In the network the power absorbed by Rx

In the network the power absorbed by R_x is 20 mW. Find R_x.

Image from: Irwin, J. David., and R. M. Nelms. Basic Engineering Circuit Analysis, Tenth Edition. N.p.: John Wiley & Sons, 2010.

Solution:

We know that R_x absorbs 20mW of power as stated in the question. We also know the following equation, P=I^2R where P is power, I is current and R is resistance.

Now, we can write the following:

20\times 10^{-3}W=(2\times 10^{-3}A)^2R_x

(remember that 1 mA is equal to 1\times 10^{-3}A)

R_x=\frac{20\times 10^{-3}W}{4\times 10^{-6}A}

R_x=5000\Omega=5k\Omega

This question can be found in Basic Engineering Circuit Analysis, 10th edition, chapter 2, question 2.5.