Startup

A Watt = 1 Joule/ 1 second. How much energy does a 100W light bulb emit in 1 minute?

100Watts = (100 Joules / Second) (60seconds/1 min) = 6000J /min

6000 Joules of energy in 1 minute

S

Power

Power = Work done/time taken

Example: A person applies a horizontal force of 300N to a box and slides it 4m across the floor in 6 seconds.

1. How much work did she do?

2. What was her power output?

Work = Fr = 300(4) = 1200J

Power = work/time = 1200J/ 6s = 200 Watts

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Efficiency, e

e = Work output/Work input

or

e = Power output/ Power input

Example: An electric stove requires 1000Watts of electrical energy to produce 500Watts of cooking heat. What’s the efficiency of the stove?

e = Power output/ Power input = 500/1000 = .50 or 50% efficiency

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Example

A car engine is 30% efficient. For every 1000J of energy that comes fromburning gasoline, how much energy does the car gain?

Work out = e (Work in) = .30(1000J) = 300J

This means 700J are lost to heat energy.P

Example

Sunlight delivers 1000Watts to every square meter of surface it shines on.

1. A solar panel is 8% efficient and has an area of .4m2. What is the power output of the panel when sunlight hits it?

2. How many panels would you need to produce 2 kW?

Power out = e (Power in) = .08( 1000W/m2)(.4m2) = 32 Watts

2000W/(32 Watts/panel) = 62.5 panels or 63 panels.

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Electricity CostsThe kilowatt-hour

( kW.hr) = 1000Watts ( 3600seconds)

= 3,600,000 Joules

Power companies usually charge around .05$ / kW hr for electricity.

Cost = Rate . Time used . power ($/kw hr) (hr) (kW)

ExampleHow much does it cost to leave a 150W bulb on for 30 daysIf the power company charges .05$/kW.hr ?

Cost = Rate . Time used . power ($/kw hr) (hr) (kW)

Cost = (.05$)(30)(24)(.150) = $5.40

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Application

A

Work the power/efficiency problems in groups.

Connection

List some drawbacks and advantages of solar panels.

C

Exit

Without using formulas, describe the meaning of power and efficiency.