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 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

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