S3.3 Power : the rate of working
Preamble
Reveal
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Preamble
Time considerations are not involved in the definition of work. In many instances however it is necessary to consider the rate at which work is done, as well as the total amount of work accomplished.[A] Definition
- Power is defined as the rate at which work is done. If an amount
of work W is done in time Δt, the average power over
that interval is defined to be
Key Point 3.5
Average Power: - The instantaneous power is the instantaneous rate of doing work:
Key Point 3.6
Instantaneous Power: - The SI unit of power is the watt (W), which is one joule per second (Js-1).

Commentary
- Avoid the misconception that there is something inherently electrical about the watt (or kilowatt)
- Power may be used to define new units of work eg. One kilowatt-hour is the work done in one hour by a force working at the constant rate of one kilowatt.
- Power is not just an abstract physical concept but has real monetary value (it can be bought and sold on the Stock Markets in units of kilowatt-hours!)
Using the definition of Work Equation 3.1 it is easy to show that
Learning Resources
![]() | HRW chapter 7.9 |
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