Time constant for the current through the inductor

AI Thread Summary
The discussion focuses on calculating the time constant for the current through an inductor in a circuit with a voltage of 6 volts, a resistance of 10 ohms, and an inductance of 100 mH. The correct formula for the time constant is established as τ = 2L/R. A participant initially suggests that the equivalent resistance is 2R, leading to a time constant of L/2R, but this is corrected to R/2, confirming the original time constant of 2L/R. The key takeaway is that the time constant for the current through the inductor is indeed 2L/R.
syhpui2
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Homework Statement



wjD8Y.png


http://i.imgur.com/wjD8Y.png

In the circuit below, V = 6 volts, R = 10 ohms, L = 100 mH. The switch has been open for a long time. Then, at time t = 0, the switch is closed.



What is the time constant for the current through the inductor?

2L / R


Homework Equations



TAU=l/R

The Attempt at a Solution



I am not sure how do I find the equvialent resistance in this case.
 
Last edited:
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Kill the source (short the voltage supply) and remove the inductor. Find the equivalent resistance at the terminals where the inductor was.

attachment.php?attachmentid=40334&stc=1&d=1319604076.gif
 

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gneill said:
Kill the source (short the voltage supply) and remove the inductor. Find the equivalent resistance at the terminals where the inductor was.

attachment.php?attachmentid=40334&stc=1&d=1319604076.gif


Equvialent resistance is 2r then time constant should be L/2R?

instead of 2L/R?

Thanks
 
syhpui2 said:
Equvialent resistance is 2r then time constant should be L/2R?

instead of 2L/R?

Thanks

Nope. Those two resistors are not in series with respect to the open terminals. So the resistance is R/2. That makes your time constant 2L/R.
 
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