PowerPower

PowerPower

Power (P)Power (P) – rate at which work is – rate at which work is done.done.• Also, rate at which energy is Also, rate at which energy is

transformed.transformed.

P = P =

SI Units:SI Units:

WorkWorktimetime

JJss

kg*mkg*m22/s/s22

ss== kg*mkg*m22/s/s33== Watts Watts

(W)(W)==

Be Careful!Be Careful!

Be careful not to confuse Work with Watts.Be careful not to confuse Work with Watts.• The Difference:The Difference:

Work is a quantity.Work is a quantity.•W = FdW = Fd

Watts are units.Watts are units.•P = 100 WP = 100 W

PowerPower

Power = Power =

Power = Force x velocityPower = Force x velocity• P = FvP = Fv

Assuming Assuming FF and and vv are in the same are in the same dimension.dimension.

Force x distanceForce x distance

timetimedistancedistance

timetime

PowerPower

John does 400 J of work in 10 John does 400 J of work in 10 seconds. How much power does John seconds. How much power does John develop?develop?• P = W/tP = W/t• P = (400 J)/(10 s)P = (400 J)/(10 s)• P = 40 WattsP = 40 Watts

PowerPower

If the power needed to operate a If the power needed to operate a 100.-Watt100.-Watt light bulb is redirected to lift a light bulb is redirected to lift a 1.00-kg1.00-kg crate crate at constant speed, how far is the crate lifted at constant speed, how far is the crate lifted in in 60.0 seconds60.0 seconds??• P = W/tP = W/t• P = Fd/tP = Fd/t• P = mgd/tP = mgd/t• (100. W) = (1.00 kg)(9.81 m/s(100. W) = (1.00 kg)(9.81 m/s22)d / (60.0 s))d / (60.0 s)• (100. W) = (9.81 N)d / (60.0 s)(100. W) = (9.81 N)d / (60.0 s)• (6.00x10(6.00x1033 J = (9.81 N)d J = (9.81 N)d• d = 612 m (!!!)d = 612 m (!!!)

PowerPower

An elevator weighing An elevator weighing 1000. N1000. N rises rises vertically at a constant vertically at a constant 2.00 m/s2.00 m/s. . What power does the elevator What power does the elevator develop? (Neglect friction).develop? (Neglect friction).• P = W/tP = W/t• P = Fd/tP = Fd/t• P = FvP = Fv• P = (1000. N)(2.00 m/s)P = (1000. N)(2.00 m/s)• P = 2.00x10P = 2.00x1033 W = 2.00 kW W = 2.00 kW

What Do You Pay For?What Do You Pay For?

The “power” company doesn’t charge you for The “power” company doesn’t charge you for power...power...• They charge you for energy.They charge you for energy.

Measured in kilowatt-hours.Measured in kilowatt-hours.• 1 kWh = energy changed (work done) by a 1000-1 kWh = energy changed (work done) by a 1000-

W power source for one hour.W power source for one hour.• 1 kWh = 1000 W x 3600 s = 3,600,000 J1 kWh = 1000 W x 3600 s = 3,600,000 J

It costs as much to run a 1000-W heater for It costs as much to run a 1000-W heater for an hour as it does to run a 100-W light bulb an hour as it does to run a 100-W light bulb for 10 hours.for 10 hours.

By the , I have the power!WWGreyskullGreyskull

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By your Fcombinedv, I am Captain Planet!

WWmightymighty

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