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Electrical Quizzes - QUIZ 8 POSTED

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LAN_403

HV Enthusiast

Sun Feb 26 2006, 10:49PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Heres another capacitor problem . . .

You have an oil filled capacitor that consists of two plates (submerged in the oil) where the dielectric constant of the oil is 10. The measured capacitance of the capacitor is 10uF and it is charged to 10V (supply removed afterwards) The oil is then drained from the capacitor case and the capacitance drops to 1uF.

Three parts:

1. What is the voltage on the capacitor after the oil was drained?

2. What is the energy stored by the capacitor in each case? If the energy changed, where did the energy go?

3. Finally, under what circumstance would there be no change in energy stored in the capacitor?

Penguin7471

Mon Feb 27 2006, 06:20AM

Registered Member #71 Joined: Thu Feb 09 2006, 08:23AM
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Posts: 63

um, in response to a bit of the 2nd part, the lost energy is lost due to the kinetic energy required to tip the oil out?

WaveRider

Mon Feb 27 2006, 09:44AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

1. 100V
2. Gravity "pulling the oil out" from between the plates supplies the energy. Before removing oil E=500uJ, after E=5000uJ..
3. If the plates moved together until the distance between them was 10% of the original distance. or..the oil remained between the plates by the enhanced capillary action caused by the presence of the electric field between the plates...

HV Enthusiast

Mon Feb 27 2006, 01:41PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Congratulations Waverider. You could also do #3 by reducing gravity so the oil would maintain itself between the plates. Nice job.

WaveRider

Thu Mar 02 2006, 09:07AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

OK Next quiz...a bit of classic circuit theory.. It shouldn't be too hard for all those DRSSTC builders out there!

1. You have two identical LC circuits (i.e. each isolated LC circuit has exactly the same resonant frequency). What happens to the resonant frequency as you increase the coupling between the two LC circuits? Hint: As you vary the frequency of Io, what happens to Va, Vb?

2. Assume the coils/capacitors are not perfect (there is some parallel resistance). What effects does this have on the resonance (i.e. how does it change the frequency response Vout/Io)?

Enjoy!

PS - Not looking for maths :-@ ... Just describe what happens......

IamSmooth

Mon Mar 06 2006, 08:04PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
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Posts: 1567

I'm not too good with coupling theory.
Is there a time limit before an answer should be posted and we move on?

WaveRider

Tue Mar 07 2006, 08:31AM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

I would like to leave this quiz for a little while.. It is fundamental to many of the tesla coil and RF projects presented on this board.... Why don't you dig up a good analog circuit design text...or, if you have access to measurement gear, do the experiment? We do this for fun, right?

HV Enthusiast

Tue Mar 07 2006, 05:55PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

You can leave it up here, but i'm going to go ahead and let the next quiz begin. So if anyone has a quiz they want to post, go ahead and post it.

Thanks
Dan

IamSmooth

Tue Mar 07 2006, 09:02PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I know I am not up in all your leagues, but I will post a quiz and hope it is ok.

There are two conductive spheres, A and B. A is smaller with a radiius r; B is larger with a radius R. A sits in the center of B, which is hollow, and is suspended by a piece of non-conductive thread.

The question is this: if there is a charge Qa on sphere A and a charge Qb on sphere B, which sphere is at the higher potential and why? If one makes a conductive connection between the two, which way does the charge flow?

...

Wed Mar 08 2006, 01:05AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

if there is the same charge on both spheres I would say that sphere A is at a higher voltage (due to smaller surface area), so if you were to connect the two spheres together you would have e- flowing from the larger sphere to the smaller one... (assuming that the spheres are positively charged, if they were negatively charged the e- would flow the the other way).

not sure though...

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