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Homework Help: Estimating Resistance

  1. Nov 13, 2012 #1
    "Estimating" Resistance


    So, suppose I have a bulb labelled 10 V, 12 W. This is connected to a 4 V e.m.f battery. Can I calculate the Resistance from 10 V, 12 W (Using P = VI and R = V/I) and assume that it is the resistance when it connected to a 4V battery?

  2. jcsd
  3. Nov 13, 2012 #2
    Re: "Estimating" Resistance

    Generally speaking, the resistance of a bulb vary with temperature. If ignore this feature, I think your answer is right.
  4. Nov 13, 2012 #3


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    Re: "Estimating" Resistance

    If the bulb is rated 12W at 10V, that likely means that it produces a reasonable of light at 10 V. The filament of the bulb, under such conditions, will be near its maximum operating temperature.

    At 4 V, the same bulb would be operating at a considerably lower temperature. The resistivity of tungsten is very temperature dependent, so the resistance of the bulb would be much less than when it operates at 10 V .
  5. Nov 13, 2012 #4
    Re: "Estimating" Resistance

    Without a graph, however, is there any more precise method of estimation?
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