All About Circuits Forum Help me calculate the power developed by this simple circuit.
 Register Blogs FAQ Members List Today's Posts Search Today's Posts Mark Forums Read

 Homework Help Stuck on a textbook question or coursework? Cramming for a test and need help understanding something? Post your questions and attempts here and let others help.

#1
06-10-2007, 02:21 PM
 NichA Junior Member Join Date: Jun 2007 Posts: 13
Help me calculate the power developed by this simple circuit.

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?

#2
06-10-2007, 02:28 PM
 hgmjr Super Moderator Join Date: Jan 2005 Location: Tennessee, USA (GMT-6) Posts: 9,030 Blog Entries: 11

One thing you can say with certainty is that with the answer key stating the answer in terms of Watts then your initial question is answered outright.

hgmjr

Last edited by hgmjr; 06-10-2007 at 02:33 PM.
#3
06-10-2007, 02:29 PM
 NichA Junior Member Join Date: Jun 2007 Posts: 13

Quote:
 Originally Posted by hgmjr One thing you can say with certainty is that with the answer key stating the answer in terms of Watts then you initial question is answered outright. hgmjr
haha... good point.
#4
06-10-2007, 02:56 PM
 hgmjr Super Moderator Join Date: Jan 2005 Location: Tennessee, USA (GMT-6) Posts: 9,030 Blog Entries: 11

Since power is additive and always a positive quantity, then I am at a loss as to how the answer can be 220 Watts since you already have 1760 Watts. This is well in excess of the value given in the answer key.

Either I have overlooked something or the answer in the answer key is for a different problem or the circuit as drawn has an error.

hgmjr
#5
06-17-2007, 05:25 AM
 Ratch Banned Join Date: Mar 2007 Posts: 1,068

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
#6
08-02-2011, 09:05 AM
 Laralike New Member Join Date: Aug 2011 Posts: 1

I don't suppose the thing that says 3i is in fact a 660V source?
Try that.
#7
08-02-2011, 10:59 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,948

I would assume there are an infinite range of possible circuit conditions but the nett energy loss is always zero.

Assume any voltage from +∞ to -∞ across the 3i controlled source and you have always have power balance as the voltage magnitude increases with either polarity.

Take the case of +20 volts across the controlled source.

The 8A source would have zero voltage drop and the 16A source would have -80V drop.

Source Powers in that case

20V source = +160W [supplying power]
100V source = +1600W [supplying power]
8A source = 0 W
16A source = -1280W [absorbing power]
Controlled source = -480W [absorbing power]

So the total power input balances the total absorbed power.

Take the case of -50V across the controlled source.

The 8A source would have -70V drop
The 16A source would have -150V drop

Source powers in that case

20V source = +160W as always
100V source = +1600W as always
8A source = -560W
16A source = -2400W
controlled source = +1200W

Once again the supplied & absorbed powers balance.

Seems a rather pointless question.
#8
08-03-2011, 11:17 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,948

Perhaps a final comment ....

Imagine a very high resistance value shunting the controlled source. Let's say it's 100MΩ.

No current would flow in this resistor since the controlled source just balances the total independent current source output. By the same reasoning the shunting resistor would add no power loss to the system.

Given there is no current then there would be zero voltage drop across the resistor. So the controlled source potential difference would be zero. The controlled source would neither absorb or generate any power.

In that case the two independent current sources would absorb the 1760W derived from the two independent voltage sources. No other losses would need explanation since none would exist.

One can probably equally well reason that the shunting resistor could be any value (including 0Ω ...?) and there would be no power loss due to the presence of the resistor, irrespective of its ohmic value.

The proposed answer alluding to the mysterious & unaccounted loss of 220W continues to puzzle me. One could, for instance, place a resistance of 0.382 ohms in series with the controlled source and thereby produce the required 220W.

I wonder if the OP has inadvertently omitted any circuit elements in transcribing the schematic...?

Last edited by t_n_k; 08-03-2011 at 11:44 AM.
#9
08-03-2011, 04:35 PM
 ErnieM Senior Member Join Date: Apr 2011 Location: Lon Guyland, Noo Yawk Posts: 5,314

AFAIK an ideal current source has zero volts across it. Thus the left leg has 20V across it, and the middle leg has 100V across it.

But as these two legs are in parallel, there cannot be two different voltages.

Thus the question is in error in at least 2 places.

Addendum: Power in watts is equivalent to joules per second. Joules is energy. One *could* ask how much energy does it consume/emit in some given time. But without specifying how much time then the energy is indeterminate.
__________________
“Freedom is never more than one generation away from extinction. We didn’t pass it to our children in the bloodstream. It must be fought for, protected, and handed on for them to do the same, or one day we will spend our sunset years telling our children and our children’s children what it was once like in the United States where men were free.”
#10
08-03-2011, 09:51 PM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,948

Quote:
 Originally Posted by ErnieM AFAIK an ideal current source has zero volts across it.
That's incorrect.

Imagine an ideal current source of 1A driving a 1Ω load. The source has 1V across it. An ideal current source adjusts its terminal voltage to keep the current in the load constant.

Last edited by t_n_k; 08-04-2011 at 04:56 AM.

 Tags calculate, circuit, developed, power, simple

 Related Site Pages Section Title Worksheet AC power Textbook AC bridge circuits : Ac Metering Circuits Textbook Mutual inductance and basic operation : Transformers Worksheet Energy, Work, and Power Textbook An electric pendulum : Resonance

 Similar Threads Thread Thread Starter Forum Replies Last Post earlt The Projects Forum 7 11-05-2009 02:19 AM fish4fun The Projects Forum 3 03-03-2009 04:04 AM danf100 The Projects Forum 8 01-12-2009 08:46 PM exidez Homework Help 5 09-16-2008 02:20 AM Houdini General Electronics Chat 1 12-13-2007 01:23 PM

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Electronics Forums     General Electronics Chat     The Projects Forum     Homework Help     Electronics Resources Software, Microcomputing, and Communications Forums     Programmer's Corner     Embedded Systems and Microcontrollers     Computing and Networks     Radio and Communications Circuits and Projects     The Completed Projects Collection Abstract Forums     Math     Physics     General Science All About Circuits Commmunity Forums     Off-Topic     The Flea Market     Feedback and Suggestions

All times are GMT. The time now is 04:51 AM.