You have a 76.1-mF capacitor initially charged to a potential difference of 10.9
ID: 2030921 • Letter: Y
Question
You have a 76.1-mF capacitor initially charged to a potential difference of 10.9 V. You discharge the capacitor through a 3.01- resistor. What is the time constant? After what multiple of the time constant does the potential difference reach 2.81 V?
You have a 76.1-mF capacitor initially charged to a potential difference of 10.9 V. You discharge the capacitor through a 3.01-2 resistor. What is the time constant? After what multiple of the time constant does the potential difference reach 2.81 V? Time constant: Number Multiple of time constant: NumberExplanation / Answer
A.
Time constant is given by:
Tau = RC
tau = 3.01*76.1*10^-3 = 0.229 sec
tau = 0.23 sec
B.
In RC circuit Voltage is given by:
V = Vo*(1 - exp(-t/tau))
When V = 2.81 V, then
2.81 = 10.9*(1 - exp(-t/0.23))
exp(-t/0.23) = 1 - 2.81/10.9 = 0.742
exp(-t/0.23) = 0.742
take log both side
t = -0.23*ln 0.742
t = 0.069 sec
t = (0.069/0.23)*tau = 0.3*tau
multiple of time constant = 0.3
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