# Homework Help: Conceptual Capacitor question

1. Jun 11, 2012

### NewtonianAlch

1. The problem statement, all variables and given/known data
http://img803.imageshack.us/img803/382/74211498.jpg [Broken]

Basically when the switch has been closed for a long time, the inductor acts as a short-circuit and the capacitor as an open-circuit so current only flows through the resistor and then back into the negative terminal of the voltage source.

In the solutions the polarity of the capacitor is marked as opposite to how it's marked for the voltage source, I'm not too sure why.

Last edited by a moderator: May 6, 2017
2. Jun 11, 2012

### Staff: Mentor

If it's a polar capacitor (which it would be for so large of a value), then its +/- markings must match the voltage source. Can you post something that shows the incorrect polarity?

Last edited by a moderator: May 6, 2017
3. Jun 11, 2012

### Staff: Mentor

True.
In the solutions? Maybe you are looking at their answer to "Mark the capacitor polarity when the switch opens at t=0"?

4. Jun 11, 2012

### Staff: Mentor

Yikes, good point! Without a catch diode, that big polar cap is going to be toast if that switch gets opened...

5. Jun 11, 2012

### NewtonianAlch

The actual question was:

The switch in the circuit has been closed for a long time, but is opened at t = 0. Determine i(t) for t > 0.

Then it says for t < 0, the equivalent circuit is as follows, with a re-drawn circuit with a shorted inductor, and an open-circuited capacitor with the polarity reversed on the capacitor.

The only reason I can think of is because when a capacitor starts discharging the current flow is negative?

Last edited: Jun 11, 2012
6. Jun 11, 2012

### NewtonianAlch

I don't understand, is that because the value of the inductor is also rather high thus releasing a large amount of energy when that switch is opened? Which way would the current actually flow anyway if there was no diode?

7. Jun 11, 2012

### NewtonianAlch

This is it:

http://img855.imageshack.us/img855/8157/40697987.jpg [Broken]

Last edited by a moderator: May 6, 2017
8. Jun 11, 2012

### vela

Staff Emeritus
My guess would be because of the direction they assumed the current i(t) is flowing. By convention, you want the positive side to be the end where the current flows into the capacitor. At t=0, vc=-12 V.

9. Jun 11, 2012

### Staff: Mentor

Vela's probably hit the nail on the head: the diagram (post #7) is specifying a polarity by which you are to interpret the voltage across the capacitor, not the potential at a particular instant. Why they so specify it is not clear, since it appears that they're looking for the current...

When the switch opens the current that's flowing through the inductor will have no choice but to find its way through the capacitor path. At t=0 the circuit will "look like" a typical LRC oscillator caught partway through a cycle. Component values should tell you whether it's underdamped, overdamped, or critically damped. That should give a pretty good idea of the general shape of the response to expect.

10. Jun 11, 2012

### Staff: Mentor

Were they to show the left vertical bus grounded, then there would be no room for speculation. It would follow that Vc was defined.

11. Jun 16, 2012

### rude man

I don't think the intent of the question was to indicate polarity markings on the capacitor; the existence of polar caps is not usually part of an introductory course ... if you think 1/4F is a big cap (not really, with supercaps now available) what about a 1/4 H inductor?

BTW a supercapacitor would not blow up if reverse-biased. It would just build up its ESR.

12. Aug 24, 2012

### sumanth248

i want to ask a basic question about capacitor.even though capacitor plates are not connected directly then how current starts to flow in circuit?

Last edited by a moderator: May 6, 2017
13. Aug 24, 2012

### Staff: Mentor

Hi sumanth248, welcome to Physics Forums.

A capacitor consists of two parallel plates very close to each other. If electrons are pushed onto one plate, their electric field repels an equal number from the other plate. So if N electrons go into one terminal of a capacitor, N electrons are seen to emerge from the other terminalâ€”thus it appears that current has gone right through it.