Chapter 2 | 103respectively. If the pressure rise of oil in the pump is mea-
sured to be 400 kPa and the motor efficiency is 90 percent,
determine the mechanical efficiency of the pump.
2–78E A 73-percent efficient pump with a power input of
12 hp is pumping water from a lake to a nearby pool at a rate
of 1.2 ft^3 /s through a constant-diameter pipe. The free surface
of the pool is 35 ft above that of the lake. Determine the
mechanical power used to overcome frictional effects in
piping. Answer:4.0 hp
Energy and Environment
2–79C How does energy conversion affect the environ-
ment? What are the primary chemicals that pollute the air?
What is the primary source of these pollutants?
2–80C What is smog? What does it consist of? How does
ground-level ozone form? What are the adverse effects of
ozone on human health?
2–81C What is acid rain? Why is it called a “rain”? How
do the acids form in the atmosphere? What are the adverse
effects of acid rain on the environment?
2–82C What is the greenhouse effect? How does the excess
CO 2 gas in the atmosphere cause the greenhouse effect?
What are the potential long-term consequences of greenhouse
effect? How can we combat this problem?
2–83C Why is carbon monoxide a dangerous air pollutant?
How does it affect human health at low and at high levels?
2–84E A Ford Taurus driven 15,000 miles a year will use
about 715 gallons of gasoline compared to a Ford Explorer
that would use 940 gallons. About 19.7 lbm of CO 2 , which
causes global warming, is released to the atmosphere when a
gallon of gasoline is burned. Determine the extra amount of
CO 2 production a man is responsible for during a 5-year
period if he trades his Taurus for an Explorer.
2–85 When a hydrocarbon fuel is burned, almost all of the
carbon in the fuel burns completely to form CO 2 (carbon
dioxide), which is the principal gas causing the greenhouse
effect and thus global climate change. On average, 0.59 kg of
CO 2 is produced for each kWh of electricity generated from a
power plant that burns natural gas. A typical new household
refrigerator uses about 700 kWh of electricity per year.
Determine the amount of CO 2 production that is due to the
refrigerators in a city with 200,000 households.
2–86 Repeat Prob. 2–85 assuming the electricity is pro-
duced by a power plant that burns coal. The average produc-
tion of CO 2 in this case is 1.1 kg per kWh.
2–87E Consider a household that uses 11,000 kWh of elec-
tricity per year and 1500 gallons of fuel oil during a heating
season. The average amount of CO 2 produced is 26.4
lbm/gallon of fuel oil and 1.54 lbm/kWh of electricity. If this
household reduces its oil and electricity usage by 15 percent
as a result of implementing some energy conservation mea-
sures, determine the reduction in the amount of CO 2 emis-
sions by that household per year.
2–88 A typical car driven 12,000 miles a year emits to the
atmosphere about 11 kg per year of NOx(nitrogen oxides),
which cause smog in major population areas. Natural gas
burned in the furnace emits about 4.3 g of NOxper therm, and
the electric power plants emit about 7.1 g of NOxper kWh of
electricity produced. Consider a household that has two cars
and consumes 9000 kWh of electricity and 1200 therms of
natural gas. Determine the amount of NOxemission to the
atmosphere per year for which this household is responsible.12 cmPump∆P = 400 kPaMotor35 kW8 cm0.1 m^3 /sOilFIGURE P2–7711 kg NOx
per yearFIGURE P2–88Special Topic: Mechanisms of Heat Transfer
2–89C What are the mechanisms of heat transfer?
2–90C Does any of the energy of the sun reach the earth by
conduction or convection?
2–91C Which is a better heat conductor, diamond or silver?
2–92C How does forced convection differ from natural
convection?
2–93C Define emissivity and absorptivity. What is Kirch-
hoff’s law of radiation?
2–94C What is blackbody? How do real bodies differ from
a blackbody?