Question

A charged capacitor, C = 40 micro F, is connected to a resistor, 120 ohms. How much time will elapse until the voltage falls to 25% of its original (max) value?

im not even sure what equation to use for this, so if someone could help me to figure that out it wou
ld be greatly appreciated!

Answers

Henry
R = 120 Ohms.
C = 40 uF = 40*10^-6 Farads.

RC = 120 * 40*10^-6 = 4.8*10^-3 seconds

t/RC = t/4.8*10^-3 = 208.3t

1/e^208.3t = 0.25
e^208.3t = 4
208.3t*Ln e = Ln 4
208.3t = 1.386
t = 0.00666 s. = 6.66 Milliseconds

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