APES Energy Problems
1. How much energy, in kJ,
does a 75 Watt light bulb use then it is turned on for 25 minutes?
2. The Kilowatt Hour, or kWh,
is not a unit of power but of energy.
Notice that kilowatt is a unit of power and hour is a unit of time. E = P x t (rearranged from above). A kilowatt hour is equal to 1 kW delivered continuously for
1 hour (3600 sec).
Assume your electric bill
showed you used 1355 kWh over a 30-day period.
a.)
Find the energy used, in kJ, for the 30 day period.
b.)
Find the energy used in J/day.
c)
At the rate of $.0749/kwh, what will your electric bill be for this month?
3. A 100 Watt light bulb is 20% efficient.
a.)
How much energy does it use in 12 hours of operation?
b.)
How much energy does the bulb convert to light during 12 hours?
4. An electric clothes dryer
has a power rating of 4000 W.
Assume a family does 5 loads of laundry each week for 4 weeks. Assume each dryer load takes 1 hour.
b.) Find the
operating cost for 4 weeks. Assume
cost is $.0758/kWh
5. Refrigeration is costly in
terms of energy usage. A
single-door, manual defrost refrigerator uses 600 kilowatt hours/year
(abbreviated kWh/yr). A large, 20
cu ft two-door automatic defrost refrigerator uses 1880 kWh/yr. How many kcal/yr do each type of
refrigerator use? 1kWh = 860kcal.
6. Assume you use an air
conditioner for a total of 137 days, 24 hours per day, at a rate of 7.25 kWh
per hour. Assume the cost per kWh
is $.0825 and 1 kWh = 3400 BTUs.
a.)
Calculate the total number of kWh used per year.
b.)
Determine the cost of air conditioning for one year.
c.)
How many kcal are used per year?
d.)
How many BTUs are used in one year?
7. Suppose your electric lights
use 400 watts per hour and average four hours per day, every day for one year.
b.) If replacing the lights with a fluorescent bulb
would save 60 w per night, what savings in kWh does this represent in one year?
c.) If the fluorescent bulb costs $18 but lasts for 10
years, would you consider it a wise investment over incandescent bulbs? Explain your answer.
8. Transportation energy is
costly.
a.) Calculate the
gallons of gas use just for going to the supermarket in one year if you take 5
trips to the store per week, traveling 7.5 miles roundtrip, and your car gets
22 miles per gallon.
b.) Convert the
gallons to kcal/year if there are 32000 kcal per gallon.
c.)
What are some of the other energy uses associated with getting foodstuffs to
the consumer other than transportation costs?
d.)
What suggestions would you make to the “average American” to help save energy
and money relating to the answers you gave in C?
9. (This question is
extracted word for word from the 1998 APES exam) The environmental impact of washing a load of dirty dishes
in an electric dishwasher differs from that of washing them by hand in a
sink. Use the information and data
below to answer thee questions that follow. Show your calculations.
Assume:
i.
The dishes all
fit in one load.
ii.
The water coming
into the water heater for the sink and into the water heater in the dishwasher
is at 50o F.
iii.
The water heaters
for the sink and the dishwasher are both 100% efficient.
iv.
In one complete
cycle, the electric dishwasher uses 10 gallons of water heated to 140o
F and the dishwasher also uses 0.500 kWh of electrical energy for its
mechanical operation.
v.
Washing the
dishes by hand requires 20 gallons of water heated to 110o F.
Other Information:
1 gallon of water =
8 lb. water
1 BTU = the amount
of energy needed to raise the temperature of 1 pound of water by 1o F.
1 kWh = 3400 BTUs.
a. Calculate the total energy
(in BTU’s) used both to heat the water and run the electric dishwasher to wash
a load of dishes.
c. Discuss the economic and
environmental costs and benefits of
i. using the
electric dishwasher (including its manufacture and disposal)
ii. washing the
dishes by hand
