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**[SOLVED] R-L Circuit - Simple**

**1. Homework Statement**

In the figure below, suppose that the switch is initially open, and at time t=0, the switch is closed. Let t1=aL/R be the time that the current through the inductor L is 70.0 percent of its value when t is infinity. Find the dimensionless number a.

http://cse.unl.edu/~ejones/Images212/LR.gif [Broken]

**2. Homework Equations**

[tex]i=I_{0}e^{-(R/L)t)[/tex]

**3. The Attempt at a Solution**

I tried and no luck:

Given the problem, when t is 0 the current is initially 0. So I_{0} is 0.

And I want to find [tex].7i[/tex] (70% of i) and I already know that t1=aL/R

So plugging all the info in:

[tex].7i=0*e^{-(R/L)(aL/R)}[/tex]

[tex].7i=0[/tex]

[tex]i=0[/tex]

This is obviously wrong, and makes no sense to me. Help would be appreciated.

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