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Investigating The Characteristics Of A Light Bulb.

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C/W        Ruth Knox        29/01/03

Investigating The Characteristics Of A Light Bulb (V-I)


When we do experiments, we must always think about the safety hazards involved.  I would work away from water, make sure my hands were dry, as water is a conductor and could cause shorting and electric shocks.  We should make sure that the equipment is not faulty, and that the wires are not frayed because it could cause electric shocks.  When you turn on a light bulb, you mustn’t leave it for too long, because you could get burnt if you touch it.


When we do an experiment, we must always predict what we think will happen.   The resistance is a resist of electrons flowing through an electrical component.  I think that if the voltage and/or the current increases, the light bulb filament will get brighter, and the resistance will go up because the temperature will change.  This breaks Ohm’s law as the temperature changes, so therefore the resistance will change.  When the resistance increases the current and the voltage will increase.  

Scientific Explanation

When the electrons (-) move through the wire, they collide with the (+) ions, giving them some energy.  This energy, then makes the (+) ions vibrate more rapidly, and the wire heats up.  We are able to see the wire heating up as the light bulb filament will glow.  When the (+)

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                                     2.02        0.83

                                        3.20        1.02

                                     4.67        1.23

                                     7.85          1.59

                                                11.18        1.90

As a result of this preliminary work, I decided to use the power supply on 9 volts.  I will go from 0.50 amps to 1.70 amps.  I found that when I turned the power supply to 9 volts, the results were the best.  On the graph, the line was bent, which means that the voltage increased.

Experiment we used a 3-½ volt bulb and 4 was the maximum voltage we could use on the power supply.  First, I turned the power supply to 2 volts.  There was a faint glow at the beginning, and as I slid the rheostat up the bulb got fairly bright.  2 volts:image00.png

                                                             V/V           I/A

                                                             0.52        0.10

                                                            1.14        0.14

                                                             2.11        0.20

I found that these results were quite good, but I wanted to see if the bulb could get any brighter, so I turned the power supply to 4 volts and recorded the results.

First we decided to go up every 0.10 volts to see if we could gain good results.

 4 volts:

                                      V/V               I/A


                                    1.65              0.18

                                    1.75              0.18

                                    1.85              0.19

                                    1.95              0.19

                                    2.05              0.20

                                    2.15              0.20

                                    2.25              0.21

                                    2.35              0.21

                                    2.45              0.21

                                    2.55              0.22

I found that there was a trend from the current when I went up by 0.10, but the results were too close together so I decided to try going up by 0.20 volts.

4 volts, going up by 0.20:

                                        V/V                I/Aimage02.png

                                1.65                0.18

1.85               0.19

  1. 0.20
  1. 0.21
  1. 0.22
  1. 0.23
  1. 0.24

3.05                0.25        

  1. 0.26
  1. 0.27

The bulb was fairly light at the beginning, but as I moved the rheostat along the bulb got much brighter.  At the end it was extremely bright.  

As a result of the preliminary work with the bulb (3 ½-volts)

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There are always errors in experiments and that can’t be avoided unless you have very accurate equipment.  Further work to this experiment could have been done with a new, reliable rheostat, which would give me more accurate results.  The rheostats are very hard to move which could make it harder to achieve the accurate voltage/current reading.  Also when I try to read the ammeter it fluctuates rapidly before settling on a final result, which also makes the results less accurate.  Also the light bulbs flicker a lot which makes the current and voltage readings flicker.  We can avoid this by having new bulbs.

My results were quite accurate because I didn’t get any anomalous results; they were always on the usual trend of the line.  When I repeated the experiment with the 12-volt bulb the majority of the results are very close to the original set, as you can see from the graph.  My graph was quite accurate because the points joined up to a smooth curve, I think overall my experiment was quite effective.

To extend this experiment by choosing to look at other variables, which would affect the rate of resistance, such as the materials in which the wire is made from, the cross sectional area of a wire, the number of components in a circuit, or the length of a wire.  A different component such as an electrical heater could also be investigated with the variable of voltage.  If we had more time to do the experiment, then the results could have been more accurate and the graphs would have been better.

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