Machines transfer energy from one store to another at a rate called power, measured in watts as joules per second. Power can be calculated using the equation for work done divided by time. Efficiency measures how well a system transfers energy and is defined as the useful output energy divided by the total input energy, expressed as a percentage.

1. •Machines, such as car engines, transfer energy from one energy store to
another every second.
•The rate of this energy transfer, or the rate of work done, is called power
Since power is defined as the rate of doing work.
•It can be expressed in equation form as

2. • 1 watt is the power of an agent, which does work at the rate of 1
joule per second.

7. 6. A crane does 20000 J of work in 40 s. Calculate its power.
7. A man carrying a bag of mass 25 kg climbs up to a height of 10 m in 50 seconds.
Calculate the power delivered by him to the bag.
8.Calculate the power of a motor that transfers 1200 J of energy every 5 s.
9. A man of mass 70 kg and a boy of mass 35 kg run up a set of stairs at height of 10 m in
the same time. Explain why the man is twice as powerful.
10. a) A crane lifts a mass of 60 kg to a height of 5 m. How much work does it do ?
b) The crane takes 1 minute to do this. Calculate the power of the crane.

8. Efficiency
The efficiency of a system is a measure of how well energy is transferred
in a system
Efficiency is defined as:
The ratio of the useful power or energy transfer output from a system
to its total power or energy transfer input
If a system has high efficiency, this means most of the energy
transferred is useful
If a system has low efficiency, this means most of the energy transferred
is wasted
The overall efficiency of a typical thermal power station is approximately
30%
This means that 70% of the energy produced is wasted energy
Energy is used to heat water to produce steam, that turns a turbine
which generates electricity
At each stage of this process, energy is dissipated to the surroundings

11. Calculating Efficiency
Efficiency is represented as a percentage, and can be calculated using the equation:
•The energy can be of any form e.g. gravitational potential energy, kinetic energy
•Where power is defined as the energy transferred per unit of time