capacitance

Capacitance measures how much electric charge a device can store for each volt you apply, the way a bucket's width measures how much water it holds for each centimetre of depth. A big capacitance means the object soaks up a lot of charge without its voltage climbing much; a small one fills up quickly. It answers: how good is this object at hoarding charge?

Capacitance C is defined as the charge stored divided by the resulting potential difference: C = Q / V. Its SI unit is the farad (F), where 1 farad = 1 coulomb per volt. The farad is a huge unit, so real capacitors are usually measured in microfarads (10^-6 F) or picofarads (10^-12 F). Capacitance depends only on the geometry and materials, not on how much charge you happen to store: for a parallel-plate capacitor, C = epsilon_0 A / d, larger with bigger plate area A and smaller plate spacing d.

Because a charged capacitor stores energy U = (1/2) C V^2, capacitance also tells you how much energy a device can bank in its electric field. Capacitance appears everywhere: in the timing of circuits, in touchscreens that sense the capacitance of your finger, in camera flashes that dump stored charge in an instant, and in the memory cells that hold the bits inside a computer.