometer 0-25mm (? 0. 01mm) Meter rule 0-100cm (? 0. 1cm) Nickel-chrome wire 1-2m Cellotape Method I shall construct a circuit which includes an ammeter, voltmeter, battery’s and the wire chosen for experimentation.

It would be setup like the following diagram. I plan to test a 1m piece of the wire at 10 cm intervals between 100-10 cm. By measuring the voltage across the wire and current through it every 10 cm I should get enough data to make a valid conclusion and find out the resistance at each length. I will cello tape down the ends of the meter wire to the wooden meter rule to make it easier to take readings. The current should stay constant the whole time.

By changing only 1 variable, the length of wire, I aim to keep this test fair. All other variables such as voltage , type of wire, diameter of wire and the accuracy of my apparatus will all be kept constant throughout the experiment. One possible source of error is due to a temperature rise due to current passing through the wire over a long period of time. This heat will increase the error in my data because the heat will increase the resistance of the wire. By allowing current to flow through the wire for only short periods of time I hope to minimize error. Another source of error is in the inconstant diameter of the wire.

To reduce this possible source of error I plan to measure the diameter of my wire along the length between 10-100cm and find an average to use in my workings. I also plan to collect all of my data in one go so I can keep all variables I want to, the same. If I collected my data in separate goes, I would end up using different apparatus at each attempt i. e. different battery. This would make my collected data unfair because some of variables that I want to keep constant are slightly changed. Prediction I predict that the longer the piece of wire, the greater the resistance.

This is due to the idea of the free moving electrons being resisted by atoms in the wire. There would be more collisions in a longer piece of wire, which explains the increased resistance. The relationship between the wire lengths and resistance should be proportional leaving me with a straight line graph. This is because in a wire twice the length of another, there would be double the number of atoms causing resistance. Also when doing my preliminary experiment I predict that the larger the cross sectional area is the lower the resistance. Preliminary Experiment Alessandro Bizzarri

To help me choose the right wire diameter I shall do some preliminaries. I need a wire, which has a low current flow in order to keep the temperature low and eliminate one source of error. The following diagram is the one used when obtaining my preliminary results. The following results are all from Nickel chrome of a length of 1m and the resistances were found using a multi-meter. Resistance (? ) Diameter (mm) To keep current low, and therefore the temperature of the wire low I have chosen the diameter 0. 19mm and a P.d. of 3. 0V.