# RC circuit time constant help

1. Feb 28, 2004

### Shaina

Hello there,

I m very new to the forum n doesn't know that much manners of this forum..

I actually wanna do a question...

we did a practical in our university in which we determined the unknown resistance by using a neon flash bulb...

I have to submit a report on that..i have to focus on RC circuit as well as TIME CONSTANT... i have searched a lot but didn't find much...plz help me ..

2. Feb 28, 2004

### chroot

Staff Emeritus
Can you clarify your exact question for us? Are you asking "what does 'time constant' mean?"

- Warren

3. Feb 28, 2004

### Shaina

Sure..

yes i surely wanna ask that..if u could suggest any URL related to the topic ..i will ne highly thankful to you...I have to submit reort on that as i earlier said that... so the ans i want must not be a definition of 2 line...

any help from u will be highly appreciated...

4. Feb 28, 2004

### chroot

Staff Emeritus
Try using google, or a textbook. Without a specific question, I'm afraid we can't help.

- Warren

5. Feb 28, 2004

### KingNothing

I'm not sure, but maybe it means that it's a constant used as a measure of time. Like for instance, my science teacher once said that lights, although we can't tell, flash about 60 times per second. If you were to judge things based on how many flashes a bulb makes, you might say you were using it as a "time constant". That's just my little interpretation.

6. Feb 28, 2004

### chroot

Staff Emeritus
7. Feb 28, 2004

### Integral

Staff Emeritus
Consider a simple RC circuit, the voltage drops in the circuit are iR due to the resistance, and q/C due to the Capacitance. Combined we get

iR + q/C = 0

but we have

$$i= \frac {dq} {dt}$$

which gives a differential equation

$$R \frac {dq} {dt} + \frac q C =0$$

This has solution

$$q = q_0 e^{- \frac t {RC}}$$

RC is called the Capacitive time constant.