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Fully understanding a capacitor circuit!

by mklein
Tags: capacitor, circuit, fully
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mklein
#1
Nov22-07, 11:37 AM
P: 43
Dear forum users

Firstly, I wanted to post in the General Physics area, but it wouldn't let me create a new thread there!!

I am a secondary school science teacher, teaching physics up to A-level.

I was in the process of revising capacitors with the class and it occurred to me that I may have a bit of a gap in my understanding.

Consider a simple series circuit with a cell, capacitor and resistor. While charging, charge collects on the capacitor plates until the voltage over the capacitor equals the voltage of the cell. While a current flows there can be a voltage over the resistor too.

When fully charged, the voltage of the cell will equal that of the capacitor. But there will be no voltage over the resistor.

Of course I am happy with the result, but I am having trouble explaining exactly WHY there is only a voltage over the resistor while a current flows – and then there is no voltage over it when the current stops.

Does this have something to do with the difference in meaning between an EMF and a voltage? i.e. the cell and capacitor create an EMF (like a ‘push’ due to static charge on the plates) whereas with the resistor just experiences a loss of potential due to resistance?

Any explanation of this result would be really appreciated

Matt Klein
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marcusl
#2
Nov22-07, 01:20 PM
Sci Advisor
PF Gold
P: 2,084
Welcome to the Forum!

The reason a voltage appears across the resistor has to do with forcing current flow. A voltage drop can be thought of as an energy potential difference needed to drive current through the resistor, which, aptly enough, resists the flow. Sometimes an analogy to water flowing in a pipe is helpful. A small pipe "resists" flow through friction with its walls. To produce a flow, you must drive the water through with a pressure differential across the pipe.

Here is a useful link:
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

Click on "Ohms Law", "Resistance" and "Resistivity" on the index at right, and explore the further links on those pages.
DaleSpam
#3
Nov22-07, 02:02 PM
Mentor
P: 17,541
Welcome Matt,

Don't forget Ohm's law:

V = I R

If I = 0 then V = 0, you cannot have one without the other.

Fullperson
#4
Apr12-09, 05:45 PM
P: 18
Fully understanding a capacitor circuit!

The way I see it is I imagine the capacitor with just two plates for simplicity. When the capacitor is charged, the plates are at the same voltage. Thus, there is no difference in voltage between the plates and so the charges don't flow. No flow of charge means no current.


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