Joule

A joule is the SI base unit for energy. It is equal to $1\frac{kg\cdot m^2}{s^2}$. In physical terms, lifting an apple one meter takes 1 joule of energy. This should not be confused with a watt which is a unit of power and a rate of how fast energy is used.

While the joule is the SI base unit for energy, when speaking in real-world terms, we often use the kilowatt-hour (kWh) instead. This is due to the fact that a joule is an extremely small amount of energy. To put how small a joule is into perspective, a liter of gasoline has 31,536,000 joules of energy in it. Using a single 100 W incandescent light bulb for ten hours (0.1 kW x 10 hrs = 1 kWh) would take 3,600,000 joules. Human activities take many joules of energy!

Figure 1. A 250 g baseball traveling at 20 m/s (72 kph) has 50 joules of kinetic energy.

Conversions

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