Determining voltage across resistor and current direction

  • Thread starter LongApple
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Homework Statement



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?
[/B]
upload_2015-1-15_17-4-3.png


Homework Equations



V=IR

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?
 

Answers and Replies

  • #2
NascentOxygen
Staff Emeritus
Science Advisor
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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.
 

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