Problem:
How can you use your home's electric meter to measure the total power of all the electric devices operating at any given time?
Background:
You might have noticed that your electric meter has a circular, flat metal plate that is circular and rotates. There is a black mark on the plate so that you can count or time the rotations. The rate of rotation, usually measured in rotations per minute (r/min.), relates directly to the power being used at the time.
On the face of the meter look for a marking such as Kh6. This is known as the meter constant. The meter constant allows you to calculate the power in watts being used at any time in your home. Mathematically the power is found by:
P(watts) = (r/min.) x 60 x Kh
r/min. = the number of rotations of the plate in one minute
60 = the number of minutes in one hour
Kh = the meter constant
P(watts) = (the number of rotations of the plate in one minute) x (minutes in one hour) x (meter constant)
For example, if you counted two revolutions of the plate in one minute, and the meter constant was 6, then the power reading would be:
P(watts) = 2 x 60 x 6 = 720 W
Practice Problems:
1. If your meter constant (Kh) is 2.5 and your plate rotates seven times per minute, what is the power used per hour?
2. Your plate rotates nine times per minute and the meter constant (Kh) is 1.9, what is the power used per hour?
3. Your teacher will give you a worksheet to use with your home electric meter.
Worksheet
1. What is the meter constant (Kh) of your electric meter at home? Kh =____________
2. Choose a period of time to record four meter readings at one hour intervals.
Taking a reading:
1. Using the second hand on a watch, determine how many times the metal plate rotates in one minute.
2. Record the number of rotations in a table.
3..Repeat these steps three more times at one hour intervals.
4. Record types of electrical devices being used at the time of each reading.
Remember: Power (watts) = (r/min.) x 60 x Kh
What conclusions can you draw from your data?
Teacher Notes
Objectives:
The students will measure energy use in their own homes.
Background:
On the face of the meter look for a marking such as Kh6. This is known as the meter constant. The meter constant allows you to calculate the power in watts that is being used at any time in your home. Mathematically the power is found by:
P(watts) = (r/min.) x 60 x Kh
r/min. = the number of rotations of the plate in one minute
60 = the number of minutes in one hour
Kh = the meter constant
P(watts) = (the number of rotations of the plate in one minute) x (minutes in one hour) x (meter constant)
For example, if you counted two revolutions of the plate in one minute, and the meter constant was 6, then the power reading would be:
P(watts) = 2 x 60 x 6 = 720 W
Materials:
Copies of student sheet (one per student)
Electric meter (for example, if one is available)
Watch with second hand
Calculators (optional)
Teaching Suggestions:
1. Go over background information with students.
2. Practice using formula with the practice problems.
a) meter constant (Kh) = 2.5
Number of rotations/minute (r/min.) = 7
Answer: Power (watts) = 7 x 60 x 2.5 = 1050
b) Kh = 1.9
r/min. = 9
Answer: Power (watts) = 9 x 60 x 1.9 = 1026
Extended Activities:
1. Choose another time frame of the day and repeat activity.
2. Do activity at another location -- nursing home, industry, school. Be certain to ask permission to do this first.
Problem:
What is the cost of using an electrical appliance for one month?
Materials:
Graph paper
Colored pencils, markers, or crayons
Ruler
Calculator
Activities:
The table lists the average power and energy usage for common household electrical appliances. Electric energy is measured in kilowatt-hours (kWh). You can find out how much it costs per month to use these appliances. Record your data in a table.
Frequently Used Appliances
Small Appliance and Wattage Rating
Blender: 300 watts = .300 kilowatt (kW)
Can Opener: 100 watts = .100 kW
Clock: 2 watts = .002 kW
Corn Popper: 600 watts = .600 kW
Curling Iron: 40 watts = .040 kW
Food Processor: 450 watts = .450 kW
Hair Dryer: 1200 watts = 1.200 kW
Juicer: 90 watts = .090 kW
Knife Sharpener: 100 watts = .100 kW
Mixer (stand): 150 watts = .150 kW
Sewing Machine: 75 watts = .075 kW
Shaver: 15 watts = .015 kW
Waffle Iron: 1200 watts = 1.200 kW
Water Softener: 2 watts = .002 kW
1. Pick five appliances and calculate an average number of hours each is used in one month.
2. Find out from your local utility the average cost of 1 kWh of electricity. Use this number to find out how much it costs to operate the five appliances you chose for one month.
Worksheet
Make a table that shows for five different appliances: the kilowatt rating, time used per month, kWh used per month, and the cost of using that appliande for one month.
Make a bar graph to compare the monthly energy use of six appliances you would consider a convenience but not absolutely necessary to have.
Teacher Notes
Objectives:
1. Students will learn which electrical appliances use the most kWh of energy per month.
2. Students will calculate the monthly energy cost of various appliances.
3. Students will create a graph using their collected data.
Background:
1. Many appliances do not list a wattage rating. However, they will list the voltage and the current. You can use these two values to calculate power consumption of the appliance:
Power = Current x Voltage
For example, if a small appliance is rated at a current of 10 amps and a voltage of 120 volts, the power rating is found by:
Power = 10 amps x 120 volts = 1200 watts
Divide by 1000 to find kW rating.
1200/1000 = 1.2 kW
2. If a hair dryer is used ten minutes a day for 30 days, the total usage for the month is 300 minutes or 5 hours. At a rating of 1.2 kW the total energy consumed is:
Energy = 1.2 kW x 5 hours = 6.0 kilowatt hours (kWh)
Total cost if utility charges 8 cents per kWh:
6 kWh x $.08/kWh = $.48
Frequently Used Appliances
Small Appliance and Wattage Rating
Blender: 300 watts = .300 kilowatt (kW)
Can Opener: 100 watts = .100 kW
Clock: 2 watts = .002 kW
Corn Popper: 600 watts = .600 kW
Curling Iron: 40 watts = .040 kW
Food Processor: 450 watts = .450 kW
Hair Dryer: 1200 watts = 1.200 kW
Juicer: 90 watts = .090 kW
Knife Sharpener: 100 watts = .100 kW
Mixer (stand): 150 watts = .150 kW
Sewing Machine: 75 watts = .075 kW
Shaver: 15 watts = .015 kW
Waffle Iron: 1200 watts = 1.200 kW
Water Softener: 2 watts = .002 kW
Materials:
Student sheets
Graph paper
Colored pencils, markers or crayons
Rulers
Calculators
Teacher Suggestions:
1. Do Activities #1 and #2 in class.
2. Assign #3 as homework.
Extended Activities:
1. Call different utilities companies to find cost of kWh usage.
Calculate cost of electricity for other rates. As a class compare these rates.
2. Have students choose appliances from home which they use frequently and calculate the monthly energy cost.
3. Calculate energy cost per year for appliances.
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