Which of the following is the correct definition of a watt-hour? A) A watt-hour is the amount of energy transferred by a process that has a power of one watt and acts for one hour. B) A watt-hour is the amount of time it takes for a one-watt device to transfer one joule of energy. C) A watt-hour is a measure of the power of a process that is equivalent to the transfer of one joule of energy in one hour. D) A watt-hour is the amount of energy that an electrical device transfers in one hour. E) A watt-hour is the amount of time it takes for a process to increase in power from zero watts to one watt.
As we consider what a watt is, we recall that it’s a unit of power equal to a joule of energy passing by a point every second. In this exercise, we’re interested in the unit not of watts but of watt-hours, watts multiplied by the unit of time.
If we multiply both sides of this units equation by a time unit, we see that that unit cancels out on the right-hand side, leaving us with the units of joules, energy. This means that as we scan through our candidate definitions of a watt-hour — A), B), C), D), and E) — we’ll look for one that gives watt-hours as an energy, something with units of joules.
We see that three of the definitions don’t meet the standard. Definition B) calls a watt-hour an amount of time, which it’s not. Definition C) calls watt-hour a measure of a power, which it’s not. And definition E) calls watt-hour an amount of time, just like B). This leaves us with options A) and D) remaining.
Both these definitions talk about a watt-hour being an amount of energy that’s transferred in a process that lasts one hour. It’s definition A) though that specifies that this amount of energy transferred over an hour has a power of one watt. Definition D) doesn’t specify the amount of power involved and so is not as good a definition as option A). A watt-hour then is the amount of energy transferred by a process that has a power of one watt and acts for one hour.