time constant question..

[transgalactic]

i understood every thing except at the last 30 seconds
http://www.youtube.com/watch?v=o9uJiyuajOM
he says "after 5 time constants t=rc" current will be 0
i cant understand by what mathematical equation he got this number 5
??

[transgalactic]

intuitively i know that after the capacitor will be filled with charge
it will stop being a conductor

but how he came up with the "5"
??

[Hao]

The 5 he choose is an example of a 'magic number'.

We know that current decays as I = I_{initial} e^{-\frac{t}{R C}}.

Hence, as time passes, I gets smaller and smaller.

After 1 time constant,
\frac{I}{I_{initial}} = e^{-1} = 0.368
After 2 time constants,
\frac{I}{I_{initial}} = e^{-2} = 0.135
After 3 time constants,
\frac{I}{I_{initial}} = e^{-3} = 0.050
After 4 time constants,
\frac{I}{I_{initial}} = e^{-4} = 0.018
After 5 time constants,
\frac{I}{I_{initial}} = e^{-5} = 0.007

So a more accurate statement is, "After 5 time constants, the current is approximately 0."

He could have easily said 4 or 6 or 100. They key point is that after about 3-5 time constants, the current is effectively 0.

Edit: Formatting error, sorry,

[transgalactic]

The 5 he choose is an example of a 'magic number'.

We know that current decays as I = I_{initial} e^{-\frac{t}{R C}}.

Hence, as time passes, I gets smaller and smaller.

After 1 time constant, \frac{I}{I_{initial} = e^{-1} = 0.368}
After 2 time constant, \frac{I}{I_{initial} = e^{-2} = 0.135}
After 3 time constant, \frac{I}{I_{initial} = e^{-3} = 0.050}
After 4 time constant, \frac{I}{I_{initial} = e^{-4} = 0.018}
After 5 time constant, \frac{I}{I_{initial} = e^{-5} = 0.007}

So a more accurate statement is, "After 5 time constants, the current is approximately 0."

He could have easily said 4 or 6 or 100. They key point is that after about 3-5 time constants, the current is effectively 0.
xyzt

[transgalactic]

thanks