Physics 161 - Energy and Environment.

Homework 6. Due Monday, May 19

Please use separate sheets for your answers

I. Energy versus Power. Review question.

1a. Define and compare Joules, watts (kilowatts, kw) and kilowatt hours (kwh). What is the difference between a watt, a Joule and a kilowatt hour?  Why do we need kwh (or, if not, why are kwh convenient)? For which situations would you use each unit? Give a specific example of each.

b. Sometimes it is convenient to use the units Btu and Btu/hour. Which unit should be compared with kilowatts and kilowatt-hours, and why? Give a specific conversion factor where possible.

c. Under what situation might it be more convenient to use Btu and Btu/hour than Joules and watts and why? Give a specific example of each.

II. Home heating

2. Air has a heat capacity of 0.018 Btu/cubic foot (that is, it takes 0.018 Btu to heat 1 cubic foot of air by 1 degree F). Suppose a typical house has 2000 square feet of floor space and the ceilings are 9 feet above the floor. We use the formula Q = mc deltaT but here m = volume of air in cubic feet, c = heat capacity and deltaT = temperature difference in degrees F.

a. How much energy (Btu) will it take to heat the air in the house from 50 degrees F (outside temperature) to 70 degrees F?.

b. A good house design allows for air changes of about 0.5 - 1.5 per hour (folks have to breathe!). Assume the house in part 2a is designed for 1 air change/hour, with the air coming in at the outside temperature of 50 degrees F. What does it cost to heat this house per year (8760 hours) if natural gas heat costs $5.00 per MBtu (MBtu= 1 million Btu)? NOTE: this does not take into account heat transfer through walls and windows!

c. If the house is drafty, and the air changes 4 times per hour as is typical in older homes, what does it then cost to heat the house for one year?

III. Chapter 6.

p. 193. Questions 12, 15, 16.

IV. Chapter 9.

p. 315-316. Questions 1, 3, 4, 8, 14.