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Time Constant Calculations
52 questions By Tony R. Kuphaldt
Page 18
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Question 52 of 52
A 470 μF capacitor begins in a charged state of 270 volts, and discharges through a 100 kΩ resistor. How long will it take before the capacitor’s voltage will fall to a relatively safe value (30 volts or less)?
Reveal answerIt will take 103.3 seconds for the voltage to fall to 30 volts.
Notes:This question incorporates electrical safety into an RC circuit time calculation. Safety is something you should periodically revisit with your students, because it is so important. Ask your students how a “safe” value of voltage may be determined, and if there are any environmental circumstances impacting this determination.
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Maybe this will help someone else. The general formulas for V(t) and I(t) in question 25 (and the x(t) versions in question s 23 and 14) contain typos (or maybe hypertext coding glitches). They should actually be V(t) = (Vf-Vo)(1-e^(-t/𝛕)) + Vo, I(t) = (If-Io)(1-e^(-t/𝛕)) + Io in question 25. Those are correct in the PDF download version. In questions 23 and 24 the equations are x = xinitial + ( xfinal − xinitial ) ( 1 − e[(−t)/(τ)] ).