4.3 Power is the rate at which work is done.


Power can be calculated from work and time

       Power- The rate at which you do work

       Common meanings of the word power :

-     Source of energy (power plant)

-     Strength (powerful engine)

       Scientific definition of power- Doing work over a period of time

Calculating power from work

       To find power, divide the amount of work by the time it takes to do the work

Power= Work


P= W


       Watt- Unit of measurement for power

       Watt= One joule of work done in one second

       Measured in watts or kilowatts


       The amount of work a horse can do in one minute

       Read page 132 for background on Charles Watt

Power can be calculated from energy and time

       Power= Energy P= E

time t

       You can think about power as any kind of transfer of energy in a certain amount of time

       Also measured in watts (joules per second)

Everyday power

       Many appliances rely on electricity for energy

       How does a clothes dryer work?


Practice Problems

1. An Antarctic explorer uses 6000J of work to pull his sled for 60s. What power does he need? See page 132 for the answer


2. A light bulb used 600J of energy in 6s. What is the power of the light bulb? See page 134 for the answer