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Preview of Period 9: Power
9.1 Measuring the Power of Appliances
How much power do appliances use?
9.2
Measuring Your Power
How much power do you use for daily
activities?
9.3
Measuring the Cost of Electricity
How much does electricity cost?
9.4
Growth in Electricity Use
How has electricity use increased over the
past century?
9-1
Review of Work and Energy
Work is done on an object against a force.
♦ An object is lifted against the force of
gravity
♦ A box is pushed across the floor against
the force of friction.
W=F D
Objects gain energy when work is done on
them.
♦ A raised object gains gravitational
potential energy.
♦ A box pushed across the floor gains
kinetic energy of motion.
Epot = M g h
2
Ekin = ½ M v
Machines make tasks easier by reducing the
force we must apply (Fin), but they cannot
reduce the amount of work needed.
9-2
What is Power?
Power is a rate describing the
amount of work done per unit of time:
Power = Work/time elapsed
P
=
W
t
We can also think of power as the amount
of energy transferred per unit time:
Power = Energy transferred/time elapsed
P
=
E
t
Power is measured in units of joules/second.
One joule/second = 1 watt
Power is also measured in ft-lbs/sec or horsepower (hsp).
One horsepower = 746 watts
One ft-lb/sec = 0.0018 hsp = 1.36 watts
9-3
Act. 9.1- 9.2: Power Requirements
• Light the bulbs using power generated by your
muscles. Which energy conversions take place
as you crank the generator?
• Do the power requirements of the appliances
match their wattages? Which has the largest
power requirement?
• How does increasing the “load” on the drill
affect its power requirement?
• Time yourself climbing the stairs. How much
gravitational potential energy do you gain?
Calculate the power required.
• How much energy is required to light the
bulbs connected to the exercise bicycle?
9-4
Measuring Power with a Wattmeter
1)
2)
Plug the wattmeter into the power strip
and turn it on.
Press the “Watt I” button.
3)
Clear the meter by adjusting the Zero
Adjust knob until the display reads 0 0 0.
4)
Plug the appliance into the outlet in the
cord attached to the wattmeter.
5)
Read the power requirement.
6)
Turn the meter OFF when you finish!
On/Off switch
000
Zero Adjust knob
Watt I
Plug in appliance
9-5
Act. 9.3: The Cost of Electricity
Power companies charge for electricity in units
of kilowatt hours (kWh). One kilowatt hour is
1,000 watts of energy provided per hour.
♦ The energy used equals the power required by
the appliance multiplied by the operating time.
♦ Find the energy used by an appliance by
solving the power equation for energy, E.
P
=
E
t
or
E
=
P
=
power (in watts)
E
=
energy (in joules)
t
=
time (in seconds)
P t
(Example)
How many kilowatt hours of energy are used
when a 100 watt light bulb burns for 6½
hours?
100 watts x
1 kilowatt
x 6.5 hours = 0.65 kWh
1,000 watts
9-6
Calculations with Electricity Costs
Example 1: Find the total cost of electricity
If electricity costs 7.5 cents/kWh, what is
the cost of 0.44 kWh of electricity?
0.44 kWh x $0.075 = $0.033
kWh
Example 2: Find the cost per kilowatt hour
If 0.44 kWh of electricity costs 3.3 cents,
what is the cost of electricity per kWh?
$0.033 x
1
=
0.44 kWh
$0.075
kWh
Example 3: Find the kWh used.
If electricity costs 7.5 cents/kWh, how
many kWh can you buy for 3.3 cents?
$ 0.033 x
1 kWh
$0.075
= 0.44 kWh
9-7
Minimizing Electricity Costs
Payback time is the time it takes to recover
the additional cost of purchasing an energy
efficient, more expensive appliance from the
savings in energy costs.
(Example 9.5)
A 25 watt CF bulb costs $5 to purchase and
lasts for 10,000 hours. A 75 watt incandescent
bulb costs $0.50 to purchase and lasts for
about 750 hours.
If electricity costs
$08.5/kWh, what is the cost of purchasing and
operating each type of bulb for 10,000 hours?
Compact Fluorescent Bulb
1.
Operating cost of CF bulb:
25 watts x
1 kilowatt
$0.085
x 10 ,000 hrs x
1,000 watts
kilowatt hour
=
$21.25
2. Purchase price of CF bulb = $5.00
3. Total cost = $21.25 + $5.00 = $26.25
9-8
Minimizing Electricity Costs, Continued
Incandescent Bulb
1.
Operating cost of incandescent bulb:
75 watts x
2.
