Suppose we have a simple circuit with an input voltage source $V$ and two resistors.

Now if we want to find the power dissipated by each resistor using the formula $V^2/R$, should the voltage be the input voltage or the voltage across that specific resistor.

I have this question because, in a series connection, voltage across different components is different.

  • 2
    $\begingroup$ Adding a simple procedure to go with RedGrittyBrick's answer: First, find the total resistance. That's easy, just add 'em up. Then find the current. That's easy too: Ohm's Law. Now calculate the power dissipated in each resistor as $I^2R$. $\endgroup$ Apr 6 '15 at 16:14

should the voltage be the input voltage or the voltage across that specific resistor

The latter.

generally when you want to know something about a particular component, you work with the conditions applying at the boundary of that specific component.

I have a desk lamp with a LED. The other end of the lamp plugs into a 240 V AC power outlet. If I want to know the power being dissipated by the LED I need to measure the voltage and current at that LED, not at the wall outlet, not at a power plant that you might consider part of the circuit.

  • $\begingroup$ Why the drive by downvote? This seems a perfectly good answer to me. $\endgroup$ Apr 6 '15 at 15:57
  • $\begingroup$ so if there are two resistors R and r, the voltage across say r is Ir where I is V/(R+r) and this value is used in the power formula for the power dissipated by r? $\endgroup$
    – user34304
    Apr 6 '15 at 16:02
  • $\begingroup$ Yes user34304 that is correct. The same current I flows in each series-connected component. $\endgroup$ Apr 6 '15 at 16:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.