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.
- Coffee DrinkerLv 74 weeks ago
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.