Electronic Circuits and Projects Forum

A circuit consisting of a 12 V voltage source and two 6 ohm resistors can be analyzed as follows.
12= 6I+6I ---> I=1 or 0=-12 + 6I +6I which is the generic form of the equation

This also means the the voltage across each resistor is 6V

Now Power = VI
If we did the same thing we would have (12)(1)W = 6(1) W+6(1) W
In the generic form of the equation
0=-12 W + 6W + 6W

Power is a negative quantity.

Power generated, by convention has a negative sign. Power dissipated, a positive sign.

Yea, it might make sense to have it the other way, but just like conventional current that's the way it is.

The analysis is generic and is generally known as a "loop equation". The algebra gets messy, but it is appropriate for multiple adjacent loops.

We don't talk about -10 MW generators, but when we analyze, they are negative. Your 100 W light bulb dissipates say 100 W of energy as heat.

Alright!!. Now where were we...

Originally Posted by KeepItSimpleStupid

I have to interject, the concept of negative power.

Positive power is power dissapated.
Negative power is power generated otherwise...

If you had a 10 MW generator and 1 million customers consuming 10,000 W
10 MW = 1000 * 1,000,000
0 = -10MW + 1000 * 1,000,000

or
Power consumed = power generated

All very true. However:

Any motor, for instance, by its very nature, while turning, is also a generator. Electric Motive Force (EMF) is said to be the "power" that causes the motor to turn. All well and good. But as it turns, it's also generating what's called a Counter-EMF (CEMF) that flows in opposition to the EMF (this can set up so called "Standing Waves", [which are really bad for causing energy losses] but that really is down the road).

Anyway, as in KISS's example, yes, there is power generated that does no useful work due to losses that are abundant. But keep in mind that a 10MW plant does not always produce at full capacity. The main power producing apparatus is throttled to the load. This does not eliminate losses, but it does reduce unnecessary loses. And, in essence, CEMF can be a power saving phenomenon.

So, for DC, since by way of a bridge rectifier system (and other ways) we can move the negative element of the sine wave to the positive side of ground we are, thereby, now making it part of the "power" side of the equation. This is as you saw in the excepts you introduced in a previous post.

It's good to see that that your curiosity is leading you to search out the answers to some of the questions you have.

Sometimes it's necessary to explain a fundamental concept in such a way is isn't exactly complete or in all cases true. Certainly in a number of fields thee are ambiguities and nuances that are difficult to grasp until a certain underlying concept is understood.

That's when you know better what questions to ask.

And you are rapidly getting there.

Hello! Anybody home??...

See, Graham? Another addiction to deal with...

Your right, KISS. Prob has been that our DSL link to the world has gotten very bad, lately. Keep checking the stupid thing and wasting time doing it.

On to Triac play, yeah!!