Capacitance and the farad

Overview

Charge stored on a capacitor, construction of a simple capacitor and how a capacitor becomes charged when connected to a supply.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Define capacitance.

   Model answer

   Capacitance is the charge stored per unit potential difference across the capacitor.

2. 2

   Give the defining equation for capacitance and define each symbol.

   Model answer

   C = Q/V, where C is the capacitance in farads, Q the charge stored in coulombs and V the potential difference in volts.

3. 3

   Define the farad.

   Model answer

   One farad is the capacitance of a capacitor that stores one coulomb of charge for each volt of potential difference across it.

4. 4

   Why is the farad an inconveniently large unit?

   Model answer

   Storing a coulomb per volt is enormous for a practical component, so real capacitors are usually rated in microfarads, nanofarads or picofarads.

5. 5

   Express 470 μF in farads.

   Model answer

   470 × 10⁻⁶ F, which is 4.70 × 10⁻⁴ F.

6. 6

   What is meant by the charge stored on a capacitor?

   Model answer

   The magnitude of the charge on either plate, since one plate carries +Q and the other −Q and the capacitor as a whole is neutral.

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