1 kilowatt
$0.085
x 10 ,000 hrs x
1,000 watts
kilowatt hour
1 bulb
= 14 bulbs
750 hours
Purchase price of bulbs:
14 bulbs x
4.
$63.75
Number of bulbs needed for 10,000 hrs:
10 ,000 hours x
3.
=
$0.50
bulb
=
$7.00
Total cost = $63.75 + $7.00 = $70.75
Compare the cost of the two types of bulbs:
Cost of Incandescent
Cost of CF
= $70.75
$21.25
= 3.3
Using an incandescent bulb for 10,000 hours
costs 3.3 times more than using a CF bulb.
9-9
Estimated Power Requirements of Appliances
Appliance
Average
Wattage of
Appliance
Annual
KilowattHours
Annual Cost at
$0.10 per
Kilowatt-Hour
FOOD PREPARATION
Blender
Coffee Maker
Dish Washer
Freezer (frostless, 15 cu ft )
Frying Pan
Mixer
Oven, microwave
Oven, self cleaning
Range
Refrigerator (frostless, 12 cu ft)
Toaster
Waste Disposal
720
900
1200
440
1200
130
1500
3000
8200
320
1100
450
15
100
360
1800
200
13
300
1100
1200
1200
40
30
$1.50
$10.00
$36.00
$180.00
$20.00
$1.30
$30.00
$110.00
$120.00
$120.00
$4.00
$3.00
5000
1000
500
2500
1000
140
100
4200
$100.00
$14.00
$10.00
$420.00
1600
200
300
170
1300
180
100
15
400
1400
150
400
140
180
160
180
27
720
$140.00
$15.00
$40.00
$14.00
$18.00
$16.00
$18.00
$2.70
$72.00
1000
14
7
14
2
5
$1.40
$0.20
$0.50
70
50
100
56
40
225
$5.60
$4.00
$22.50
LAUNDRY
Clothes Dryer
Iron (hand)
Washing Machine
Water Heater
COMFORT
Air Conditioner (room)
Electric Blanket
Dehumidifier
Fan (rollaway)
Heater (portable)
Humidifier
Lamp, Incandescent
Lamp, Fluorescent
Waterbed Heater
HYGIENE
Hair Dryer
Shaver
Toothbrush
INFO/ENTERTAINMENT
Television
Stereo
Personal Computer
Act. 9.4: Growth Rates
Linear Growth
♦ Linear growth is constant.
straight line.
Its graph is a
♦ The same amount is added during each time
period.
♦ The amount added is independent of the initial
amount.
♦ The amount added is independent of the
number of elapsed time periods.
Exponential Growth
♦ Exponential growth is not constant. Its graph
is an upward curving line.
♦ The amount added changes with each time
period.
♦ Exponential growth doubles the amount of the
quantity during each time period.
♦ The amount added depends on the initial
amount and on the number of time periods.
♦ The doubling time is the length of time
required for the quantity to double.
9-10
Act. 9.4: Growth Rates
Electricity Production in the U.S.
3000
2500
2000
1500
1000
500
1995
1990
1985
1980
1975
1970
1965
1960
1955
1950
1945
1940
1935
0
1930
Kilowatt Hours (billions)
3500
9-11
Period 9 Summary
9.1:Power is the rate at which work is done or
energy is transferred.
P = W/t
or
P = E/t
Power is measured in joules/second, or
watts, in metric units and in ft-lbs/second
or horsepower in English units.
9.2:Electrical energy provided to homes is
measured and billed in kilowatt hours.
9.3:Payback time is the time it takes to recover
the additional cost of purchasing an energy
efficient, more expensive appliance from the
savings in energy costs.
9-12
Period 9 Summary, Continued
9.4:Electricity production in the U.S. has grown
rapidly – at times linearly and at other
times exponentially.
♦ Linear growth adds a constant amount to
the initial amount each time period.
♦ Linear growth rates are independent of the
initial amount and the number of
time periods.
♦
Exponential growth doubles the amount each
time period.
♦ Exponential growth rates are dependent on
the initial amount and on the number
of time periods elapsed.
♦ The time needed to double a quantity is
called its doubling time.
9-13
Period 9 Review Questions
R.1 What is power?
equations.
Give a definition and
R.2 Which of the following appliances would be
likely to use the most power per year:
microwave oven,
washing machine,
refrigerator, toaster, or range?
R.3 What is a kilowatt-hour? What quantity
does the electric company bill you for –
power, energy, or some other quantity?
R.4 How could it be profitable for a power
company to encourage customers to use
less electricity?
R.5 How can you determine whether a graph is
increasing linearly or exponentially?
9-14

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