# Help me calculate the power developed by this simple circuit.

#### BrandonSayareh

Joined Jul 21, 2021
2
Hello, I know this was a while back but I have found a YouTube video on the problem.

#### bertus

Joined Apr 5, 2008
21,534
Hello,

The video is a private video that needs a login.
I do not have a login.

Bertus

#### BrandonSayareh

Joined Jul 21, 2021
2
Hello,

The video is a private video that needs a login.
I do not have a login.

Bertus

Hi, sorry about that, the author of the video says it will be live on July 23, 2021 at 11 am.

Also, the problem is slightly different, it is only to find the power delivered/ absorbed by the 20V source which is -160W.
So the video might not be totally useful, my bad.

#### dcbingaman

Joined Jun 30, 2021
334
NichA,

I would say that no power is dissapated in the circuit because no resistance exists. Please post the solution when you receive it. Ratch
I have to agree with that. Energy is being exchanged but not used. In the real world the 'wires' within the circuit have resistance thus there is energy being used but the amount all depends on the resistance of the wire which is not known. An ideal LC tank circuit has energy moving continuously back and forth between the capacitor and the inductor, but no energy is being used.

#### MrAl

Joined Jun 17, 2014
8,475
Power is sometimes measured a bit differently depending on load.
For a resistor, of course it is V*I or I^2*R, etc., but for a charging circuit there is still a power measure even though energy is being transferred and not immediately dissipated. Note that modern cell phone chargers have a power rating associated with them that gives an indication about how fast they charge, like 10 watts, 15 watts, 20 watts, etc., and higher ratings than that usually mean it can charge more than one phone at a time but with today's technology it could be for just one device.

The power going to say a 12v battery that is actually at 13 volts and has 2 amps flowing INTO it is:
13*2=26 watts.
Note i emphasized the word "INTO" because if it is going OUT OF then the power transfer is in the opposite direction. The difference in direction is handled by assigning either a positive power or negative power to the quantity.

• dcbingaman

#### MrAl

Joined Jun 17, 2014
8,475
Homework question: Find the total energy developed in the circuit. The first thing I need help understanding is: is this question supposed to be asking for power instead of "energy." How can you calculate the energy developed without some type of time constraint?

Ok, so assuming we are talking about power developed, here is what I know:

First, I know that i = -8 A so the dependent current source is always equal to -24 A.

Second, I must calculate the power absorbed by each element by using the formula I*V. (Of course we will use the formula -I*V if the current is moving in the direction of the voltage rise). So the power absorbed by the 20V voltage source and the 100V voltage source is -160 Watts and -1600 Watts. And since these numbers are negative that tells us that power is being delivered.

So I figured out that these two elements are developing 1760 W of power, but I can't find the power created or absorbed by the current sources because I can't create 3 independent equations to find the three remaining voltages across the terminals of the current sources. (KVL only yield two such equations)

How do I find the power absorbed or delivered by these 3 remaining circuit elements?

I note that it does not say "dissipated" anywhere so power is not dissipated, but it does suggest that power is flowing and energy is being delivered and that is still measurable so you can get the answer from careful circuit analysis.

#### dcbingaman

Joined Jun 30, 2021
334
Power is sometimes measured a bit differently depending on load.
For a resistor, of course it is V*I or I^2*R, etc., but for a charging circuit there is still a power measure even though energy is being transferred and not immediately dissipated. Note that modern cell phone chargers have a power rating associated with them that gives an indication about how fast they charge, like 10 watts, 15 watts, 20 watts, etc., and higher ratings than that usually mean it can charge more than one phone at a time but with today's technology it could be for just one device.

The power going to say a 12v battery that is actually at 13 volts and has 2 amps flowing INTO it is:
13*2=26 watts.
Note i emphasized the word "INTO" because if it is going OUT OF then the power transfer is in the opposite direction. The difference in direction is handled by assigning either a positive power or negative power to the quantity.
Good points. I did get thinking about this a bit more and you are correct. Power is being transferred from one place to another. The example of charging the battery is well taken. Even in the case of a resistor it is getting transferred from one place to another, specifically heating up the air around the resistor. First law of thermodynamics, mass/energy is neither created or destroyed but can be transferred from one place or form to another.

#### crutschow

Joined Mar 14, 2008
27,718
I would say that no power is dissapated in the circuit because no resistance exists
But the question asks for the energy (power) developed, not dissipated.
And certainly power can be absorbed as well as generated by some of the elements.

#### dcbingaman

Joined Jun 30, 2021
334
Thanks Ghar,

I have a different solution to the power balance problem ....
i has to be -8A it has no choice being fed by 8A constant current source.
if i is -8A naturally 3i is -24 Amps (8A+16A) and makes sense because i is -4 and CDCS is 3*(-4) = -24A

The 20V source is being discharged 20V*
Homework question: Find the total energy developed in the circuit. The first thing I need help understanding is: is this question supposed to be asking for power instead of "energy." How can you calculate the energy developed without some type of time constraint?

Ok, so assuming we are talking about power developed, here is what I know:

First, I know that i = -8 A so the dependent current source is always equal to -24 A.

Second, I must calculate the power absorbed by each element by using the formula I*V. (Of course we will use the formula -I*V if the current is moving in the direction of the voltage rise). So the power absorbed by the 20V voltage source and the 100V voltage source is -160 Watts and -1600 Watts. And since these numbers are negative that tells us that power is being delivered.

So I figured out that these two elements are developing 1760 W of power, but I can't find the power created or absorbed by the current sources because I can't create 3 independent equations to find the three remaining voltages across the terminals of the current sources. (KVL only yield two such equations)

How do I find the power absorbed or delivered by these 3 remaining circuit elements?