# The resistance introduces the element of time in the

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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 .