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Q & A: capacitor discharge

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Most recent answer: 07/08/2009
Q:
What would happen if we charge or discharge a capacitor without a resistor in the circuit?Will there still be heat loss?
- Nimish (age 17)
Mumbai, India
A:
That's an interesting question. In practice, there will always have to be some heat generated, but there's no fundamental minimum.

Say that the capacitor is discharged through a DC electric motor, hooked up to lift a weight as it turns. Such motors can be fairly highly efficient. Most of the energy then goes into the potential energy of the lifted weight.

Mike W.

If there is some inductance involved, and there usually is, there will be oscillations that will transform all of the stored energy of the capacitor into electromagnetic radiation. 

LeeH

(published on 07/06/2009)

Follow-Up #1: loss from capacitor

Q:
Would the heat lost in the two cases-with the resistor and without it, amount to cv^2/2? If yes, how? And also about the coupled motor-is its efficiency more than 40%(just a rough idea please if possible)?
- Nimish (age 17)
Mumbai, India
A:
The capacitor is going to lose all its energy, CV2/2, either way. In the case where a motor is used to store the energy in another form (say as gravitational potential) only a fraction of that energy is thermalized, i.e, 'turned to heat'.

Electric motors can be very efficient, so better than 40% conversion is indeed possible. The main problem is that the voltage gradually decreases, so that simple motor won't run at its optimum voltage. In principle there are ways around that.

Mike W.

(published on 07/08/2009)

Follow-up on this answer.