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How much power does your circuit use?

How much power does your circuit use?

A couple of days ago, I got the following email from Jacob:

“I have been reading your “Getting Started with Electronics” book and have a question over problem 3.1. While I understand using I = V/R to find 0.07A which includes using 7V in the equation related to 9V battery and 2V LED, but why do we not use the 7V again in the equation P = I*V? Instead, in the answers, you use 9V.

When figuring power consumption of a circuit, do we only use the voltage being supplied and not what is being used in the circuit ie. the LED? I hope I’m not asking a stupid question.


BTW: I’m enjoying your book very much so far.”

“Hi Jacob,

Great question!

First we find the current in the resistor, which is 0.07A. Since there is only one path for the current, the same amount current will also pass through the LED. That means the 0.07A is the current in the whole circuit. The voltage of the whole circuit is provided by the 9V battery, so to find the power consumption of the whole circuit you need to use 9V in the P = I*V formula.

If you use 7V in the power formula, you only get the power consumed by the resistor. If you use 2V, you get the power consumed by the LED. 9V gives you for the whole circuit.

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