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

time

P= W

t

· Watt- Unit of measurement for power

· Watt= One joule of work done in one second

· Measured in watts or kilowatts

Horsepower

· 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