Time constant tao Which resistance do I use?

[frozenguy]

Homework Statement

The top circuit is an example my professor did in class, and how he solved Tao. What I'm concerned with is how he is figure R for L/R

The second example is one I found online.

How do I figure the R to use? Is it simply the entire thevinin equivalent or a specific equivalent?

The Attempt at a Solution

 

[asp_physics]

Find Thevenin resistance. That's the value you need to put.

In the second case, short the voltage source.
Thevenin resistance = 0.5k || (2k+3K)

In the first case, short the voltage source.
The vertical 6k becomes ineffective as there is a short across it. The 3k and the horizontal 6k at the top are in parallel with regard to the inductor. Hence thevenin resistance = 6k || 3k.

 

[frozenguy]

Find Thevenin resistance. That's the value you need to put.

In the second case, short the voltage source.
Thevenin resistance = 0.5k || (2k+3K)

In the first case, short the voltage source.
The vertical 6k becomes ineffective as there is a short across it. The 3k and the horizontal 6k at the top are in parallel with regard to the inductor. Hence thevenin resistance = 6k || 3k.

Oh man.. I forgot about shorting them out ahhh.. Thanks I appreciate it!