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Electrical characteristics of a lamp. Experiment.

  1. Jan 29, 2008 #1
    [SOLVED] Electrical characteristics of a lamp. Experiment.

    GCSE Science...
    Electrical characteristics of a lamp.
    The Aim is to investigate the relationship between current, voltage and resistance in a lamp.
    1) Set up the circuit as in the diagram (with three cells)
    2) Set the variable resistor to maximum
    3) Measure the voltage and current of the circuit
    4) Repeat as many times as you see fit and record the data, decreasing the resistance using the variable resistor.

    I couldn't get the accurate results... and I seriously don't get it.
    What's supposed to happen in this experiment?

  2. jcsd
  3. Jan 29, 2008 #2
    I suppose your investigation is about the relationship of V=IR in a lamp?

    Do you mean you can't get accurate results in the way that it is different from the real value or do you mean that you didn't get any concordant results?

    If you mean you are not getting results near the real value, it may be because you have some random and systematic errors. When you plot a line and the line is above or below the origin or at a different gradient than expected; then you have a systematic error. If your points are just jumping up and down and you can't even plot a line, you probably have a random error.

    If you have a systematic error, it could be because you set up your experiment wrong, bad equipment or something is affecting your experiment all way (e.g. heat of the lamp or something). You could try setting your experiment up again, using different equipment, trying another method or doing it in more controlled enviroments. If you have random errors, i am not quite sure what you can do. You could try doing your experiment again though...

    Somethings i want to remind you of: do at least three trials for GCSE level; i think its part of the markscheme...

    By the way, i can't promise that the suggestions I made are going to work...
  4. Jan 29, 2008 #3
    Did you obtain data? Can you plot V vs I over a range from very dim (maybe even dark) to very bright? What does that look like?
  5. Jan 29, 2008 #4


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    What relationship were you expecting? What relationship did you actually observe?

    - Warren
  6. Jan 30, 2008 #5
    Thanks for the posts.
    The results I got are:
    1.6V, 160mA=10ohms at the dimmest lamp
    1.9V, 180mA=10.6ohms
    2.4V, 210mA=11.4ohms at the brightest lamp

    What I don't get it that when they are dimmer, I thought it should have more resistance (Volt), but it has the most Ω when it has the brightest lamp...
    Does this mean I got the wrong results
  7. Jan 30, 2008 #6
    The results of a carefully done experiment are just that - the results, neither right nor wrong. If you don't trust the results, you must either repeat the experiment or improve the technique.

    I notice you have only three data points and the range of current is only 160 mA - 210 mA. If I were doing this, I would try to cover a range perhaps from 50 mA - 210 mA and take maybe ten readings in that range. Then, I would repeat the experiment at least once for this bulb and would, if possible, try another bulb.

    But, all that aside, did you plot your data? There are a couple of ways of doing that: V vs I; or, given your data, perhaps R vs V. Do you see any apparent relationship ? I'm pretty sure you already did.

    What is the reason you think your data are bad? As a way of helping you think about that, have you ever noticed WHEN the light bulbs in your house most often burn out?
  8. Jan 30, 2008 #7


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    Your data is probably accurate. As the lamp gets brighter, the filament gets hotter. Higher temperatures correspond to higher resistances.

    - Warren
  9. Feb 1, 2008 #8
    does that mean as the filament gets hotter, there is more resistance?
  10. Feb 1, 2008 #9
    Is that what your data show? Trust yourself.
  11. Feb 2, 2008 #10
    I realized that I need to investigate the resistance in a lamp. But I calculated the resistance of the variable resistor. Does this mean that the resistance of the lamp decreases? (because the resistance of variable resistor increases). Sorry, I am really really confused.
  12. Feb 2, 2008 #11
    If your picture is correct, you placed the voltmeter across (in parallel with) the lamp. Current is the same everywhere in a simple series circuit like this, so it doesn't matter where the ammeter is.

    So you measured the current through the lamp and the voltage drop across the lamp. If you use those two numbers to calculate resistance, it is the resistance of the lamp.

    To measure the resistance of the variable resistor, you would have to move the voltmeter to be in parallel with the variable resistor.
  13. Feb 2, 2008 #12
    OHH YES. Thank you so much.
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