5.5 POWERP. 250-254

When carrying a load up some stairs, you do the same amount of work whether you walk or run up the stairs. Power is the rate at which work is done.

Power

The unit of power is the joule per second, also known as the watt.

• One watt (W) of power is expended when one joule of work is done in one second.

• One kilowatt (kW) equals 1000 watts.

• One megawatt (MW) equals one million watts.

Power

POWER

Power (P) - the rate at which work is done, or the rate at which energy is transferred.

The formula for power is:

SAMPLE PROBLEM 1 An elevator and its occupants have a mass

of 1300 kg. The elevator motor lifts the elevator to the 12th floor, a distance of 40.0 m, in 74.8 s. What is the average power output of the elevator motor?

We can calculate the energy consumed of any device by rearranging out equation:

THE KILOWATT HOUR

The joule is the SI unit used to measure energy.

A joule per second, or watt, is used to measure power.

The kilowatt hour (kWh) is a larger unit of energy used by electric utilities to measure energy consumption.

Section 5.5

Calculating Energy Rating and Cost

What is the cost of operating a 1100 W microwave oven for 5.0 min if the price of electrical energy is 6.0¢/kWh? Use the red data from the problem to fill in the given information. Then, use the blue variables to complete the formula below. Click the screen for a hint.

power time interval

Required: ΔE; then cost

price of electrical energy =

Given: P = Δt =

Analysis: =

Section 5.5

Calculating Energy Rating and Cost

What is the cost of operating a 1100 W microwave oven for 5.0 min if the price of electrical energy is 6.0¢/kWh? Use the red data from the problem to fill in the given information. Then, use the blue variables to complete the formula below. Click the screen for a hint.

power time interval

Required: ΔE; cost

price of electrical energy =

Given: P = Δt =

Analysis: =

1100 W 5.0 min

6.0¢/kWh

PΔE

Δt

Section 5.5

Calculating Energy Rating and Cost

What is the cost of operating a 1100 W microwave oven for 5.0 min if the price of electrical energy is 6.0¢/kWh? Use the red data from the problem to complete the calculation. Then, complete the final statement.

= ( )(

)(1 KW/1000W)(1h/60 min )

ΔE = kWh

Solution: P =Δt

ΔE = PΔt

cost = kWh ( )

cost = ¢

ΔE

Statement: It costs to operate a 1100 W microwaveoven for 5.0 min.

Section 5.5

Calculating Energy Rating and Cost

What is the cost of operating a 1100 W microwave oven for 5.0 min if the price of electrical energy is 6.0¢/kWh? Use the red data from the problem to complete the calculation. Then, complete the final statement.

= ( )( )(1 KW/1000W)(1h/60 min)

ΔE = kWh

Solution: P =Δt

ΔE = PΔt

cost = kWh ( )

cost = ¢

ΔE

Statement: It costs to operate a 1100 W microwaveoven for 5.0 min.

1100 W 5.0 min

0.092

0.092 6.0¢/kWh

0.55

0.55¢

HOMEWORK

P. 251 practice #1-3P. 252 practice #1P. 253 practice #1P. 254 questions #1,2,4,5

Ch. 5 Review

P.261 ch. 5 self quiz #1-15 P. 262-267 ch. 5 review #1-23, 25-40, 42-44,

47,49-51, 57, 59

Unit review ch. 5 P. 363 -363 unit self quiz #1-6, 18-23, P. 364-371 #1-7, 33-42, 44, 45, 47, 73, 74,

79