For the circuit shown in the figure above, C = 7.50 MuF, = 100 V, and R = 550 Om

Question

For the circuit shown in the figure above, C = 7.50 MuF, = 100 V, and R = 550 Omega. After having been at contact a for a long time, the switch throw is rotated to contact b. (a) What is the charge on the upper plate of the capacitor just as the switch throw is moved to contact b? ? MuC (b) What is the current just after the switch throw is rotated to contact b? ? A (c) What is the time constant of this circuit? ? ms (d) How much charge is on the upper plate of the capacitor 7.50 ms after the switch throw is rotated to contact b? ? MuC

Explanation / Answer

(a) initial charge on the capacitor   Q0 =CV0 Here C   = 7.5*10-6 C V0   = 100 V Q0   = 7.5*10^-4 C (b) Initial current  I0 = V0 /R   =   100 V / 550 = 0.18 A (c)Time constant = RC = ( 550)(7.5*10-6 C)   =4.125*10-3 s (d) at time t = 7.5*10-3 s This is the case of dicharging of a capacitor Q = Q0 e-t/RC    =  Q0 e-t/     plug all values we get charge on the upper plate of the capacitor after t = 7.5*10-3 s is Q = 1.22*10-4 C

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