Hi!
I have no idea how to approach this problem: what is the energy of the 3V voltage source at 15 minutes? Does the 3V voltage source provide energy or consume it? I know this DC stuff is supposed to be on the easy side, but I really have no idea where to begin and I really need to understand how this is done. Also apologies if I have chosen poor wording for the problem as English is my 3rd language.
Thank you for any help in advance!

 

The voltage source provides energy.

Is problem asking for total energy consumed by circuit in 15 min ?

Some definitions of energy (instantaneous) and over time -

https://www.physics.uci.edu/~silverma/units.html



Regards, Dana.

 

My best guess is that the problem is asking for the total amount of energy supplied by the voltage source over a fifteen minute time span.

But forget about energy questions for the time being and just analyze the circuit to find the voltages and currents in it.

If you are having trouble getting that far, show your work so that we can help you figure out where you are going wrong.

 

Thank you danadak and WBahn for your helpful replies!

I've been very busy with my undergrad (->Bs. Eng, ICT) studies in general and haven't been able to get back to you. Please forgive me for not thanking you earlier, as I'm also a parent of 3 small children and thus I'm rather schedule-challenged..

 

Hello there,

Is this a trick question? Energy can not be transferred in zero time except under highly theoretical situations.

I ask because the energy of a 3v source at 15 minutes is zero. The energy of a 3000v source at 15 minutes is zero also. The energy of 3 megavolt source at 15 minutes or 15 hours is still zero.

However, the energy of a 3v source over the time period of 0 to 15 minutes is only zero if there is no current flow. If there is current flow, then the energy is finite if the current is finite and can be plus or minus.

Thus either this is a trick question or they mean over the time period from 0 to 15 minutes which is more likely.

Energy is 'power' times 'time' and under most circumstances if the time is zero then the energy is zero.

Simple non zero 'time' example:
100 watts for 3 hours is 300 kilowatt hours. If your service provider charged you 10 cents per kw hour that would have cost you 3 cents.
Challenge:
See if you can figure out the cost over a one year period where there are exactly 365 days in that year, and using the same rate of 10 cents per kw hour and that same 100 watts.

 

...

Hi!
Thanks for your good and very valid insight!
I worded the question poorly, but then again, it was originally worded pretty poorly by my teacher. It is supposed to be over the time period from 0 to 15 minutes.

 

However, the energy of a 3v source over the time period of 0 to 15 minutes is only zero if there is no current flow.

Not quite true. There can be non-zero current during the entire time interval (except at the instances when the direction changes, if it does) and yet have zero energy delivery over a finite amount of time. This happens all the time with reactive components where they alternate between absorbing energy and delivering energy.

 

Not quite true. There can be non-zero current during the entire time interval (except at the instances when the direction changes, if it does) and yet have zero energy delivery over a finite amount of time. This happens all the time with reactive components where they alternate between absorbing energy and delivering energy.

Hi,

Good point, An ideal LC circuit can transfer energy back and forth forever and we might average it to zero over an integer multiple of cycles.
However, there's no way it can get that energy to begin with except as applied over a finite time interval if you want to get more technical. Thus over some time interval the energy was non zero. This is usually the startup part of the time analysis.
I do realize this is a more detailed look at this though, so i still accept your description and i think it is good to think about that aspect of it too.

Also, i thought we were talking about a DC circuit as that was what was brought up in the first post. I still like the AC view too though that you point out, very good to think about also.

 

Hi!
Thanks for your good and very valid insight!
I worded the question poorly, but then again, it was originally worded pretty poorly by my teacher. It is supposed to be over the time period from 0 to 15 minutes.

Hello again,

Well since it was a DC circuit it was a matter of looking at it logically as much as we possibly can barring some mistake in the questioning.
You may also like to consider the AC cases as WBahn had pointed out once you understand the DC case.