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Capacitors

Uses of capacitors in circuits

  • 3 things to know
  • Unit A2 2

Overview

Effect of using too small a smoothing capacitor, working voltage of a capacitor and how it depends on the plate separation.

What you need to know

Answer each one before you reveal it.

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Key term

Why does a capacitor block direct current but pass alternating current?

Key term

What decides how well a smoothing capacitor works?

Key term

What is the effect of using too small a smoothing capacitor?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Why does a capacitor block direct current but pass alternating current?

    Model answer

    A steady voltage charges it once and then no more charge flows, whereas an alternating voltage keeps charging and discharging it so current continues in the circuit.

  2. 2

    How is a capacitor used to smooth a rectified supply?

    Model answer

    It is connected across the output and charges at the peaks and discharges between them, so the voltage falls much less between one peak and the next.

  3. 3

    What decides how well a smoothing capacitor works?

    Model answer

    The time constant of the capacitor with the load, which must be long compared with the time between peaks.

  4. 4

    What is the effect of using too small a smoothing capacitor?

    Model answer

    The capacitor discharges substantially between peaks, so a large ripple voltage remains on the output.

  5. 5

    How is a capacitor used in a timing circuit?

    Model answer

    It charges through a resistor and the circuit switches when the potential difference reaches a set value, so the delay is set by the time constant.

  6. 6

    A timing circuit must give a delay of about 5 s using a 100 kΩ resistor. Suggest a suitable capacitance.

    Model answer

    C = τ/R = 5 ÷ 100 × 10³, which is 5 × 10⁻⁵ F, so a 47 μF capacitor is a suitable standard value.

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