Transcript SU-Home_Heating_Multiple_Choice_Questions

ENERGY
USE AT
HOME
Home Heating
Multiple-Choice Questions
Physics and Astronomy Outreach Program at the University of British Columbia
Question 1
Home
Heating
You come into a cold room and switch on the space
heater. Which statement is true during the time
when the temperature in the room is rising?
A. The power delivered by the heater is larger
than the heat lost from the room.
B. The power delivered by the heater is smaller
than the heat lost from the room.
C. The power delivered by the heater is the same
as the heat lost from the room.
Physics and Astronomy Outreach Program at the University of British Columbia
Question 1 Answer
Home
Heating
You come into a cold room and switch on the space
heater. Which statement is true during the time
when the temperature in the room is rising?
A. The power delivered by the heater is larger
than the heat lost from the room.
B. The power delivered by the heater is smaller
than the heat lost from the room.
C. The power delivered by the heater is the same
as the heat lost from the room.
Physics and Astronomy Outreach Program at the University of British Columbia
Question 2
Home
Heating
How could you decrease the heat loss through a
window on a cold day?
A. Increase the thickness of the glass
B. Use a glass with a smaller k (thermal
conductivity)
C. Use a smaller window
D. Decrease the temperature inside your house
E. Do all of the above
Physics and Astronomy Outreach Program at the University of British Columbia
Question 2 Answer
Home
Heating
How could you decrease the heat loss through a
window on a cold day?
A. Increase the thickness of the glass
B. Use a glass with a smaller k (thermal
conductivity)
C. Use a smaller window
D. Decrease the temperature inside your house
E. Do all of the above
Physics and Astronomy Outreach Program at the University of British Columbia
Question 2 Solution
Home
Heating
How could you decrease the heat loss through a
window on a cold day?
Heat: Q = kA𝛥T
x
where k is the thermal conductivity
A is the area of the window
x is the thickness of the window
𝛥T is the difference between the
inside and outside temperature.
So the heat loss is decreased by decreasing k, A or
𝛥T or by increasing x.
Physics and Astronomy Outreach Program at the University of British Columbia
Question 3
Home
Heating
You are working with some friends on a problem
asking you to make sense of the high temperatures
inside your car when it is parked in full sun and all
of the windows are closed. Your friends want to
use an energy balance model and make the a
series of assumptions (see next slide). You are the
critic in the group so you point out that one of these
assumptions is not reasonable. Which one?
Physics and Astronomy Outreach Program at the University of British Columbia
Question 3
Home
Heating
A. The sun’s radiation energy can enter the car
through the windows
B. The cars interior absorbs almost all of the
radiation energy from the sun and the emissivity
is close to 1
C. Due to the high temperature inside the car, the
car radiates energy back to the environment
through the windows.
D. Heat conduction through the car’s windows can
be neglected
E. If there is no wind, heat loss due to convection
can be neglected
Physics and Astronomy Outreach Program at the University of British Columbia
Question 3 Answer
Home
Heating
A. The sun’s radiation energy can enter the car
through the windows
B. The cars interior absorbs almost all of the
radiation energy from the sun and the emissivity
is close to 1
C. Due to the high temperature inside the car, the
car radiates energy back to the environment
through the windows.
D. Heat conduction through the car’s windows can
be neglected
E. If there is no wind, heat loss due to convection
can be neglected
Physics and Astronomy Outreach Program at the University of British Columbia
Question 3 Solution
Home
Heating
The sun’s radiation energy will enter the car through
the windows and the car’s interior will absorb almost
all of the radiation energy from the sun. As the car
warms up, heat will be both radiated and conducted
back to the environment through the windows. If
there is no wind, heat loss to convection will be
negligible.
Physics and Astronomy Outreach Program at the University of British Columbia
Question 4
Home
Heating
An apartment building, consisting of concrete walls
insulated with 10cm thick fiberglass (R-13), is 10m
by 10m.
Q) What is the thermal conductivity of the fiberglass?
A. 22 W/mK
B. 1.3 W/mK
C. 0.77 W/mK
D. 0.077 W/mK
E. 0.044 W/mK
Physics and Astronomy Outreach Program at the University of British Columbia
Question 4 Answer
Home
Heating
An apartment building, consisting of concrete walls
insulated with 10cm thick fiberglass (R-13), is 10m
by 10m.
Q) What is the thermal conductivity of the fiberglass?
A. 22 W/mK
B. 1.3 W/mK
C. 0.77 W/mK
D. 0.077 W/mK
E. 0.044 W/mK
Physics and Astronomy Outreach Program at the University of British Columbia
Question 4 Solution
Home
Heating
An apartment building, consisting of concrete walls
insulated with 10cm thick fiberglass (R-13), is 10m
by 10m.
Q) What is the thermal conductivity of the fiberglass?
R = 13 BTU/(h ft2 oF)
we know that 1 BTU/(h ft2 oF) = 0.176 m2K/W
R = 13 * 0.176 m2K/W = 2.29 m2K/W
we also know that k = x / R
k=
0.1m
= 0.044 W/mK
2.29 m2K/W
Physics and Astronomy Outreach Program at the University of British Columbia
Question 5
Home
Heating
An apartment building, consisting of concrete walls
insulated with 10cm thick fiberglass (R-13), is 10m
by 10m and each floor is 2.5 m apart. On a cloudy
day, the temperature inside is 20oC and the
temperature outside is 10oC. Heat coming from the
floor below equals the heat lost to the floor above.
Q) How much heat is lost to conduction?
A. 200 W
B. 300 W
C. 450 W
D. 600 W
E. 800 W
Physics and Astronomy Outreach Program at the University of British Columbia
Question 5 Answer
Home
Heating
An apartment building, consisting of concrete walls
insulated with 10cm thick fiberglass (R-13), is 10m
by 10m and each floor is 2.5 m apart. On a cloudy
day, the temperature inside is 20oC and the
temperature outside is 10oC. Heat coming from the
floor below equals the heat lost to the floor above.
Q) How much heat is lost to conduction?
A. 200 W
B. 300 W
C. 450 W
D. 600 W
E. 800 W
Physics and Astronomy Outreach Program at the University of British Columbia
Question 5 Solution
Home
Heating
How much heat is lost to conduction?
Heat: Q = kA𝛥T
x
k = 0.044 W/mK
A = 10 m x 10 m = 100 m2
𝛥T = 10 K
x = 0.1 m
Q = (0.044 W/mK)(100 m2)(10 K) = 440 W
0.1 m
Physics and Astronomy Outreach Program at the University of British Columbia