Capacitor is made of say two plates seperated by dielctic
material. When volage is apllied the ions on the plate are
zero so initially more current will flow like in short
circuit.
As time passes the charges(+ve and -ve) accumalate on
plates over a time and it opposes current flow. Now we say
capacitor charged fully. Now the time taken to charge fully
is called time constant of a capacitor. This time can be
varied by connecting a resistor in series with capacitor.
When high resisor is connected voltage drop across
capacitor is reduced so it take long time to charge fully.
Vica-versa for low resitor. The relation is exponetial
charging and discharging. Discharging process is also
similar way.  |