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Charging Of A Capacitor: Quick Question watch

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    When a capacitor is discharging it never quite loses all of it's charge. This situation seems to have parallels with the charging of a capacitor. If I connect a capacitor across a 6V supply for example, will the potential difference across the capacitor actually ever reach 6V? Or will it just tend towards 6V?

    Yes indeed the maths, with the exponential function, says that the capacitor only fully charges or discharges when the time taken has reached infinity.
    The same maths tells us that a swinging pendulum never actually stops as a result of air resistance, or a mass on a spring never stops oscillating!

    Of course in practice, the value eventually gets so close to to zero (or its target value of 6V) that the difference falls below the limit of what can be measured.
    In the case of the capacitor the time is determined by the resistance in the circuit (as well as the actual capacitance). That's why it's common to talk about "time constants", where you specify how long it takes to drop (or rise) to a particular fraction of the original value.
    You also see this demonstrated with radioactive decay where you talk about half life. The exponential function governing this decay is essentially the same as the discharge of the capacitor.
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Updated: April 2, 2011
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