Capacitance

A capacitor is a device that stores electric charge on two separated conductors, like a pair of metal plates. The more voltage you push across it, the more charge piles up — and capacitance tells you exactly how much charge you get for each volt. Bigger plates or a smaller gap between them let a capacitor hold more charge at the same voltage.

Q = C · V

• Q — charge (C): the amount of electric charge stored on the capacitor
• C — capacitance (F): how much charge the capacitor stores per volt applied
• V — voltage (V): the electrical push, or potential difference, across the capacitor

A capacitor with a capacitance of 3 farads is connected across a 4 volt battery. How much charge does it store?

• C = 3 F
• V = 4 V

1. Q = C · V
2. Q = 3 F × 4 V

Q = 12 C

The same 3 farad capacitor is now connected to a 6 volt battery instead. How much charge does it store?

• 24 C
• 2 C
• 18 C

For a parallel-plate capacitor, if you double the area of the plates but keep the gap the same, what happens to the capacitance?

• It doubles
• It stays the same
• It is cut in half