Power delivered to resistors

  1. A question on my homework is asking about power delivered to resistors. These resistors are on an open circuit. While P=IV, it is also equal to (V^2)/R, so my question is if resistors are on an open circuit is it possible for power to be delivered to them? There is a source supplying voltage to the circuit.

    Thanks in advance

    [edit]:Move this thread if necessary-no this is not actually a question on my homework, but rather a confusion of mine about how to approach the problem.
    Last edited: Nov 9, 2008
  2. jcsd
  3. What is the current through an open circuit?
  4. cepheid

    cepheid 5,189
    Staff Emeritus
    Science Advisor
    Gold Member

    If the resistors are open circuited, charge is not flowing (indeed, cannot flow) through them. In other words, there is no current. If the current is zero, what happens to P? This should be pretty intuitive. You're not going to dissipate energy by...doing nothing.

    The formula P = V2/R is derived by assuming that the current in the resistor is V/R (which is true for a resistor that happens to obey Ohm's law). In which case, P = IV = (V/R)V = V2/R.

    In other words, this formula is, very specifically, "the power dissipated in a resistor of resistance R with voltage V across it." It is not valid under any other circumstances.

    Edit: DaleSpam was much more succinct than I. I hope he's not mad that I gave away this answer. This is not a homework thread. The OP was confused about a concept, so I clarified it. That having been said, I can see the value of asking the OP pointed questions in order to allow him to reason his way to the answer himself.
  5. Integral

    Integral 7,288
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    If the circiut is open then current is 0. If the current is 0 then so is the voltage drop across the resistor. Power is the same using either relationship.
  6. mgb_phys

    mgb_phys 8,809
    Science Advisor
    Homework Helper

    An open circuit means an infinite resistance so no current. No current = no power.
    What confuses a lot of students is the V^2/R, it seems to say you should have power with no current. But the V is the voltage DIFFERENCE accross the resistor. If there is no current through the resistor then there is no voltage difference accross it and so V is zero.
  7. Thank you, I am aware that the current through an open circuit is zero, and assumed that the (V^2)/R method is there when there is current assumed to be through it, but I was unsure. So I thank you for your help.
  8. Not at all, that was pretty fun to see all of the replies in such a short time.
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