Engineering Solving Circuit Analysis Problem: Find V - Power of 5Ω Resistor is 20W

[hackett5]

Homework Statement

Hello, I was hoping someone could walk me through the steps to solve this problem:

The power absorbed by the 5Ω resistor is 20W. Find V
http://img191.imageshack.us/img191/6411/14966985.jpg

Homework Equations

V= I x R
P = V²/R

The Attempt at a Solution

I first found the voltage across the 5Ω resistor to be 10V using V= I x R and substituting 20W / V for the current. How do I find the total voltage supplied?

 

[schlunk]

You know the voltage across the resistor, good. So you know the voltage of the node above the 5Ohm resistor. Can you tell us how much current goes in and out of that node? Where does it come from? Where does it flow to?

 

[hackett5]

Well, using I= V/R, I get 2A (I₁) going through the 5Ω node. The 2Ω and 3Ω resistors are in series so I can combine them as 5Ω. So through that node the current would also be 2A (I₂) and voltage would be 10V. Then I₂ + I₁= I_total or 2A +2A= 4A? The total resistance would be 2.5Ω and voltage = 10V?

So the total voltage would be: V= 10V + the voltage across the 2Ω resistor.

V= 10V + (2Ω x 4A)
V=18V

 

[schlunk]

Bingo!

 

[hackett5]

Awesome! Thanks for the help