# Homework Help: Transient analysis (RC) question problem

1. Jun 5, 2010

### munna007

hello friends....

i am having following problem in this question.

first i dont 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.

File size:
27.7 KB
Views:
191
File size:
28.3 KB
Views:
161
2. Jun 5, 2010

### n.karthick

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.

3. Jun 5, 2010

### 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.

4. Jun 5, 2010

### n.karthick

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.
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.

5. Jun 6, 2010

### munna007

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

thanks friend , u have helped me a lot.

6. Jun 6, 2010

### munna007

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