Negative power? (Simple)

1. Sep 9, 2010

jegues

1. The problem statement, all variables and given/known data
I've got a gap in my understanding of power in circuit. I have a feeling my problem has something to do with sign convention or something. Anyways, here's the question,

Two electrical devices are connected at an interface shown below(see figure). Using the reference marks shown in the figure, find the power transferred and state whether the power is transferred from A to B or B to A, when v = -33 V & i = -1.2 mA.

2. Relevant equations

$$P = IV$$

3. The attempt at a solution

Okay, I know that since the current is negative it is simply going in the opposite direction than depicted in the diagram. (See figure) With this I can conclude that the power is being transfered from B to A, correct?

Now for the part I'm moreso confused about, the negative voltage. All these means is that the voltage that is being measured has a greater negative potential than the ground node (reference node) right? How do I deal with the negative sign when making calculations in equations such as,

$$P = IV$$

?

Do I simply sneak a negative in like so,

$$P = -IV$$

?

Any clarification on this would be extremly helpful!

Thanks again!

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2. Sep 10, 2010

CEL

Power is voltage times current. A positive sign means that the power is being dissipated at the branch. A negative sign means that the power is being furnished by the branch.

3. Sep 10, 2010

jegues

So do I keep the negatives when making my calculations for power using,

$$P = IV$$?

Or do I simply include the negatives for voltage and current in my calculations?

So if my power ends up being positive then the power will be transferred from A to B, and vice versa, if my power ends up being negative then the power will be gained by device A, meaning that the power is being transfered from B to A, correct?

4. Sep 10, 2010

CEL

Simply include the negatives of voltage and current.

If your power is negative at A, it will be positive at B. Then the voltage is furnished by A and dissipated at B.
Remember that in the source, the current flows from the minus to the plus terminal. So the power is negative.
In the load, the current flows from the plus to the minus terminal, so power is positive.

5. Sep 10, 2010

jegues

So if I was given the same situation with say, v= 12 V and i = -2.2 A

Then,

$$P = IV$$

$$P = (-2.2 A)(12V) = -26.4 W$$

So I have -26.4 W of power, flowing from B to A because my power is negative, correct?

This part really confused me. How do I know which device I'm measuring from. I make the calculation like I did above, but this doesn't tell me if I'm measuring power at device A or device B.

Can you clarify some more?

EDIT:
After reading it over, this part makes perfect sense, but the question and confusion still remains at which device am I measuring the power at? Device A or Device B?

Last edited: Sep 10, 2010
6. Sep 10, 2010

CEL

No, you have 26.4 W of power flowing from A to B.

According to your picture, the reference current flows from A to B. So, device A is the reference.

7. Sep 10, 2010

jegues

If device A is the reference and I obtain a negative power in my calculation,

i.e.

$$P = (-2.2 A)(12V) = -26.4 W$$

Then I should have 26.4 W flowing from B to A. A is asorbing power, correct?

8. Sep 10, 2010

CEL

No, it´s the reverse. Power is flowing from A to B.

9. Sep 10, 2010

jegues

So does a negative power mean that the reference device is absorbing power, or dissapating power?

If it's absorbing, then the power must be flowing into it correct?

What am I misunderstanding?

EDIT: Here's the source of my confusion.

This is incorrect. If the power has a positive sign, then power is being absorbed. If the power has a negative sign, then it is supplying power.

Now it makes sense that the power is being transferred from device A to device B.

Last edited: Sep 10, 2010
10. Sep 10, 2010

jegues

Okay after doing some review on passive sign convention I think I have a better idea at what's going on so I'm going to give a few scenarios a shot.

a) v = 12 V & i = -2.2 A

Here we have 26.4 W of power, and the power is being transferred from A to B.

b) v = -33 V & i = -1.2 mA

Here we have 39.6 mW of power, and the power is being transferred from B to A.

c) v = 15 V & i = 40 mA

Here we have 600 mW of power, and the power is being transferred from B to A.

d) v = -37.5 V & i = -43 mA

Here we have 1612.5 mW of power, and the power is being transferred from B to A.

Is this correct? Do I finally have my head on straight?

11. Sep 11, 2010

CEL

If your device has memory (capacitor, inductor, battery) it absorbs and stores power. If it has no memory (resistor) it dissipates power. In both cases the power is positive.

12. Sep 11, 2010

CEL

Yes, this is correct.