# R-L Circuit

1. May 3, 2009

### georgedaisuki

1. The problem statement, all variables and given/known data
Inductor 12 Henry, energy supply is Emf knot.

At t=0, the switch is opened which leads to rapid discharge along a resistor. The flash from the discharge lasted 50 ms. What is the resistance of the resistor?

2. Relevant equations
I know to use i=Ie^(-R/L) t but what would I use for the final current since it goes to zero?

Ln 0 is undefined.

3. The attempt at a solution

I thought it would be enough to say the final current is .0000001 and the initial current is 1. But taking ln of the final current gives many different numbers depending on accurate you make the initial current. Is there a better way to solve this>?

2. May 4, 2009

### nickjer

Well ideally it takes an infinite amount of time for the current to drop to 0 A. But in reality it is usually some factor of the time constant. Does your book mention something about how long it takes for the current to be considered 0 A? For example, $$5\tau$$. Where:

$$\tau = L/R$$