• The resistance introduces the element of time in the changing and discharging of a capacitor. • When a capacitor charge or discharge through the resistance, a certain time is required for the capacitors to charge fully or discharge fully. • The voltage across the capacitor cannot change instantaneously because a finite time is required to move charge from one point to another. • The time constant of the series RC circuit determines the rate at which the capacitor charge or discharge. The RC time constant is a fixed time interval that equals the product of the resistance and the capacitance in the series RC circuit.
Capacitors in dc circuits RC Constant Time
Capacitors in dc circuits When a capacitor is charged through a series resistor and dc source, the charging curve is exponential. C R I initial t 0 (b) Charging current V final t 0 (a) Capacitor charging voltage
Capacitors in dc circuits When a capacitor is discharged through a resistor, the discharge curve is also an exponential. (Note that the current is negative.) t t I initial 0 (b) Discharging current V initial 0 (a) Capacitor discharging voltage C R
Capacitors in dc circuits The same shape curves are seen if a square wave is used for the source. V S V C V R C R V S What is the shape of the current curve? The current has the same shape as V R .