WELCOME TO PERIOD 12
Homework Exercise #11 is due today.
PHYSICS 1103 – PERIOD 12
•What is power?
•How is electrical energy measured?
•How can you save money when using
electricity?
•Remember to put away your phone.
No calls or texting during class.
Power
Power is a rate describing the amount of work done or
energy transferred per unit of time:
Power =
Work
time
OR
Power = Energy
time
Power is measured in units of watts.
1 watt = 1 joule/second
In English units, power is measured in footpounds/second (ft-lbs/sec) or horsepower (hsp).
Power requirements
Light the bulbs using power generated by your
muscles.
Compare the appliance wattages (given on the
appliances) to the measured wattage.
How does increasing the “load” on the drill affect its
power requirement?
Measuring power with a wattmeter
Plug in
appliance
1) Plug the wattmeter into
the power strip.
2) Plug the appliance into the
outlet in the front of the
wattmeter.
3) Turn the appliance on.
The appliance wattage
appears in the display
screen.
Select
Mode
Human power
How much power is generated when a person climbs
the classroom stairs?
Power =
Epot = M g h
time
t
What is the weight (M g) of the person?
How high did the person rise (h)?
How long did it take to climb the stairs (t)?
Energy to ride the bike
How many watts of power light the bulbs connected
to the exercise bicycle?
How long were the bulbs lit?
Find the energy to light the bulbs by solving the
power equation for the energy, E.
P = E
t
Measuring electricity
• Electric companies charge for electricity in units of
kilowatt hours (kWh).
• One kilowatt hour = 1,000 watts of power provided for
one hour.
• The spinning disc on the kilowatt-hour meter
measures the electricity provided.
• The disc is connected with gears to the hands on the
kilowatt-hour meter dials that show the kWh used.
Calculating the cost of electricity
On an electric bill, find:
1) The monthly cost of electricity and
2) The number of kilowatt hours of energy used.
Use a ratio to find the cost per kilowatt hour (kWh):
monthly cost of bill = cost
monthly kWh used
kWh
Using ratios to find electricity costs
Find the total cost of electricity used:
If electricity costs 15 cents/kWh, what is the cost of 244
kWh of electricity?
244 kWh x $0.15 = $36.60
kWh
Find the kilowatt-hours used:
If electricity costs 15 cents/kWh, how many kWh can
you buy for $45.00?
$45.00 x
1 kWh
$0.15
= 300 kWh
Example: the cost of using a light bulb
1) How many kilowatt hours of energy are used when a
100 watt light bulb burns for 1,200 hours?
100 watts x
1 kilowatt
x 1,200 hours  120 kWh
1,000 watts
2) If electricity costs $0.10/kWh, how much does this
120 kWh cost?
$0.10 x 120 kWh = $12.00
kWh
Find the cost of using the incandescent and compact
fluorescent bulbs. Subtract to find the cost savings.
Payback time
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.
• An energy-efficient dishwasher costs $80 more than a
less efficient dishwasher.
• The energy-efficient dishwasher saves $20 each year
in operating costs.
• What is the payback time for the dishwasher?
additional cost =
savings each year
$80 x 1 year = 4 years
$20
Cost of purchasing and using appliances
A washer costs $950 to purchase and $75/year to
operate.
What is the total cost of this washer after 5 years?
Total cost = purchase price + (cost x # of years)
year
Total = $950 + ($75 x 5 yrs) = $950 + $375 = $1,325
yr
Find the total cost of using two appliances for the
given number of years. Subtract to find the savings.
A windmill generates electricity
A backyard windmill generates an average of 628 watts.
How many kilowatt-hours per year does it generate?
628 watts x 1 kilowatt x # of hours/year = ? kWh/yr
1,000 watts
How much would it cost to purchase 10 years of these
kWh from an electric company at $0.12/kWh?
$0.12 x ? kWh x 10 yrs = ??
kWh
yr
Payback time for the windmill
How much would it cost to purchase and operate this
windmill for 10 years?
Total cost = purchase price + (cost x # of years)
year
Which costs less after 10 years – generating
electricity with the windmill or buying electricity from
the electric company? How much less?
Appliance
Average
Wattage
of
Appliance
Annual
KilowattHours
Annual Cost
at $0.10 per
Kilowatt-Hour
FOOD PREPARATION
Table 12.1 on
textbook
page 93
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
Audio equpment
Personal Computer
BEFORE THE NEXT CLASS…
Read textbook chapter 13
Complete Homework Exercise 12
Bring a blank Activity Sheet 13 to class.
Watch video 3 Edison’s Miracle of Light for
discussion during period 15