CapacitanceHard

Question

The switch S shown in figure is kept closed for a long time and then opened at t = 0, then the current in the middle 20 Ω resistor at t = 0.25 ms is :
                                                      

Options

A.0.629 A
B.0.489 A
C.0.189 A
D.23 μA

Solution

                            
Switch is kept closed for a long time,
Current through 20Ω resistor i =
q0 = 25 × × 20 = 150 μC 
at t = 0 , switch is opened, i = i0e−t/t  
                            t = 0  i = i0 e−t/t  
                            t = RC =  20 × 25  =  500 μS
Current
i = e-1/2 =  0.189 A
                                       

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