- #1

marsupial

- 45

- 2

## Homework Statement

The switch in the circuit in has been closed for a long time. At the switch is opened. a) Find i

_{0}for t>=0 (b) find v

_{0}for t>=0

I don't understand the effect of the switch in the position it is in. It's acting as a short circuit, I think, so presumably the capacitor doesn't charge. But I don't understand how they have calculated the initial current. Also, for the second part of the solution, they seem to have inserted a value of 0.1 for p in the voltage near the inductor. I am not sure where they got that from either.

## Homework Equations

The solution is given below, but there are a couple of things I don't understand:

## The Attempt at a Solution

I really wasn't sure why they did what they did. I tried to do a voltage divider to get the voltage at v(0) but then realized it was irrelevant anyway as it acted as a short when the switch was closed so the inductor wasn't charged anyway.

I now realize where they got 0.1 from - the initial current, as the capacitor is shorted and the inductor also acts as a short, so the initial current is 0.1 (from 80/800). But I still don't understand where they got their equation from for initial current, namely: io(0−) = 20/4000 = 5 mA

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