What distinguishes a unit of energy from a unit of power?

• A. A unit of energy measures energy; a unit of power is the rate of energy use
• B. A unit of energy measures temperature
• C. A unit of power measures distance
• D. There is no difference between energy and power

Answer: (A)

Energy is the total amount of work that could be done or energy that could be transferred, while power describes how fast that energy is being used or moved. In units, energy is measured in joules, whereas power is measured in watts, and 1 watt equals 1 joule per second. This means you can use a certain amount of energy over a period of time, and the rate at which you do that work is the power.

A simple way to see it is through the relationship: Power = Energy ÷ Time (and Energy = Power × Time). For example, a device that delivers 60 joules of energy each second has a power of 60 joules per second, or 60 watts. If you keep using energy at that same rate for two seconds, you’ve transferred 120 joules of energy. This distinction explains why energy and power are not the same thing: one is the total amount available or used, the other is the speed at which that amount is used.

The other statements don’t fit because temperature measures how hot or cold something is, distance measures how far something travels, and saying there’s no difference ignores the clear relationship between an amount of energy and the rate at which it’s consumed.