# Thread: Rather simple question... (What is "time constant"?)

1. ## Rather simple question... (What is "time constant"?)

So a math question says "An unspecified quantity of a radioactive element is stored in a safe vault in the year 2000. Six years later, we find there is only half the amount there was originally." one of the subquestions is to find the "TIME CONSTANT..."

I know how to do these types of questions (radioactive decay and all that jazz) so I don't need the actual answer or any explanations... rather I just need to know what exactly time constant IS. Is it the original quantity when time is zero? Or is it the half-life? Or what? How is it defined?

Thanks

2. the following general equation models exponential growth or decay (base e) ...

$\displaystyle A = A_0 e^{kt}$

$\displaystyle A$ = amount of material present at any time t

$\displaystyle A_0$ = amount of material at time t = 0

k is the time constant

3. Originally Posted by skeeter
the following general equation models exponential growth or decay (base e) ...

$\displaystyle A = A_0 e^{kt}$

$\displaystyle A$ = amount of material present at any time t

$\displaystyle A_0$ = amount of material at time t = 0

k is the time constant

Are you sure? I'm pretty certain k is the DECAY constant (because the question asks for both decay constant and time constant)...

4. not sure ... I do know that k is the time constant when dealing with RC and RL circuits.

here is a wiki link that seems to explain the difference ...

http://en.wikipedia.org/wiki/Exponential_decay