Resistance of a Wire

The resistance of a wire

Planning:-

Introduction:

To investigate the resistance of a wire at different lengths to see whether or not a longer wire has a larger resistance than a shorter wire.

Apparatus:

Wire

Voltmeter

Ammeter

Battery

2 crocodile clips

Metre rule

Variable resistor

Diagram:

Variables:

Length

Thickness

Resistance

Material

The material and diameter of the wire needs to be kept constant to keep the test fair as we are investigating resistance. The independent variable will be the length of wire as we will be changing it to see whether or not the resistance changes. The dependent variable will be the resistance of the wire as that is what is going to change due to the length of wire.

The length of wire will be measured accurately using a metre rule. The wire will be stuck onto the rule using tape. This means that the wire will not move about so we can measure the length accurately. Repeating measurements improve accuracy as you can spot any anomalies and the results can be averaged to give more reliable results. The voltmeter will give the reading for voltage ( V ) and the ammeter will give the reading for current ( I ). The resistance will then be calculated using the formula R=V/I. A variable resistor will be used to give three sets of voltage and current readings so we can work out the resistance of the wire. These three values will be taken for each length of wire. Each value will have a resistance. These three resistances will be added together and then divided by three to give an average resistance.

Preliminary measurements:

These preliminary measurements are taken so we can choose the wire which gives a range of resistance values as the length is changed. A high resistance means less current so there is less chance for the wire to get hot. The maximum current must not cause the wire to get hot. This because it will give unreliable results because the resistance increases. Also, the use of preliminary work is to find suitable ranges for the voltmeter and ammeter.

The types of wire that are going to be used are:

32 swg constantan

34 swg constantan

36 swg constantan

32 swg nichrome

36 swg nichrome

Length (cm) V (mV) I (mA) R (ohms)

32 C: 100.0 887 112 7.92

30.0 473 190 2.49

34 C: 100.0 1008 87 11.59

30.0 594 173 3.43

36 C: 100.0 1130 70.1 16.12

30.0 800 160 5.00

32 N: 100.0 1125 63.5 17.72

30.0 742 142.8 5.20

36 N: 100.0 1390 33 42.12

30.0 1070 88 12.16

These results show that the 36 swg nichrome wire has the greatest resistance/metre and it give a wide range of resistance values as the length is changed. This also shows the suitable ranges for the voltmeter and ammeter, and the number of cells that are needed. The voltmeter will be set to the 2000 mV (2V) range because the voltage is always less than 2 volts and this range is most accurate. The ammeter will be set to the 200 mA range because the current will always be between this range (e.g. 30 mA to 100 mA), also this range is most accurate. I will be using I cell because this reduces the current in the wire and so reduces heating. The range of lengths that I will be using are 30.0 cm to 100.0cm because six values can be obtained, and the measurement of length is more accurate as the wire will be stuck onto a metre rule (100.0cm).

Method:

The circuit will be set up as shown in the diagram. Then a length of wire over a metre long is sellotaped to a metre rule. The positive crocodile clip is attached at 0cm. And the negative is moved up and down the wire, stopping at different lengths ranging from 30.0cm and 100.0cm. Each time reading the ammeter and voltmeter to work out resistance R = V/I. This is using 36 swg nichrome wire. Other variables, voltage, thickness, and temperature will be kept constant, although the temperature will rise slightly once current is passing through it, which will cause the atoms in the wire to vibrate, and so obstruct the flow of electrons, so the resistance will increase, creating an small error. The voltage will be kept the same at 1.5V from a battery pack. To gain more accurate results, three values will be taken for each length of wire and the resistances will be calculated and averaged. The results will then be entered in a results table and a graph will be drawn.

Prediction:

"Resistance is proportional to the length of wire". This is because there are more ions in a longer wire which causes more collisions between the electrons and ions, hence increasing resistance. If the wire length is doubled, the numbers of ions doubles, and so the resistance doubles. If we plot a graph of length of wire against resistance, we should get a straight line through the origin. If this prediction is to be upheld, the temperature of the wire must not arise room temperature or the resistance will increase. Heat energy is produced in the wire while current is flowing through it. This is because electrons collide with ions and increase the kinetic energy of the ions. The wire must lose this heat energy at the same rate as it is produced, for the temperature to remain constant. This is why we must use a very low current and not have the wire too short.