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Chapter 5 Mechanical and Electrical Power, Work, and Energy 113

FIGURE 5-9 Ceramic capacitors (left) and electrolytic capacitors (right)

The amount of energy a capacitor can store is called its capacitance (C) and is
measured with a unit called a farad. We normally deal with tiny capacitors that are
measured in millionths of a farad, or microfarads (μF). Compared to other means of
energy storage such as batteries, the energy density of capacitors is low.

The energy a capacitor can store is calculated as follows:

Energy = 1/2 × Capacitance (C) × Voltage (V) 2

This gives you energy in a unit we haven’t talked about yet, called a joule. A joule is a
standard unit equivalent to the ft-lbs we measured mechanical power in earlier.

The advantages of using capacitors over batteries are that they charge much faster,
are very efficient, and give you high power over a short amount of time when they
discharge. The disadvantages are that they can get very big and expensive for a
capacitor equivalent to just an AA battery because of their poor energy density. For
example, you would need a soda-can-sized capacitor just to hold enough charge to
light a standard flashlight for a minute or so.