# An experiment to find the resistivity of nichrome

An experiment to find the resistivity of nichrome

A. Planning

Plan of the method to be used: -

The resistivity of nichrome can be determined using the equation ??=?RA/L

Where:

R:- Is the resistance of the wire in ?"ohms" and can be determined using the equation R=V/I where "V" is voltage in volts and "I" 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 "d" 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 "m" 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 it's diameter. Therefore the resistivity of nichrome can be calculated.

?

Diagram of the circuit used in this experiment

List of the apparatus used: -

- 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

0-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 readings 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 weren't 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 wouldn't 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.

GCSE Physics Resistance of a Wire

Before starting my coursework I have decided to chose a factor that will affect the resistance of a wire.

I shall do this by going through all of the factors that affect the resistance of a wire and how I would

measuring each factor to find out which would be the most effective and easiest factor to measure.

Below is a list of factors and reasons why they affect the resistance of a wire. From this list of factors I

shall only pick one factor to investigate. To explain the how the factors would affect the resistance of a

wire I have drawn a diagram to show how resistance occurs.

WIRE

ATOMS=

ELECTRONS=

Resistance occurs when the electrons travelling along the wire collide with the atoms of the wire.

These collisions slow down the flow of electrons causing resistance. Resistance is a measure of how

hard it is to move the electrons through the wire.

Factors

.Temperature: If the wire is heated up the atoms in the wire will start to vibrate because of their

increase in energy. This causes more collisions between the electrons and the atoms as

the atoms are moving into the path of the electrons. This increase in collisions means that

there will be an increase in resistance.

2.Material : The type of material will affect the amount of free electrons which 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 must 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

and the resistance will increase.

3.Wire length : If the length of the wire is increased then the resistance will also increase as the

electrons will have a longer distance to travel and so more collisions will occur. Due to this

the length increase should be proportional to the resistance increase.

4.Wire width : If the wires width is increased the resistance will decrease. This is because of the

increase in the space for the electrons to travel through. Due to this increased space between

the atoms there should be less collisions.

To chose which factor I am going to investigate I am going to consider how I would measure each

factor and which factor would be the best and easiest to record.

To measure the wire width I would use different widths of the same length and same material of wire

e.g. thin , medium and thick copper wire with thin and thick constantin wire. To record the difference

in widths I would use the same voltage and measure the resistance for each thickness. Although it

would be easy to obtain and record the data the graphs that I would be able to draw up would not be

interesting.

For the temperature of the wire I would not be able to carry out a fair test because it is extremely

difficult to produce and control the range of temperatures needed without the correct equipment.

If I chose to measure the difference in the resistance in different materials I would chose a number of

different materials and using the same voltage I would record the resistance given by each wire of the

same length and width. Although once again it would be simple to record these results the graphs that

could be drawn would not show any connection between the material and the resistance because of the

limited number of materials I could test with the equipment available.

The final factor is the length of the wire. To measure and record the findings for this factor would be

simple and the results collected could show a connection between the length of the wire and the

resistance given by the wire. This is why I have chosen to investigate this factor.

Prediction

I predict that if the length increases then the resistance will also increase in proportion to the length. I

think this because the longer the wire the more atoms and so the more likely the electrons are going to

collide with the atoms. So if the length is doubled the resistance should also double. This is because if

the length is doubled the number of atoms will also double resulting in twice the number of

collisions slowing the electrons down and increasing the resistance. My graph should show that the

length is proportional to the resistance.

The diagrams below show my prediction and shhould explain it more clearly

Because the length of the wire is only half the length of the wire below there should be half the number

of collisions between the electrons and the atoms.

The wire below is twice the length of the wire above and so there should be twice the number of atoms

resulting in twice as many collisions and a predicted doubling of the resistance.

Preliminary Method

In this preliminary experiment I will select a wire that will be used in my main experiment when

investing the connecting between the length of the wire and the resistance of the wire.

To ensure a fair test whilst carrying out my preliminary experiments I am going to be very careful

when selecting my independent variables which are the width of the wire and the wire material. I am

going to use a constant voltage of 2 volts and a constant length of 50 cm.

Apparatus: Meter ruler ¡V To measure the wire being tested to ensure a fair test.

Selection of wires ¡V Different materials and widths but the same length.

Crocodile clips ¡V To connect the wire being investigated to the rest of the circuit.

Voltmeter & Ammeter ¡V To measure the resistance.

Wires ¡V To connect the above items and to complete the circuit.

To measure the resistance of the wire I am going to use the equation RESISTANCE=VOLTS

CURRENT I will obtain the voltage and current readings from the voltmeter and ammeter. ...