1. Physics and Astronomy Outreach Program at the University of British Columbia Physics and Astronomy Outreach Program at the University of British Columbia Multiple-Choice Questions ENERGY USE AT HOME Home Heating

2. Physics and Astronomy Outreach Program at the University of British Columbia Physics and Astronomy Outreach Program at the University of British Columbia Question 1 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. Home Heating

3. Physics and Astronomy Outreach Program at the University of British Columbia Physics and Astronomy Outreach Program at the University of British Columbia Question 1 Answer 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. Home Heating

4. Physics and Astronomy Outreach Program at the University of British Columbia Physics and Astronomy Outreach Program at the University of British Columbia Question 2 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 Home Heating

5. Physics and Astronomy Outreach Program at the University of British Columbia Physics and Astronomy Outreach Program at the University of British Columbia Question 2 Answer 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 Home Heating

6. Physics and Astronomy Outreach Program at the University of British Columbia Physics and Astronomy Outreach Program at the University of British Columbia Question 2 Solution How could you decrease the heat loss through a window on a cold day? Heat: Q = kAT x where k is the thermal conductivity A is the area of the window x is the thickness of the window T is the difference between the inside and outside temperature. So the heat loss is decreased by decreasing k, A or T or by increasing x. Home Heating

7. Physics and Astronomy Outreach Program at the University of British Columbia Physics and Astronomy Outreach Program at the University of British Columbia Question 3 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? Home Heating

8. Physics and Astronomy Outreach Program at the University of British Columbia 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

9. Physics and Astronomy Outreach Program at the University of British Columbia 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

10. Physics and Astronomy Outreach Program at the University of British Columbia 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.

11. Physics and Astronomy Outreach Program at the University of British Columbia Physics and Astronomy Outreach Program at the University of British Columbia Question 4 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 An apartment building, consisting of concrete walls insulated with 10cm thick fiberglass (R-13), is 10m by 10m. Home Heating

12. 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 Physics and Astronomy Outreach Program at the University of British Columbia Question 4 Answer An apartment building, consisting of concrete walls insulated with 10cm thick fiberglass (R-13), is 10m by 10m. Home Heating

13. Physics and Astronomy Outreach Program at the University of British Columbia Physics and Astronomy Outreach Program at the University of British Columbia Question 4 Solution 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 ft 2 o F) we know that 1 BTU/(h ft 2 o F) = 0.176 m 2 K/W R = 13 * 0.176 m 2 K/W = 2.29 m 2 K/W we also know that k = x / R k = 0.1m = 0.044 W/mK 2.29 m 2 K/W Home Heating

14. Physics and Astronomy Outreach Program at the University of British Columbia Physics and Astronomy Outreach Program at the University of British Columbia Question 5 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 20 o C and the temperature outside is 10 o C. 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 Home Heating

15. Physics and Astronomy Outreach Program at the University of British Columbia Physics and Astronomy Outreach Program at the University of British Columbia Question 5 Answer 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 20 o C and the temperature outside is 10 o C. 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 Home Heating

16. Physics and Astronomy Outreach Program at the University of British Columbia Physics and Astronomy Outreach Program at the University of British Columbia Question 5 Solution How much heat is lost to conduction? Heat: Q = kAT k = 0.044 W/mK xA = 10 m x 10 m = 100 m 2 T = 10 K x = 0.1 m Q = (0.044 W/mK)(100 m 2 )(10 K) = 440 W 0.1 m Home Heating