Question about capacitors and charge. Repeated.?

Question about capacitors.  Some one asked this, I spent time answering, then the question was deleted. So, to avoid wasting all that work, here is the question repeated.

1. A cap is charged to 60 volts. and discharged thru a 75 Ω resistor. What is the current when the cap has lost 90% of its charge.

2. when it has lost 90% of it's energy.

1. Q = CV

voltage is proportional to charge, so it has lost 90% of it's voltage, so the voltage is 10% of 60 volts, or 6 volts

I = V/R = 6/75 = 0.08 amps = 80 mA


Energy U = ½CV² = ½C60²

10% of that is C60²/2•10

and the voltage at that energy is

V = √(2E/C) = √(2(C60²/20)/C) = 60√(1/10) = 19.0 volts

I = E/R = 19/75 = 0.253 amps = 253 mA


question and answer, that is.

1 Answer

  • 4 weeks ago

    Nobody cares,

    If the original poster still needed an answer they wouldn't have closed or deleted the question. Sorry you took the time to type a detailed response, but sometimes that happens on this forum.

    Plus you posted in the wrong category. This went to the business & finance category - likely because yahoo saw the word "charged/charge" along with percentages and numbers.

    You should move this to the science & Mathematics section - or just delete it.

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