Which of the following best describes a watt?

• A. A unit of heat energy.
• B. A unit of power equal to one joule per second.
• C. A unit of force.
• D. A unit of momentum.

Answer: (B)

Power is the rate at which energy is transferred or work is done. A watt is defined as one joule per second, so if energy is delivered at a rate of 1 joule every second, the power is 1 watt. For example, a 60-watt light bulb uses energy at 60 joules each second. The other options describe different physical quantities—heat energy (joule is energy, not a rate), force (newton), and momentum (kg·m/s)—which are not measures of how quickly energy is transferred.