i have asked to do a question but i am not sure if it is right, the question is:a 35uF capacitor is charged to 500v it is then connected to a 20Mohms resistor, estimate time taken for charge to drop to 10v?my answer was around 2hrs, is this about right?thanks
the reaon i got to 2 hours was i first found out my time constant which is t=rc which is time = resistance x capacitance and my answer was 700, then i created a time curve with my time constant in 700sec increments along bottom and current up the side, my highest current value i used was 25uA because i used i = V/R. USING OHMS LAW i could find out voltage at 25ua, and as time goes on the current level droppped as did voltage, as on this curve i got 10v when time reached about 7350 secs ( 2 hrs ), am i right?

 

This is a simple calculus question. Can you do simple calculus?

 

what formula would you have used? how different would your way be compared to what i have done?

 

To get a rough figure, a capacitor discharges to 37% of its initial value in RC seconds. It discharges to 1% in about 5RC seconds.

It you want to do the proper calculation see:-
http://en.wikipedia.org/wiki/Capacitor#Current-voltage_relation

But this sort of maths always makes me lose interest!