# Capacitors charging equation help

Discussion in 'Homework Help' started by ItzKitkat, Nov 15, 2014.

1. ### ItzKitkat Thread Starter Member

Nov 9, 2014
33
0
Hi, can anyone please explain me b (determine the expression for ic)

I have tried it but it gave me 0.00045 something.

Here is what I calculated

(100/10000)e^(-0.125)/(0.0125)) = 0.000000453

I have one more question. What is -t in the image and how is it calculated?

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2. ### ItzKitkat Thread Starter Member

Nov 9, 2014
33
0
EDIT: I know how to b. I dont understand C

3. ### ItzKitkat Thread Starter Member

Nov 9, 2014
33
0
I don't know why I cant edit my post, but I know how to solve all the equations.

4. ### LDC3 Active Member

Apr 27, 2013
920
160
Just when I was about to ...
Good for you.

5. ### ItzKitkat Thread Starter Member

Nov 9, 2014
33
0
Hey I have one similar question.

In this question what is -t?

Here is what I am trying to do.

(100)e^(-125/0.125) = 0 V

Also for ic

(100/5000)e^(-125/0.125) = 0 A

EDIT: I get it now. If t is = -0 .

But I don't the last one

EDIT 2: I get all of them -.-.

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Last edited: Nov 16, 2014
6. ### LDC3 Active Member

Apr 27, 2013
920
160
t represents time. So, for the last one, t = 0.375 sec.
8t = 8 * 0.375 = 3

Vc = 100e(-8t) = 100e(-3) = ...
Ic = -20e(-8t) = -20e(-3) = ...

7. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
If you don't know what t or -t is, then how did you possibly solve the first problem?

8. ### ItzKitkat Thread Starter Member

Nov 9, 2014
33
0
@WBahn, I had to find V @ 0 seconds so t = 0

I have a similar question that I am stuck on. Can someone help me with this?

DC = 10 V
R1 = 2.4 Kilo Ohms
R2 = 1.0 Kilo Ohms
C = 100 uF

In the book it says SW1 is closed (conducting) and SW2 is open (not conducting).

I want to find the RC time constant, during discharge mode, what is the RC time constant in seconds?

Here is what the book says

This circuit has two modes of operation; when SW1 is closed (conducting) and SW2 is open (not conducting) the circuit is said to be charging, when SW2 is closed (conducting) and SW1 is open (not conducting) the circuit is said to be discharging.

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9. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Redraw the circuit with the switches configured for discharging. You can omit any components that, in this configuration, cannot affect the capacitor. What are you left with? What is the RC time constant for this circuit?

10. ### ItzKitkat Thread Starter Member

Nov 9, 2014
33
0
Since SW2 is open, there is no current/voltage going through. That leaves you with R1 and the capacitor

11. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Why is SW2 open? You said you wanted the RC time constant when discharging? What are the switch positions when discharging?

12. ### ItzKitkat Thread Starter Member

Nov 9, 2014
33
0
Thats what the book says

"This circuit has two modes of operation; when SW1 is closed (conducting) and SW2 is open (not conducting) the circuit is said to be charging, when SW2 is closed (conducting) and SW1 is open (not conducting) the circuit is said to be discharging."

13. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
I'll ask again. You say that you want to find the time constant during discharge mode. The book says that in discharge mode that SW2 is closed. But they you go and assume that SW2 is open and say that you did that because that is what the book said.

Forget about what the book says and look at the circuit. What positions do you need to put the switches in in order to discharge the capacitor?[/user]

14. ### ItzKitkat Thread Starter Member

Nov 9, 2014
33
0
Oh, I didn't read the second part.

To discharge, the switch has to be open.

15. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
So redraw the circuit with just the components that are still part of the circuit with the capacitor and then see what the time constant is.

16. ### ItzKitkat Thread Starter Member

Nov 9, 2014
33
0
So, time constant = 3400 ohms * 100 uF = 0.34 s

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17. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Are those switch positions (both closed) consistent with what is needed to discharge the capacitor? What did the text say about those switches?

Even if they are supposed to be closed, does that place the two resistors in series (as seen by the capacitor). This is another example of throwing formulas from a sheet at a problem without understanding what those formulas mean and when they do, and do not, apply.

18. ### ItzKitkat Thread Starter Member

Nov 9, 2014
33
0
As seen by the capacitor the R2 is parallel with the capacitor and series with R1

19. ### WBahn Moderator

Mar 31, 2012
18,087
4,917
Nope.

What is required for two components to be in series?

What is required for two components to be in parallel?

20. ### ItzKitkat Thread Starter Member

Nov 9, 2014
33
0
What do you mean by two components?