Solve RC Circuit Question: Time Constant & Current

[MightyDogg]

Homework Statement

Switch S has been open for a long time. It is then suddenly closed. Determine the time constant (a) before the switch is closed and (b) after the switch is closed. (c) Let the switch be closed at t=0. Determine the current in the switch as a function of time.

Homework Equations

T=RC
I=I(e^-t/rc)

The Attempt at a Solution

I actually already know the answers to this question because of my handy dandy answer key. I know part A is the simple T=RC where R = 50kΩ + 100kΩ and C = 10microF. However, I am confused on part B. Why does the resistance for the time constant change from 150kΩ to only using the 100kΩ resistor?

 

[phinds]

Why does the resistance for the time constant change from 150kΩ to only using the 100kΩ resistor?

Do you understand the concept of a short circuit?

 

[MightyDogg]

No. My professor has not spent much time on the topic. I actually did not know that this problem was a short circuit question.

 

[phinds]

No. My professor has not spent much time on the topic. I actually did not know that this problem was a short circuit question.

So now that you know, can you make use of the fact ?

 

[MightyDogg]

Since I do not have any notes on this subject I researched a bit online. From my understanding of what I read, short circuits are circuits that disregard a certain path because the current would rather go along the more unimpeded pathway? However, in this case, why wouldn't the current rather go along the 50kohm resistor since there is less resistance there?

 

[phinds]

Uh ... how do you get that 50kohm is less than zero?

 

[MightyDogg]

Wait... I think I have an error in my knowledge of what is fundamentally going on... I was comparing the resistance between 50 and 100. From your reply I am assuming that once the switch closes there can be a current flowing through that loop, so you only need the R for the loop with the least resistance? So that would be the right smaller loop and not the whole one? Sorry for my misunderstandings. I have a test coming up soon and I think I have a decent grasp of the material, it's just my professor never talked about this in class.

Edit: it's just I thought you needed to trace a path from the battery to the capacitor. I guess you do not.

 

[phinds]

Sounds like you have it now. The closed switch creates a short circuit that takes the voltage source and the 50kohm resistor out of the rest of the circuit. It isn't that the closed switch creates a path with smaller resistance, it creates a path with NO resistance --- that's what a short circuit IS.

Now, the stuff to the left of the short circuit can ALSO be analyzed as a separate circuit, in this case being just a voltage source and a resistor (and not seeing the capacitor and the other resistor at all)

 

[MightyDogg]

Sounds like you have it now. The closed switch creates a short circuit that takes the voltage source and the 50kohm resistor out of the rest of the circuit. It isn't that the closed switch creates a path with smaller resistance, it creates a path with NO resistance --- that's what a short circuit IS.

Now, the stuff to the left of the short circuit can ALSO be analyzed as a separate circuit, in this case being just a voltage source and a resistor (and not seeing the capacitor and the other resistor at all)

Okay, I understand it now. Thank you for your help!