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How does the power dissipated by a light bulb vary with voltage?

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How does the power dissipated by a light bulb vary with voltage?



For my experiment, I am going to investigate how the power dissipated by a light bulb varies with voltage. To find this out, I will need to do an experiment to test this out and repeat it another two times.

Meaning of terms

Current – Current is the flow of electric charge. An ammeter measures current in a circuit.

Voltage – Voltage is the potential difference between two points in a circuit is the electrical energy gained or lost by 1 coulomb of charge. A voltmeter measures voltage between two points in a circuit.

Resistance – If a component has resistance, it changes some of the electrical energy passing through it into another form of energy. A rheostat can increase or decrease its own resistance so in that way I can control the amount of voltage across the light bulb whilst doing the experiment.


I think that as the voltage across a light bulb increases, the power dissipated by the light bulb also increases but at a greater rate. This is because as the voltage increases, the current also increases. This is because if the current is the amount of electrons flowing through a circuit at any point in a circuit and if the voltage increases then the current must increase as the electrons flowing through that point are flowing faster. Therefore, as the voltage increases, the current also increases because P = I.

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 – The voltmeter will need to measure from 0V – 6V (because my light bulb will be a 6V one) so I will use the one which is 0 – 20V d.c. It will be precise to 2 decimal places.

Power pack – It will need to give out voltages of 0V – 6V (because my light bulb will be a 6V one).

Variable resistor or rheostat – The size of the rheostat I need will be 10 Ω and 5A. I found this out because I used this type of rheostat for my preliminary experiment and it gave me good results.


Light bulb – I will need a 6V light bulb.

Ammeter – The ammeter will need to measure from 0A to 0.29A (because from my preliminary measurements, at 6V, the current was 0.29A). So I will use the one which is precise to the nearest 1/10th of an amp and can measure from 0 – 10amps.It will be precise to 2 decimal places.

Circuit diagram


  1. I will set the circuit as shown in the circuit diagram.
  2. I will set the voltage at the power pack on 1V.
  3. I will change the position of the rheostat contact so it gives me a reading of about 0.5V.
  4. I will record the voltage and current readings.
  5. I will continue changing the voltage of the power pack and the position of the rheostat contact until I get 12 readings of voltage and current each, with the voltage readings at intervals of about 0.5V.


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        The evidence that I wrote in the conclusion for how resistance increases with temperature came from an experiment that I did earlier in the year. In the experiment, I set up a circuit with a power pack, ammeter and a coil of wire placed in a non-conducting tin with two terminals. I connected a voltmeter parallel to the coil to work out the resistance. I poured hot water into the tin and measured both the current and voltage at intervals of 10 degrees Celsius. I found that as I decreased the temperature through the coil if wire, the resistance also decreased. So therefore, I concluded that in the light bulb, the exact same thing happens for a light bulb.

        However, in the light bulb, the filament is not made of copper but tungsten. This means that tungsten may not follow the same rules as the copper. This would mean that my whole conclusion could be different because it is based on how resistance increases in copper. To make a better conclusion, I would like to do the same experiment as with the copper, but with a coil if tungsten wire instead. This would ensure that the conclusion would be based on evidence for a light bulb, and I would therefore have a stronger conclusion.

This is what my results table would look like for the experiment:


Potential difference/V


Resistance/ Ω










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Response to the question

This essay is very good, going very in-depth and explaining points clearly to the reader. There is very little that can be improved in this essay. The student has answered the question of 'How does the power dissipated by a ...

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Response to the question

This essay is very good, going very in-depth and explaining points clearly to the reader. There is very little that can be improved in this essay. The student has answered the question of 'How does the power dissipated by a light bulb vary with voltage?' well, making a prediction from the theory they have learnt and plotting graphs they predict to occur by using their knowledge of equations for power. They have then found that their results largely support their prediction from the knowledge of their theory. They have then thought about why their results do not entirely coincide with their theory, using more theory to figure out that the resistance must of increased as the wire heated up. This is a very good idea, and I would always recommended thinking of why your results do not entirely fit your theory as it could end up being something fairly simple, and will help enhance your understanding.

Level of analysis

The student has gone in-depth and has planned his experiment in advance. I think this is a very good idea and can help you from making mistakes in the experiment; it's a lot better to plan how to set up the experiment before you do it, than wasting time trying to figure out how to set it up when you could be doing the experiment. The student has done some preliminary measurements to see how high the values can go so they can make a neat table, it might seem like doing extra measurements would take up more time, but I find this can really speed things up as you won't end up making a table with more measurements than you could actually do, leaving you with a neater table and a table that is faster to make.
He also makes sure to keep his control variables the same throughout the experiment, and tries their best to make sure only the independent variable is changed as they are meant to. They also understand how to plot a graph and understand how to figure out how these graphs relate to equations.

Quality of writing

The student has made very few spelling or grammar mistakes and uses the scientific terms whenever appropriate. He has also laid out their essay very well, ending with a conclusion that links with their prediction and explains why the prediction is not entirely the same as the conclusion, I find that this helps make the essay seem more professional and help you learn your theory better.

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Reviewed by jackhli 15/02/2012

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