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Question: Joule's Law of heating.

  1. Jun 29, 2013 #1

    My text says that when the resistance is doubled, the heating effect is halved.
    Joule's law of heating states that Resistance is Directly proportional to Heat produced.
    I.E, R α H.
    ∴ 2R α 2H.
    So, shouldn't the Heat be doubled.

    Or, since the Potential Difference is constant, should I use
    H = (V2 t) / R
    instead of
    H = I2Rt
    Does Joule's law apply everywhere, or are there conditions?
    Just to be clear, this isn't homework.
  2. jcsd
  3. Jun 29, 2013 #2
    You can use both equations as both are same. Putting V = IR in first gives the second equation.

    When resistance doubled, what happens to the current through the resisrance?
  4. Jun 29, 2013 #3
    NOT always. It is applicable to a certain condition, that is?
  5. Jun 29, 2013 #4


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    Correct. That follows from..

    Power = V2/R ............. (1)

    Joules law states that the heat produced in a conductor by a constant current is equal to the product of the square of the current and resistance of the conductor and the duration of time the current passes. eg.. Q = I2 * R * T


    Power = Q/T

    Power = I2 * R .....................(2)

    It may appear that (1) and (2) are contradict each other but that's wrong. As darkxponent points out, if you change R you change I. That's because in a simple resistor circuit I is not independent of R...

    From Ohms Law..

    I = V/R

    Substutute for I in (2) gives

    Power = (V/R)2 * R

    = V2 /R

    Which is equation (1)
  6. Jun 29, 2013 #5
    Thank YOU!

    Thank You Guys!
    I knew that you can replace V = IR to get the equation.
    I didn't know joule's law stated constant current as a requirement.
    Thanks SO Much!

    Best Regards,

  7. Jun 29, 2013 #6
    Would like to add a bit more to the thread. The basic power equation is VI, derived from Electromagnetics, so Joules Law is not a 'Law' at all. This, P=VI, equation is valid everywhere, even when I varies non-linearly with V(where impedance is not constant).

    Consider a case where where I = k*V^2.
    In this case Power equation will become. p = k*V^3
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