- #1

mklein

- 43

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I am a school teacher, and whilst I KNOW the behaviour of a capacitor, I must admit I do not fully UNDERSTAND its behaviour.

Please see the left hand image – a capacitor and resistor in parallel over a battery:

http://www.mpklein.co.uk/cap_query.jpg

Textbooks will tell you that, initially, the voltage over the capacitor is zero (due to it having zero charge). Because resistor B is in parallel , the voltage across resistor B must also be zero. Therefore no current can flow through resistor B.

One problem I have with this is why the initial voltage across the capacitor must be zero. Surely if the spacing between the plates was increased a little (and perhaps the plate area reduced) this is equivalent to the other circuit – essentially a gap. Indeed, wouldn't the charge stored on a pair of leads, with a gap, be zero? Wouldn’t a gap in the circuit have the same voltage across it as the battery? How does the capacitor achieve zero volts despite being wired across the battery?

I also do not understand this logic: the voltage across the capacitor is zero, so because they are in parallel the voltage across resistor B is zero . Why is this? The capacitor is in parallel with the battery, so shouldn’t it have the same voltage across it as the battery?

The way I see it we have one route with a gap (the capacitor) and another route which should conduct (the resistor) – so why don’t we get a current through the resistor ? Please help !

Many thanks

Matt