Two identical capacitors  and 
 are charged to the same potential 
and are connected in two circuits at 
, as shown in figure. The charge on the capacitors at time 
 are respectively

                
                  Time  CR is known as time constant. It is time in which charge on the capacitor
decreases to  times of it’s initial charge (steady state charge).
In figure  PN junction diode is in forward bias, so current will flow the circuit 
charge on the capacitor decrease and in time  it becomes 
; where
In figure  junction diode is in reverse bias, so no current will flow through the circuit hence change on capacitor will not decay and it remains same, 
 
 after time 
.