# Quick Question

Discussion in 'Homework Help' started by fishguts, Sep 19, 2007.

1. ### fishguts Thread Starter Member

Sep 3, 2007
22
0
I am trying to find the time constant for a circuit.I want to make sure I am doing this right. My resistor is 10K while my capacitor is 5pf. this would give me:

T=RC
T=10X10^3X5X10^-12
T=10,000X.000000000005
T=.00000005s

then .00000005s would convert to .05ns right?

2. ### niftydog Active Member

Jun 13, 2007
95
0
You were right until the last step.
0.00000005s = 50ns

I prefer to stay away from all those zeros and use the sum of the exponents. In this case you just do this;

10 x 5 = 50

3 + -12 = -9

^-9 = nano so it's 50ns

3. ### fishguts Thread Starter Member

Sep 3, 2007
22
0
alright thanks alot. I am not that great with exponents yet, but I am practicing.

4. ### fishguts Thread Starter Member

Sep 3, 2007
22
0
I have oe more question. I am trying to figure out the capacitor coltage after a certain time but I am stuck on using the formula I cant understand it at all. Does anyone know anylinks that explain how to use the formula better? I tried reading the article on this website but it was not any help.

Jun 13, 2007
95
0
6. ### fishguts Thread Starter Member

Sep 3, 2007
22
0
The part that is confusing me is the exponent part where it is t/T. I have no idea how the examples are getting the numbers they are getting in their solution at that part

7. ### niftydog Active Member

Jun 13, 2007
95
0
Small t is the time in seconds at which you want to know the voltage.

Big T (actually the greek letter tau) is the RC time constant (T = R x C).

8. ### fishguts Thread Starter Member

Sep 3, 2007
22
0
ok I got the formula down. I have one more question

I have a problem that says to plot Vc for 0<t<4t using at least for points. what does it mean by this notation of 0<t<4t? I have never seen a problem like this before

9. ### niftydog Active Member

Jun 13, 2007
95
0
t has to be defined before it can be used in this context, so unless they say "t=0.5s" or something then t is meaningless. You sure it's not "T" rather than "t"?

"0<t<4t" just means to plot the "t" axis from 0 up to "4 x t".

10. ### fishguts Thread Starter Member

Sep 3, 2007
22
0
here is an attachment if it helps
it is number 3b.

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11. ### JoeJester AAC Fanatic!

Apr 26, 2005
3,402
1,225
In your problem, you are going from time zero ( 0 < t ) to four time constants ( 4 tau ) .

You were given that tau is 2 mS ... so they wanted the plot to 8 mS.

12. ### fishguts Thread Starter Member

Sep 3, 2007
22
0
alrigt thats what I was kinda thinking. thanks for the help!

13. ### niftydog Active Member

Jun 13, 2007
95
0
Yes, look closely, the t after the 4 is actually "T" or tau, not "t".