KVL & KCL Problems

Discussion in 'General Electronics Chat' started by ELECTRONERD, Dec 18, 2009.

1. ELECTRONERD Thread Starter AAC Fanatic!

May 26, 2009
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Hello Everyone,

As one might suspect from first glance, I assure you this isn't a homework problem. I am not in college but I found these problems to help me improve upon KCL and KVL.

My first problem, as depicted in the attachment, wants you to find the power in element A. According to Ohm's Law this is simple enough: P = VI so it should be 15W. However, the answer is actually -15W! How is this so? Wouldn't negative power be considered to be power absorbed by the element rather than dissipated? Also, you may notice that the current direction and voltage polarity are both positive compared to eachother. Does this make them negative? What if I reversed either the current direction or the voltage polarity, would that make it positive?

Thanks,

Austin

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2. ELECTRONERD Thread Starter AAC Fanatic!

May 26, 2009
1,146
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Another conjecture I came up with has to do with supplying power and dissipating power. Since they ask for the power dissipated in element A it should be a negative value. However, if power is supplied it would be positive.

I hope that's right, I'd appreciate any comments or advice.

Austin

3. Ratch New Member

Mar 20, 2007
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4
ELECTRONERD,

P = VI is not Ohm's law. What is element A? Is it a resistor, current source, or voltage source? If it is a resistor, it will always be dissipating power no matter what direction current exists, or what the polarity of the voltage across it. Ratch

4. ELECTRONERD Thread Starter AAC Fanatic!

May 26, 2009
1,146
16
Ratch,

If P = VI isn't Ohm's Law, what is it?

Element A is the box with the "A" inside it. The reference says they are elements, meaning you don't know whether they are resistors, voltage source, or current source. Although, I would most likely bet it's a resistor.

Austin

5. Ratch New Member

Mar 20, 2007
1,068
4
ELECTRONERD,

It is the power formula. If you want to know what Ohm's law is, then you can read the link below. You will probably find it interesting.

You would be making a wrong bet. The voltage polarity and current direction of element A and the northeast element shows that they are sources. The east element is a wire. Element B and the two west elements are sinks. It doesn't matter whether you make element A a 5 A or 3 V source, and the northeast element a 9 A or 4 V source. You will have to decide whether you like the convention of labelling a source negative.

Ratch

http://forum.allaboutcircuits.com/showthread.php?t=11674&highlight=ohm's+law