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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.

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    $\begingroup$ 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 $I^2R$. $\endgroup$ – Solomon Slow Apr 6 '15 at 16:14
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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.

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  • $\begingroup$ Why the drive by downvote? This seems a perfectly good answer to me. $\endgroup$ – John Rennie 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. $\endgroup$ – cuddlyable3 Apr 6 '15 at 16:10

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