2. RATE OF ENERGYTRANSFER The rate at which work is done is called power. More generally, power is the rate of energy transfer by any method.
3. If the work done on an object is W in a time interval Δt, then the average power delivered to the object over this time interval is written as follows:
4. It is sometimes useful to rewrite this equation in an alternative form by substituting the definition of work into the definition of power.
5. Horsepower: The SI unit of power is the watt, W, which is defined to be one joule per second. The horsepower, hp, is another unit of power that is sometimes used. One horsepower is equal to 746 watts.
6. Example: A 193 kg curtain needs to be raised 7.5 m, at constant speed, in as close to 5 s as possible. The power ratings for three motors are listed as 1 kW, 3.5 kW, and 5.5 kW. Which motor is best for the job?
7. Efficiency: The ratio of used energy to total energy is called efficiency. useful energyoutput useful power outputefficiency = = total energyinput total power input Euseful Puseful e= = Etotal Ptotal %e = e×100