Am Wed, 02 Apr 2008 10:19:21 +0100 schrieb Invisible:

> Michael Zier wrote:
> 
>> "Much to learn you have!"
> 
> True enough...
> 
>> Take a capacitor: initially it's discharged. Now connect it to a
>> voltage source, the first instant you do, the voltage across the
>> capacitor is still zero
> 
> How on earth do you work that one out?

Out of continuity?

> If you connect a capacitor to a 9 V source, then the potential
> difference across the capacitor is... exactly 9 V. In which universe is
> that 0 V?

That's why I said "the first instant". dt (that's a differential "d") 
before you contact the cap's terminal, the voltage across the terminals 
is zero, right? Why should the potential be different one dt later 
(except for a dV)? We live in a continuous world, perhaps you dont, IDK, 
where most real-world measures are differentiable. And even if there was 
no (ohmic) resistance at all, at t=0 (contact the terminals) there must 
be a current into the cap first before the voltage across the cap can 
increase (that's causality!). And only if there's no (ohmic) resistance, 
you get the full 9V across the cap in finite time (namely 0+dt, and that 
may be mathematically sloppy to say so). In the real world, where wires 
have ohmic resistance, you'll *never* reach 9V across the cap in finite 
time: V(cap)=V(source)*exp(-t/RC)

Post a reply to this message