2. RATE OF ENERGY
TRANSFER
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.
8. Efficiency:
The ratio of used energy to total energy is
called efficiency.
useful energyoutput useful power output
efficiency = =
total energyinput total power input
Euseful Puseful
e= =
Etotal Ptotal
%e = e×100
