Transient analysis (RC) question problem

[munna007]

hello friends...

i am having following problem in this question.

first i don't understand how they have calculated R equivalent=5/6.i think its value should be 25Kohms. will it be different at 1ms and 4ms or same ?\\

i shall be thanks to u .
in one picture is the problem n in other is solution itself.

manish.

 

[n.karthick]

first i don't understand how they have calculated R equivalent=5/6.i think its value should be 25Kohms. will it be different at 1ms and 4ms or same ?

Yes, it is different before 1 ms and after that, because switch is opened at 1ms. At t=0, the voltage source becomes dead (i.e., it will be short circuited) and so the capacitor sees three resistors connected in parallel to it (effectively it is 5/6 kohm). But at t=1 ms switch is opened, the capacitor thereafter sees only one resistor (25kohm) connected to it.

 

[munna007]

thanks for the reply sir, you have solved my problem.
why is the voltage source dead at t=0. ?i m also confused with two differet time values..

v(t)=25.e^-2000t for 0<t<1ms.

tell me something about this equation...

 

[n.karthick]

why is the voltage source dead at t=0. ?

Because of the unit step function u(-t). Remember u(-t) is 1 for all t<=0 and is equal to zero for t>0.

i m also confused with two different time values..

v(t)=25.e^-2000t for 0<t<1ms.

Upto t=0, the voltage source is active and at t>0, capacitor starts discharging to all three resistors (the above equation is valid), but at t=1ms switch is opened and so after that capacitor discharges only through 25kohm.

 

[munna007]

after seeing such a marvellous reply , it came to my mouth ,wow !


thanks friend , u have helped me a lot.

 

[munna007]

CAN u provied me some pdf files on study on transient analysis ?.