V = 240 ( 1 - e -(rt)L )

given R = 1 ohm

given L = 200mH

calculate the time when the voltage is expected to be 140v

been trying for ages just cant crack it

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- Jan 19th 2009, 01:29 AMlewuk09Can anyone solve me this maths equation, please?
V = 240 ( 1 - e -(rt)L )

given R = 1 ohm

given L = 200mH

calculate the time when the voltage is expected to be 140v

been trying for ages just cant crack it - Jan 19th 2009, 02:44 AMmr fantastic
First you should make sure you post the correct equation. I assume it's a series RL circuit so note the missing vinculum I have added (in red).

Now substitute the given data into the equation (note that 200mH = 0.2 H):

$\displaystyle 140 = 240 \left( 1 - e^{-t/0.2}\right) = 240 \left( 1 - e^{-5t}\right)$

$\displaystyle \Rightarrow \frac{140}{240} = 1 - e^{-5 t}$

$\displaystyle \Rightarrow \frac{7}{12} - 1 = - e^{-5 t}$

$\displaystyle \Rightarrow \frac{5}{12} = e^{-5 t}$

$\displaystyle \Rightarrow \ln \frac{5}{12} = -5 t$

and I'm sure you can do the finishing touches.

(Since I've worked in SI units the time you get will be measured in seconds).