Determining voltage across resistor and current direction

1. Jan 15, 2015

LongApple

1. The problem statement, all variables and given/known data

Resistor conected to unknown network N. Resistor immersed in isolated water bath. Resistor resistance R = 8 ohms. It is determined the power dissipated in resistor is 11 W

a) Find the voltage across the resistor.

b) What is the current i entering the network N from the resistor?

2. Relevant equations

V=IR

3. The attempt at a solution

Ignore part a). I understand my mistake for that one.

For part b) though I am trying to get a better intuition for what in this problem would tell us that current is actually leaving the network as opposed to entering it. What is the physical intuition for why that is the case and what is the matematical reason part b) has a negative -1.17 A?

2. Jan 15, 2015

Staff: Mentor

The resistor is a passive element, it does not source current. So we know the current through the resistor is coming out of the network, and it's 1.17A.

So, 1.17A out of the network can be expressed as negative current INTO the network. The question asks for the current entering the network from the resistor, this must therefore be negative 1.17A, i.e., -1.17A. It's just the way it's done to stay mathematically correct at all times.

Power is I2.R so any negative sign disappears in the power calculation.