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# How does current vary with voltage in a light bulb?

Extracts from this document...

Introduction

Andrew Isherwood 11.7

How does current vary with voltage in a light bulb?

Preliminary Investigation

Before I start the experiment I have carried out some research into Resistance, Voltage, Current and the history on Goerge Ohm.

Resistance

In the metal inside of a wire the molecules are compact. Then when the current passes through it, it heats up the wire due to the electrons rubbing against the molecules in the wire when they either collide or skim past the molecules that are in the wire the higher the voltage is the more energy that is given to each electron so the faster it goes and the more friction it creates. So the electron's that are trying to pass through the wire at tremendous speed hit the vibrating molecules and slow the electrons down.  Therefore the greater the vibration is greater the chance there is of the electrons getting slowed down. This is called resistance.

Some metals are more resistant to others simply because some wires are more compact than others therefore don’t vibrate as much therefore it doesn’t slow the electrons down as much.

The thickness and the length of a wire causes more resistance this

Middle

(2dp)

Test 1

0.62

0.96

R=V/I

1.54

Test 2

1.16

1.81

R=V/I

1.56

Test 3

1.89

3.00

R=V/I

1.59

Test 4

2.40

3.90

R=V/I

1.63

Test 5

3.12

5.17

R=V/I

1.66

Graph

The graph is on the next page.

Analysis

From these results we can see that the resistance is constant. The voltage at test are i.e. 0.96V, when doubled gives a doubling in the current. This can be seen on the graph.

On the graph for this experiment I have circled two points that are not on the line these would be points in the graph where the wire got too hot and therefore gave me a miscalculation this might be because the meters are not accurate.

My preliminary work has proved that the wire obeys Ohms law, as long as the temperature is constant. You can see from the calculations of resistance that is relatively constant throughout the experiment only vary by 1/10 of an ohm at the higher voltages.

From my results I can see that this is an Ohmic conductor because it follows ohms law.

In the main investigation we will examine what happens in a light bulb and compare the two.

Prediction

For the main experiment where you do the same experiment as the one we did in the preliminary experiment apart from we will be using a bulb instead of a wire.

I predict that the bulb will not obey Ohms law because in order for a bulb to work electricity is passed through a thin piece of wire (the filament) and the gases inside of a light bulb force the filament to burn brightly in order for it to light up a room so the temperature is never constant it is always rising. But as it happened twice in the previous experiment the wire got hot because I did not leave it long enough to cool down and therefore it gave me an inaccurate result therefore the temp wasn’t constant. But with a bulb there is not a chance that it will be able to cool down unless you turn it off for a period of time but that would not be done in a house for it is not practical. So the filament will just keep getting hotter and hotter therefore giving you a result which will be correct to begin with where current is proportional to the voltage and then it will start flattening out because of the heat therefore I will have proved Ohm’s law, where the temperature must be constant so it does not increase resistance.

Method

When I carry out this experiment I shall follow this procedure;

1. I will set up a circuit so it looks like the diagram below.

1. Set up a power pack and set it on 1 volt.
1. Take a reading and write it on my results table then turn off the power pack for 10 seconds then turn it on and take another reading.
1. I will repeat this until I have 3 results then repeat this on voltages 2-10 volts

I am using a power pack instead of batteries and a voltmeter so my readings will be more accurate.

Results

 Voltage Volts 1 Volts 2 Volts 3 Average (Volts) 1V 0.01 0.01 0.01 0.01 2V 0.99 1.00 0.99 0.99 3V 2.09 2.09 2.09 2.09 4V 3.59 3.59 3.60 3.59 5V 4.74 4.74 4.74 4.74 6V 6.21 6.24 6.28 6.24 7V 7.47 7.45 7.44 7.45 8V 9.00 9.01 9.02 9.01 9V 10.10 10.17 10.21 10.16 10V 11.33 11.38 11.41 11.37

Conclusion

All of the results that I did fit the pattern none where really off but I could of got them better if I had left the bulb to cool down a little longer and for a set time. I think that my results on both experiments the preliminary one and the main one are good enough and reliable enough to support my conclusion.

The only improvements that I can think of for further work is to take three or fourth decimal places instead of just 2, to have a set time to leave my bulb to cool down for say 30second, to repeat the experiment up to 15volts to give you a bit more of a bigger picture, by repeating 1 volt three times then moving on to the next one that I should of done 1 volt once then 2 volts and when I got to 10 volts started it again and done it that way three times, also to use more scientific meters to record my results with and also by repeating the experiment with different light bulbs to see if you would get different results. The only reason that I did not make these changes is because I did not have enough time in the lesson to do so or the extra equipment if I did have these I think my results would be much more accurate.

This student written piece of work is one of many that can be found in our GCSE Electricity and Magnetism section.

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