POWER

Power

• Suppose you and a friend are pushing boxes of books up a ramp and into a van.

• Each box is the same weight. • You make a game…who can push

the box up faster?

Power

• The friend moves the box up faster than you. She does it in 30s you do it in 45s.

• You both do the same amount of work.• Why? How does this happen? She’s

faster.• She was faster, and had more power,

but you did the same amount of work.

Power

• Power is the amount of work done in a certain amount of time.

• Power is a rate – • The rate at which work is done.

• Power = Work/time• P = W/t

Power

• It took 20 minutes (1,200 sec) to move a box. You did 40,000 Joules of work in the process. How much power did you use?

• t = 20min = 1,200 sec

• W = 40,000 J• P = ?

•Remember = Joules / second = Watt

•Power is always measured in Watts

•P = W/t

•P = 40,000 J/1,200 sec•P = 33.33 J/sec

•P = 33.33 W

Calculating Power

• Power is measured in – Watts (W)

• The watt is named after James Watt. He helped to develop the steam engine.

• A watt (W) is 1 J/s• A watt is a pretty small unit.

Calculating Power

• Large amounts of watts are usually shown in kilowatts.

• One kilowatt (kW) is equal to how many watts?

• 1,000 W • If you were to run up one flight of stairs

it would use about 1 kW of power.

Power & Energy

• Doing work transfers energy from one object to another.

• Energy can be transferred in other ways that don’t involve something moving.

• Lightbulbs…

Power & Energy

• A lightbulb uses electrical energy to produce heat and light…but no work is done.

• Power is made or used any time energy is transferred divided by the time needed for the transfer to happen.

• Anytime energy is transferred from one object to another, power can be calculated.

• Power = Energy/time• P=E/t

Changing Energy by Doing Work

• What happens to the energy of the book when you lift it off your desk?

• You changed the height of the book so it changed the potential energy.

• Where did this increase of energy come from?

• You transferred energy to the book by lifting it up.

Changing Energy by Doing Work

• You can also increase the kinetic energy of something by doing work on it.

• For example, pushing a chair…• When you push a chair, you do work

on it and it now has kinetic energy.

Changing Energy by Doing Work

• Rub your two hands together. What happens?

• They feel warm.• Why? • The energy of your hands increased

in the form of heat caused by friction. • Anytime you do work on an object,

you cause its energy to increase.