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Capacitors: Why is this a Parallel Circuit?

by LorentzForce
Tags: capacitors, circuit, parallel
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Jun20-11, 11:53 AM
P: 4
Two capacitors are connected in parallel to each other and a battery. There are two switches, S1 between the first capacitor and the battery and S2 between the two capacitors. At first, S2 is open and S1 closed, so the first capacitor charges, then S1 is opened and S2 closed, so the charge is shared between the two capacitors.

My question is why are the two capacitor still considered to be in parallel when S1 is open. Here's a link to the actual question where the scenario came up, question 1 d:

This isn't homework, I'm just struggling witht he concept, it seems as if it should be in series when S1 is open, but apparently not?

Thank you in advanced for your explanations,

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Jun20-11, 12:18 PM
P: 5,462
Hello lorenzforce, welcome to Physics Forums.

The old question if you connect only one thing across something else is it parallel or series?

The easiest way is to perform the test in my attachment.

I have drawn two capacitors and your switch (closed).
Now consider what capacitance would be presented to the outside world by this combination at terminals T1 & T2.

Does this help to see that they are in parallel, so far as the outside world is concerned, although it is arguable whether they are in series or parallel within their loop.
But it is the face presented to the outside world that counts.

Does this help ?
Attached Thumbnails
Jun20-11, 12:19 PM
P: 505
Open means disconnected and closed means connected. When s1 is open, and s2 is closed, the capacitors are connected in parallel with eachother. Just not in parallel with battery any more.

Jun20-11, 02:04 PM
P: 4
Capacitors: Why is this a Parallel Circuit?

Thank You Studiot, I guess it's logical to look at it from an outside perspective. It also just occurred to me that when connected in series and then charged, the positive plate of one capacitor would be 'next to' the negative of the other capacitor, whereas in this case it is positive to positive, is that right?
That affects the way the circuit functions I guess. There could thus be two different amounts of charge stored on both capacitors which is not possible in a series circuit?

Thank You very much once more and thanks for your assistance Dmytry :)

Jun20-11, 02:41 PM
P: 5,462
If they are connected positive to positive it is called parallel.

If they are connected positive to negative it is called anti-parallel.
This arrangement is used with diodes and also with capacitors in loudspeaker crossover networks.
Jun20-11, 03:32 PM
P: 4
Fantastic, thank you. We just don't learn enough about this sort of thing before university (/college in the US)! I'll have to expand my electronics knowledge a little :)
Jun20-11, 03:50 PM
P: 5,462
I'm puzzled.

If you are in the US, what are you doing with last year's UK GCE paper?

I should have mentioned that anti- parallel is the posh name. They are more often called 'back to back' diodes or capacitors or whatever.
Jun20-11, 04:08 PM
P: 4
I am in the UK sorry to confuse! I just added that for clarification for any US members, I know they say uni too sometimes but just to be sure :) Thanks again :)

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