A family cuts its energy use… — AP® Environmental Science

Name: ______________________ Date: ____________ A family cuts its energy use: refrigerator and furnace (10 points)

Data

A household's energy upgrades

A family in a cold-winter state is deciding how to cut its energy use. Show every setup with numbers and units. Efficiency = useful energy out ÷ energy in × 100.

Energy data for one household (invented data for practice; the carbon dioxide factors are near published values)
Item	Value
Old refrigerator	1,100 kWh per year
New refrigerator	400 kWh per year
Electricity price	$0.15 per kWh
Carbon dioxide from the local grid	0.4 kg per kWh
Natural gas burned by the old furnace	900 therms per year
Old furnace efficiency	80%
New furnace efficiency	95%
Natural gas price	$1.40 per therm
Energy in 1 therm of natural gas	100,000 BTU
Carbon dioxide from burning natural gas	5.3 kg per therm

(a) Calculate the percent decrease in yearly electricity use when the family replaces the old refrigerator with the new one. Show your setup. [2 points]
(b) Calculate the mass of carbon dioxide, in kg, the new refrigerator avoids each year. Show your setup with units that cancel. [2 points]
(c) The old furnace burns 900 therms a year at 80% efficiency. Calculate how many therms the new 95% efficient furnace would burn to deliver the same heat to the house. Show your setup. [2 points]
(d) Calculate the money the family saves each year on natural gas with the new furnace. Use your answer from part (c). Show your setup. [1 point]
(e) Explain why burning natural gas in the new furnace still contributes to climate change, even though it is more efficient. [1 point]
(f) Propose one change, other than replacing the furnace or the refrigerator, that would further reduce the family's home heating energy, and justify how it works. [2 points]

The question

A household's energy upgrades