Upload
phungthien
View
219
Download
0
Embed Size (px)
Citation preview
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 OR Power = Energy
time time
Power is measured in units of watts.
1 watt = 1 joule/second
In English units, power is measured in foot-
pounds/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
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.
Plug in appliance
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 = 300 kWh
$0.15
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?
2) If electricity costs $0.10/kWh, how much does this
120 kWh cost?
$0.10 x 120 kWh = $12.00
kWh
kWh120hours200,1xwatts000,1
kilowatt1xwatts100
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 = $80 x 1 year = 4 years
savings each year $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 Kilowatt-Hours
Annual Cost at $0.10 per Kilowatt-Hour
FOOD PREPARATION Blender 720 15 $1.50
Coffee Maker 900 100 $10.00
Dish Washer 1200 360 $36.00
Freezer (frostless, 15 cu ft ) 440 1800 $180.00
Frying Pan 1200 200 $20.00
Mixer 130 13 $1.30
Oven, microwave 1500 300 $30.00
Oven, self cleaning 3000 1100 $110.00
Range 8200 1200 $120.00
Refrigerator (frostless, 12 cu ft) 320 1200 $120.00
Toaster 1100 40 $4.00
Waste Disposal 450 30 $3.00
LAUNDRY Clothes Dryer 5000 1000 $100.00
Iron (hand) 1000 140 $14.00
Washing Machine 500 100 $10.00
Water Heater 2500 4200 $420.00
COMFORT Air Conditioner (room) 1600 1400 $140.00
Electric Blanket 200 150 $15.00
Dehumidifier 300 400 $40.00
Fan (rollaway) 170 140 $14.00
Heater (portable) 1300 180 $18.00
Humidifier 180 160 $16.00
Lamp, Incandescent 100 180 $18.00
Lamp, Fluorescent 15 27 $2.70
Waterbed Heater 400 720 $72.00
HYGIENE Hair Dryer 1000 14 $1.40
Shaver 14 2 $0.20
Toothbrush 7 5 $0.50
INFO/ENTERTAINMENT Television 70 56 $5.60
Audio equpment 50 40 $4.00
Personal Computer 100 225 $22.50
Table 12.1 on
textbook
page 93