The push switch is first pressed and made to close so it can remove any charge on the capacitor down to 0 volts. The capacitor is then said to be "discharged".
When a positive voltage is applied to the input of the resistor, the capacitor (C) will steadily start to charge up via the resistor (R). As the
capacitor has no charge at first, the output voltage at the tip of the arrow will be at 0 volts.
Consequently a Potential Difference (P.D. or voltage ) is initially seen across
the resistor (R). This means current can easily flow through the resistor (Ohm's
Law says this) and this allows the capacitor to start to charge.
As the charge increases, the output voltage will slowly start to rise too.This causes the charging current flowing through the resistor to reduce (as the voltage across
the resistor is getting less). This can be seen in the graph on the right.
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Circuit Diagram


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