(a) In Fig. P 1.27 (a) , P1=36 W. Is element 2 absorbing or...

Key Concepts

Electrical Power

Electrical power is the rate at which electrical energy is transferred by an electric circuit. It is calculated as the product of the voltage across an element and the current through it (P = VI). A positive value generally indicates that the element is absorbing or dissipating power, while a negative value means that the element is supplying or delivering power to the circuit.

Passive Sign Convention

The passive sign convention is a method used in circuit analysis to determine whether an element is absorbing or supplying power. According to this convention, if the current enters the positive terminal of an element, the power calculated (the product of voltage and current) is taken as positive, meaning the element absorbs power. Conversely, if the current enters the negative terminal, the element is considered to be supplying power, resulting in a negative power value.

Conservation of Energy in Circuits

In analyzing electrical circuits, conservation of energy is a key principle. It ensures that the total power supplied by the sources in a circuit equals the total power absorbed by the loads. This balance is crucial for verifying circuit models and ensuring that power calculations for different circuit elements are consistent with the overall energy dynamics of the system.

Transcript

00:01 All right.

00:02 Working a problem that has to do with the concept of power absorption and supply.

00:10 And so the thing to keep in mind, and i keep it on a sticky notice on doing this kind of stuff, is that if current is flowing into the positive out of the negative, that means power is being absorbed.

00:29 And conversely, if it's out of the positive into the negative, that means it's being supplied.

00:39 So if you don't have the current direction drawn on your circuit schematic, then if you know the power is being absorbed by one particular element, then, and let's say that element does have plus and a minus terminal, then you are able to tell, oh, if you are able to tell, oh, if you're it's being absorbed, then current must be flowing that direction.

01:06 And so that gives you an indication which direction the current is flowing.

01:11 And if you also know that element's voltage and you know the power that it is absorbing, then you're able to determine the magnitude of the current.

01:23 So if that element is absorbing 36 watts and its voltage is 12 volts, then my current would just be power 5 by voltage, and so it would be 3 amps.

01:41 And that same 3 amps would be going in the same direction into the next element.

01:45 So if that next element has a voltage of 6 volts, now current is 3 amps, and so its power is going to be 18 watts.

01:59 And if current is flowing this direction into its positive terminal as well, then the power is being absorbed.

02:08 And so that would be the case for that second element.

02:16 Now, if i have a similar problem and i have two elements in series, but their poles are reversed, if i'm told that the power going through the second element, if it is negative 48 watts, that means the power here is being supplied.

02:40 That's called the passive sign convention...

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