EPS219 – Energy and the Environment
Homework#2 – Due in class, Thursday, January 29, 2015
1) (5 pts) Ch3, #Q1: Because energy is a conserved quantity, what happens to
the kinetic energy of a car after you take your foot off the accelerator?
KE goes into thermal energy of the car, road, and air due to friction.
2) (5 pts) Ch3, #Q8: If the efficiency of a coal-fired electrical generating plant is
35%, then what do we mean when we say that energy is a conserved quantity?
The remaining energy isn’t “lost” – it has gone into heating the local
environment, but it is still accounted for.
3) (10 pts) Ch3, #P3: How many pounds of coal have the same heating value as
20 gal of gasoline?
gasoline energy = 20 gal × 1.24 × 105 Btu/gal;
coal has 1.25 × 104 Btu/lb; dividing these yields 198 lbs
4) (10 pts) Ch3, #P6: If a 1000-MW power plant is shut down for one day, what
will be the loss in revenues if the utility could sell its electricity at 12¢/kWh?
1,000,000 kW x (24 hr) x $0.12/kWh = $2,880,000
5) (10 pts) Ch3, #P8: Suppose you left a 100-W light bulb on continuously for
one month. If the electricity generation and transmission efficiency is 30%, how
much chemical energy (in joules) was wasted at the power plant for this
oversight? If the fuel consumption for one meal in China using a kerosene wick
stove is 6 MJ (1 MJ = 1,000,000 joules), how many equivalent meals could be
cooked with this wasted energy?
Electricity used = (0.1 kW) x (30 days/mo)(24 hr/day) = 72 kWh/mo.
Chemical energy = (72/0.3) × (3.61 × 106 J/kWh) = 8.66 × 108 J wasted, or about 144 meals.
6) (5 pts) Ch4, #Q1: Why can’t a body contain heat?
By definition, heat is the transfer of energy from one object to another.
7) (10 pts) Ch4, #Q10: Why is a condenser needed in a steam electric plant? Why
isn’t it better just to recycle the low-pressure steam rather than condense it,
releasing heat to the environment?
The condenser is needed to turn the steam back into water. It is the expansion of
the water into steam that spins the turbines. The same steam used again would
not drive the turbines.
8) (5 pts) Ch4, #Q13: How would you explain to a group of citizens why waste
heat has to be released in the operation of a steam-generating plant?
There has to be a drop in temperature to drive the flow of heat (2nd law of
thermodynamics), so some heat has to be lost in the process.
9) (5 pts) Ch4, #Q20: Can you cool a kitchen by leaving the refrigerator door
open?
No. The kitchen will warm up as energy is added to the room by the work of the compressor.
10) (10 pts) Ch4, #P5: It takes an electric teakettle with 20°C water 5 minutes to
reach boiling (100°C). How long will it take for all the water to boil away,
assuming the same rate of heat addition?
Don’t need to compute the mass of the water. Can use the fact that it takes 4.184
kJ/kg to raise water 1°C, but 2260 kJ/kg to get it to boil away.
Need (100° - 20°) x (4.184) = 334.7 kJ/kg to bring to a boil in 5 min.
Time to boil away = 2260/335 Å~ 5 min = 34 min
11) (15 pts) Ch4, #P6: How much energy will it take to melt a block of ice of
mass 15 kg? If this same energy was used to lift this block of ice, to what height
(in meters) could it be raised? (Does the height change if the amount is more or
less than 15 kg?)
a) Energy = (335 kJ/kg for heat of fusion) x (15 kg) = 5025 kJ
b) E = mgh, so h = E/mg = (5025 kJ)/(15 kg x 9.8 m/s2) = 34 km.
c) No. The height is always the same, as the mass divides out in the two parts.
12) (10 pts) Look at WWF 2014 Living Planet Report
(http://wwf.panda.org/about_our_earth/all_publications/living_planet_report/
or on Blackboard). What was the increase in megacities between 1970 and 2010,
where was most of this growth, and give an example of how might this increase
susceptibility to natural hazards?
a) From 2 in 1970 to 23 in 2010. The growth is mostly in Asia
b) The high population densities mean that in the case of a natural disaster
(earthquake, hurricane, etc.), the inability to get help to people could increase
the death toll [other answers accepted]