Physics 100 Energy in Today’s World Homework Chs. 6 and 12
Prof. Menningen
p. 1 of 4
1.
What should be done to compensate for the fact that solar energy is dependent upon the time
of day and the season of the year?
a. Concentrate the energy received with a solar collector.
b. Lobby for a change in governmental strategy concerning solar energy.
c. Add diffusers to spread the energy received across a larger area.
d. Store the energy received for later retrieval.
2.
What is the meaning of the word insolation?
a. The absorption and reradiation of infrared light radiated from the earth.
b. The material used to prevent thermal conduction.
c. Incident solar radiation.
d. The fraction of sunlight reflected from the earth and its atmosphere.
3.
Which of the following is NOT a component of insolation?
a. The azimuthal component.
b. The reflected component.
c. The direct beam from the sun.
d. The diffuse component.
4.
Which of the following photons would have the highest energy?
a. red
b. infrared
c. blue
d. ultraviolet
5.
What is the difference between n-type and p-type semiconductor?
a. N-type semiconductors are denser than p-type and form the bottom layer
of a photovoltaic stack.
b. The charge carriers in n-type semiconductors are electrons but in p-type
the charge carriers are "holes".
c. Electron-hole pairs recombine at a faster rate in n-type semiconductors than
they do in p-type semiconductors.
d. N-type semiconductors are doped with boron and p-type semiconductors are
doped with phosphorus.
6.
Wiring photovoltaic cells in ______ will increase the electrical current produced and wiring
them in ______ will increase the voltage produced.
a. series ... parallel
b. series ... series
c. parallel ... parallel
d. parallel ... series
7.
What is the role of an inverter in a photovoltaic system for a residential dwelling (Figure 12.10)?
a. It converts thermal energy to electrical energy.
b. It changes a low AC voltage to a high AC voltage.
c. It converts DC electricity to AC.
d. It converts electrical energy to mechanical energy.
8.
Wind energy is intermittent. How might we take advantage of wind energy when the wind is
not blowing?
The energy from the wind can be stored in batteries or in the gravitational
potential energy of water that is pumped into a reservoir.
Physics 100 Energy in Today’s World Homework Chs. 6 and 12
9.
Prof. Menningen
p. 2 of 4
Solar energy can be attractive not only environmentally but also economically. If an electric
clothes dryer has a power rating of 6250 watts and is used for 1 h per day, how much money
can be saved in a month (30 days) by hanging your clothes outside to dry in the sunshine
instead of using the electric dryer? Assume electricity sells for $0.105 per kWh.
E  Pt   6.250 kW 1.00 h/d  30 d   187.5 kWh
cost  187.5 kWh  $0.105 /kWh  $19.69
10. What size flat plate collector is needed to supply a family's daily domestic water needs in
January in St. Cloud, MN? Assume 95 gallons per day are needed (1 gal = 3.785 kg),
ΔT = 70°F for the water, and that the collector-heat exchanger system has an average
efficiency of 40%. The collector tilt angle is equal to the latitude (see Appendix D).
(a) What is the required temperature change in degrees Celsius?
5
5
T  C   T  F    70C   38.9C
9
9
(b) How much energy is needed to heat the water needed for one day? The specific
heat of water is 4190 J/kg·°C.
Q  mcT   95 gal  3.785 kg/gal  4190 J/kg/C  38.9C 
 5.86  107 J  58.6 MJ
(c) Convert the insolation (from Appendix D) to units of J/m2/d.
2
BTU 1055 J  1 ft 
J
MJ
7
1410 2 

 16 2
  1.60  10
2
ft  d BTU  0.305 m 
m d
m d
(d) What collector area is required to heat the water?
Q eIA
Q
5.86  107 J/d
P 
 A 
 9.16 m2
7
2
t
t
eI  0.40  1.60  10 J/m /d 
(e) If the collector were a perfect square, what is the length of its sides in feet?
2
 1 ft 
2
2
A  L  9.16 m  
  98.4 ft  L  98.4 ft  9.92 ft
 0.305 m 
2
2
Physics 100 Energy in Today’s World Homework Chs. 6 and 12
Prof. Menningen
p. 3 of 4
11. If the insolation on a flat plate collector is 1225 Btu/ft2/d, how large must the collector be to
provide 45,000 Btu/h of interior space heating for 1 day? The collector efficiency is 50%.
P  eIA  A 
P 45,000 BTU/h  24 h/d

 1760 ft 2
2
eI  0.50  1225 BTU/ft /d 
2
 0.305 m 
2
1760 ft  
  164 m
 1 ft 
2
12. Water has the highest specific heat of any ordinary material, which means that it can store a
good deal of thermal energy. A cubic meter of water stores about 4.19 MJ/°C, so 8.50 m3
(300 ft3) of water can store 35.6 MJ/°C. Rocks have a much smaller specific heat but a much
greater density than water. If the specific heat of rock is 880 J/kg/°C and the density is 2730
kg/m3, how many cubic meters of rock are necessary to store 35.6 MJ/°C?
Q  mcT   V  cT
because density   m V so m   V
Q T
35.6  106 J/C
V

 14.8 m3
3
c
 2730 kg/m  880 J/kg/C 
13. Suppose we wish to lift 60 m3 (16,000 gal) of water through a height of 5.5 m in a period of 8
hours.
(a) What power is required to lift the water?
3
3
2
mgh  60 m  1000 kg/m 10 m/s   5.5 m 
P

 115 W
t
8.0 h  3600 s/h
(b) If the pump is 60% efficient, how much electrical power does the pump consume?
e
Pout
Pin
 Pin 
Pout 115 W

 191 W
e
0.60
(c) How many 40-W solar arrays would you need in order to operate this pump?
(Round up to the nearest integer number.)
N
Pin
191 W

 5 arrays
Parray 40 W/array
Physics 100 Energy in Today’s World Homework Chs. 6 and 12
Prof. Menningen
p. 4 of 4
14. What maximum output would you expect from a wind turbine with a blade of diameter 7.0 m
(23 ft) in a 6.7 m/s (15 mph) wind?
P   2.83  104  D 2 v3
  2.83  104 kW  s3 /m5   7.0 m   6.7 m/s   4.17 kW
2
3