Ken said:

Hi,

This is a newbi questions I suppose.

If I take a voltage source say a battery, I put a capacitor with rating of

300volts to it, the charge went up to the maximum charge of the battery

(12.4volts). I thought the charge would accumulate up to 300 volts. Why does

it go to 300 volt ?

thank you

Ken

The voltage rating on a capacitor tells you the maximum voltage you

can safely apply to it and still have it function as a capacitor.

The relation between current and voltage for a capacitor is:

I=C*(dv/dt), where I is in amperes, C in farads, and dv/dt is the rate

of change of the voltage across the capacitor in volts per second.

When you first connect the capacitor across a voltage source, like a

battery, there is a large pulse of current, limited only by the

internal series resistance of the source and inside the capacitor.

But this large current pulse results in a high rate of change of

voltage and the capacitor quickly charges up till its voltage matches

that of the source. At that point, you have two equal voltages

bucking each other, and the current heads toward zero.

This is something like attaching a small air storage tank to a much

larger one. There is a brief blast of air through the connecting

hose, and then, as the small tank pressure approaches that of the

larger tank, the flow through the hose falls toward zero. You can not

get the small tank to reach a pressure higher than that inside the

larger tank that is filling it.