Help me calculate the power developed by this simple circuit.

Hello,

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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
332
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.
 

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?

P.S. the answer key says the answer is 220 W so I know my answer of 1760 W is already way off!
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
332
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.
 

dcbingaman

Joined Jun 30, 2021
332
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?

P.S. the answer key says the answer is 220 W so I know my answer of 1760 W is already way off!
This is a strange question. According to the first law of thermodynamics mass/energy cannot be created or destroyed but only transferred around from one place to another. The 'energy developed' is zero. All energy producers 'whatever they might be' must equal the energy consumers. I would have thought the question more meaningful if we asked how much power is being absorbed or lost by the 20V battery or the 16A CC, etc. The net energy generated and consumed is always zero for the entire circuit.
 
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