5.5 p ower p. 250-254. power equals the amount of work done divided by the time interval during...
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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?
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¢
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