Calculating Inductance in an RL Circuit

AI Thread Summary
To calculate the inductance in the RL circuit, the relevant equations include the time constant τ = L/R and the current equation I(t) = (EMF/R)(1 - e^(-tR/L)). Given a 9.0 V battery, a 5.5 ohm resistor, and a current increase from 0 to 0.30 A in 0.17 seconds, the inductance can be determined by substituting known values into the equations. The attempt to find L yielded an incorrect value of 5.1, indicating a need for careful application of the equations. The discussion emphasizes the importance of correctly solving for the unknown inductance to progress to the next part of the problem.
n.hirsch1
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Homework Statement



An RL circuit has a 9.0 V battery and a 5.5 ohm resistor. When the switch is closed, the current in the circuit is observed to increase from 0 to 0.30 A in 0.17 s.
A) What is the inductance?
B) How long after the switch is closed does the current have the value 0.47 A?


Homework Equations


τ = L/R
EMF = L*I / t
I = EMF / R *(1-e^-tR/L)

The Attempt at a Solution


I don't know how to find a time constant, my text doesn't explain it, which makes me think that this question may not even use this equation. I can't get to part B without an answer to part A. By using the second equation I got L = 5.1, but that isn't correct.
 
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n.hirsch1 said:

Homework Statement



An RL circuit has a 9.0 V battery and a 5.5 ohm resistor. When the switch is closed, the current in the circuit is observed to increase from 0 to 0.30 A in 0.17 s.
A) What is the inductance?
B) How long after the switch is closed does the current have the value 0.47 A?


Homework Equations


τ = L/R
EMF = L*I / t
I = EMF / R *(1-e^-tR/L)

The Attempt at a Solution


I don't know how to find a time constant, my text doesn't explain it, which makes me think that this question may not even use this equation. I can't get to part B without an answer to part A. By using the second equation I got L = 5.1, but that isn't correct.

are you sure you don't know how?
 
No, I don't. Its asking for inductance, I have resistance, I have no idea how to get the time constant.
 
n.hirsch1 said:
No, I don't. Its asking for inductance, I have resistance, I have no idea how to get the time constant.

I(t) = \frac{v}{R} e^{\frac{-tR}{L}}
You know the value of current = 300mA at t = .17. v = 9 and r = 5.5. This leaves 1 unknown. Solve for it.
 
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