Power delivered vs. absorbed?

[Cifrocco]

I found this Problem and Solution in a textbook.

Twelve years ago I graduated and received an Associate Degree in Electronics Engineering, although Electronics has always just been a hobby for me. Nevertheless, in my training I never came across this specific discussion of power being delivered or absorbed. Perhaps it's just another way of saying what I already know, but I thought I'd throw it out here and see what the experts among you think.

 

[ljcox]

I agree

Len

 

[Styx]

P=IV plain and simple and is a scalar quantity

Take a battery. IF it is a 5V battery and it supplies 1A, then it is a "provider" of 5W.

IF there is a 5R resistor that has 5V across it, it is "dissipating" or "absorbing"

I think they are just writing in a different (be it confusing way) the difference between a load and a source. All power is positive.

 

[Cifrocco]

So a current source that absorbs power is for example, a rechargeable battery that is in the process of being recharged?

 

[evandude]

that's an example, yeah.

intuitively, you know a battery provides power and a resistor absorbs/dissipates it. at the POSITIVE end of the battery, current flows out. at the POSITIVE end of the resistor, current flows in. it's just a simple matter of associating those two points.

And yes, when a battery is being charged, current is flowing INTO the positive end, so it is absorbing power instead of sourcing it.

using positive and negative power is sort of a convention, so it depends on the context whether you use that or just always positive and denote whether it's sourcing or dissipating power.

 

[2ny]

specific discussion of power being delivered or absorbed

for "a" and "b" power is being absorbed, in case of "c" power is being delivered!

 

[Morgen]

I don't remember ever seeing power (watts) assigned as negative or positive.

When charging a battery, isn't the charger the current source and the battery the load? A battery is not a current source, it is a voltage source (i.e. a battery of voltaic cells).

 

[evandude]

You don't really see power as negative or positive when you are, for instance, giving specs for a device or something, because by the type of device you can assume whether it is dissipating or absorbing.

however, sometimes in circuit analysis you will run across some negative powers if you look at those things. It's really more of an academic thing, because if you were to then describe that circuit, you would just say that the device was dissipating or absorbing, with no sign on the quantity. In the early days of my EE classes, they looked at positive/negative powers a little in circuits... it introduces the idea of power being dissipated by a voltage/current source instead of supplied... which (obviously from the talk in this thread) isn't always obvious to all people.

 

[Morgen]

I still maintain that the charger is the current source in the given example. There's obviously avoidance in addressing that.

 

[evandude]

you're correct... I hadn't said anything because it wasn't the point of the conversation, just a minor detail... and a moot point at that, because I'm sure he meant that it was an example of a device that's normally a source of power, absorbing power instead... after all, a battery is quite obviously not a current source strictly speaking... i'm sure we've all short circuited a battery at some point, if it were a true current source it would be fine, rather than heating up and popping :wink: in other words, I imagine that by "current source" he simply meant "a device that normally supplies current/power"

 

[Morgen]

Foolish me then... I thought he was asking the question in the context of the figure that he posted.