Understanding the Negative Power Delivered in Circuits

[Mdhiggenz]

Homework Statement

Why is it when referring to the power delivered through a circuit the value is negative, and the power absorbed would be positive?

Logically it doesn't make sense to me that negative power value is being delivered through out the circuit.

Thanks

Higgenz

Homework Equations

The Attempt at a Solution

 

[CherryTrooper]

Because when the power is being delivered, its being "taken away" from the circuit. When it's being absorbed, it's "gaining" power, thus positive.

P= V x I, so if you think about it the electric potential is positive, and the current is positive. It's converting, or charging or something. When it's releasing (or delivering) power, like a battery would do to an associated item, the power in the battery's circuit would be negative (because the current is flowing out, and thus negative)

 

[Mdhiggenz]

awesome explanation thanks!

 

[milesyoung]

Whether a positive number for power means the element is "producing" or "consuming" power depends on the sign convention used.

For the passive sign convention, positive power means the circuit is consuming power. For the active sign convention, positive power means the circuit is producing power:
http://en.wikipedia.org/wiki/Passive_sign_convention#Sign_conventions

 

[Nickie]

The concept of negative power delivered in circuits can be confusing at first, but it is an important concept to understand in order to fully grasp the behavior of electrical systems. The reason why the power delivered is represented as negative is because it is a measure of the net energy flow into the circuit. In other words, it represents the energy being taken away from the source and into the circuit. This is why it is considered negative, as it is moving against the direction of the current flow.

On the other hand, power absorbed is a measure of the energy being consumed by the circuit. This is represented as positive because it is in the same direction as the current flow, and therefore, adding energy to the system.

Think of it as a bank account. When money is being deposited into your account, it is represented as a positive value because it is adding to your balance. But when money is being withdrawn from your account, it is represented as a negative value because it is decreasing your balance.

Similarly, in a circuit, the power delivered is like the money being withdrawn from the source and the power absorbed is like the money being deposited into the circuit. The negative value simply indicates the direction of the energy flow, not that it is a "bad" or negative thing.

I hope this explanation helps clarify the concept for you. It is important to remember that in circuits, power is always conserved, meaning the total power delivered must equal the total power absorbed. So even though the values may be negative or positive, they will always balance out in the end.