Peoples Physics Concepts

8.7. Power and Efficiency http://www.ck12.org

8.7 Power and Efficiency

• Define and calculate power and efficiency.

Students will learn how to calculate power and efficiency. Students will also learn the true meaning of both.

Key Equations

P=W∆t; Power is equal to the energy released per second and has units of Watts ( 1 Watt = J/s).

Eff=PPoutin ; The efficiency equals the output power divided by the input power

Guidance

Power is the rate at which energy is being transferred. Power tells you how many Joules per second of energy is being

used to drive something. Thus power is simply the change in energy divided by the time. Since work is by definition

the transfer of energy (in or out of a system), power is also equal to the work divided by the time. Efficiency tells

you how efficient something is and gives a number between 0 and 1. If the efficiency is equal to 1, then the machine

is perfectly efficient (that is all the power used to drive it goes to the out put of the machine with no energy losses). If

the efficiency is zero, then all of the input power is lost in the machine and the machine can not output any energy.

Example 1