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Homework Help: Why is the resistance lower in a bulb which is turned on?

  1. Nov 3, 2014 #1
    1. The problem statement, all variables and given/known data
    Why is the resistance lower in a bulb which is turned on? (Compared to when it has been off)
    One bulb which is off has a measured resistance of ##27.0 M\Omega##
    When it is on it has a measured resistance of #380.9375 \Omega##
    2. Relevant equations
    ##\rho (T)=\rho_o(1+\alpha \Delta T) ##

    3. The attempt at a solution
    I think this is due to the temperature dependence of resistivity. This problem has come from an experiment we had to do. But I can't believe the drastic drop in resistance.
  2. jcsd
  3. Nov 3, 2014 #2


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    I can't quote knowledgeably on your figures (except to say that they are WAY off from what I would expect) but your reasoning for why it happens is sound.
  4. Nov 3, 2014 #3


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    By any chance was the bulb a light emitting diode (LED) based "bulb" or perhaps a compact fluorescent (CLF)? These types of light-bulbs can and often do contain internal, power supply circuitry that is inherently nonlinear.
  5. Nov 3, 2014 #4


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    collinsmark makes a good point. My comments were based on an incandescent bulb.
  6. Nov 3, 2014 #5
    I had three bulbs. One was a filament bulb, the other a fluorescent bulb, the other was a LED bulb.

    Actually the bulb with the greatest change was the incandescent bulb (filament bulb). Because it went from ##27M \Omega ## all the way to ##380 \Omega##.

    My professor measured its resistance while it was off with a multimeter, by the way.
    Last edited: Nov 3, 2014
  7. Nov 3, 2014 #6


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    380 Ω sounds about right for a small incandescent bulb of around 40 W (Assuming 120 V supply). But 27 MΩ sounds way too high for its cold resistance unless that MΩ should really be mΩ.

    In general the resistance of a metal increases with temperature.
  8. Nov 3, 2014 #7
    I mailed my prof to see what he thought. He said something strange (IMHO). See:

    "Resistance in a complex circuit is not the same when it is 'on' and when it is 'off. This because it contains transistors, transformers, resistances, diodes, capacitors, etc.

    In the experiment you see this difference (on vs off)

    On the other hand energy-saving fluorescent bulbs and LED bulbs contain internal emfs."
  9. Nov 3, 2014 #8

    Simon Bridge

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    Really need the details of the experiment - how, and under what circumstances, were the measurements made?

    i.e. did you run the bulb for a while, then disconnect, then quickly use the ohmmeter on it? Or maybe you measured the voltage and the current while it was on and used Ohm's Law?

    Your profs comments would seem to suggest that at least one of the bulbs was not as simple as it looked, and that the resistance was measured for bulbs which are part of a circuit.

    The main difference between "prevously on for a while" and "off for a while" states is temperature - though that may not be the only one.
    In general, as previously mentioned, resistance increases with temperature... especially for metal. In semi-conductors, all kinds of odd things can happen. Florescent lamps exhibit "negative differential resistance" - the higher the current the lower the resistance, so some external circuit is needed to stop them blowing up.

    If all bulbs were tested in the same rig, then it is possible that the method limiting the current for the florescent is is contributing to your results.

    But it is hard to tell without knowing the details of the measurement.
    Really - it sounds like an artifact of the broader experiment.
  10. Nov 3, 2014 #9
    The procedure was as follows:

    My prof said bring three bulbs to class. I brought them. I unpackaged them and the resistance was measured for each one of them with a multimeter. Then he inserted them (no more than one at a time) into a simple circuit connected to the wall and told me the current flowing.

    After that I could calculate the resistance while it was in the circuit with Ohm's law.
  11. Nov 4, 2014 #10


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    How did you measure the resistance with the multimeter?
    It is difficult to measure the resistance of a lamp just by touching two leads from the multimeter to it. It does not make good contact. The cold resistance of an incandescent bulb is a few ohms.
    When I made the same experiment with my students, the bulb was inserted into a socket with wires connected, as if it is in a lamp. The wires from the socket were plugged into the inputs of the multimeter.
    There can be other problems, a broken wire, for example.
  12. Nov 4, 2014 #11

    Simon Bridge

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    ... in addition, what was the "simple circuit"?
    How soon after "current was flowing" did you take readings to work out the resistance? Was the current DC or AC? How were the meters connected?
    See: http://physicsed.buffalostate.edu/pubs/TPT/TPTDec99Filament.pdf
    p523 "A student experiment" for a typical result.

    However I am leaning strongly towards an artifact in the experiment setup ... like misplacement of ohmmeter leads as ehild suggests or something you may not know about the "simple circuit".
    Last edited: Nov 4, 2014
  13. Nov 5, 2014 #12


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    I think there must be a mistake somewhere.

    I've just measured a 40W 240V incandescent bulb straight out of the box and got 109 Ohms. I didn't measure the hot resistance but I would expect it to be around 1400 - 1500 Ohms based on it's power rating. (Note this is a 240V AC bulb)

    27 milli Ohms could be a result of accidentally shorting the meter probes together?
    27 Mega Ohms could be a result of the filament failing? Although my meter would probably give an out of range indication.

    For what it's worth a fluorescent bulb (with integral ballast) that I tested cold appears to be an open circuit. Likewise an LED bulb (also with integral ballast) also appears to be open circuit (regardless of which way around I applied the meter probes). You might get different results depending on the design of the ballast in the fluorescent/LED bulb.
  14. Nov 5, 2014 #13
    I appreciate your help. My guess is that my bulbs were somewhat fancy (had some internal stuff that I'm not mentioning). My prof said what I measured was OK, so I guess it's alright. I've turned in my project now. Thanks a lot.
  15. Nov 6, 2014 #14

    Simon Bridge

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    No worries - and you've just seen that real life can throw you some odd results.
  16. Nov 6, 2014 #15


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    Staff: Mentor

    27 MOhm is a resistance value you can get by touching the contacts with dry hands, if the intended connection is not working.
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