You can choose between a heat pump and an electric heater to heat your house. If you assume that the electric heater is 100% efficient at turning electrical energy into heat, an electric heat pump of what coefficient of performance (COP) will result in the same cost of electricity for the same amount of heating? Assume that the motor that runs the heat pump is 100% efficient
1.0.25
2.0.5
3.1.0
4.2.0
3
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A long wooden rod at 38°C with a 2.5 cm OD is placed into an airstream at 600°C. The heat transfer coefficient between the rod and air is 28.4 W/(m2 K). If the ignition temperature of the wood is 427°C, ? = 800 kg/m3, k = 0.173 W/(m K), and c = 2500 J/(kg K), determine the time between initial exposure and ignition of the wood.
GIVEN
• A long wooden rod is placed into an airstream
• Rod outside diameter (D) = 2.5 cm = 0.025 m
• Initial temperature of the rod (To) = 38°C
• Temperature of the airstream (T?) = 816°C
• The heat transfer coefficient (hc) = 28.4 W/(m2 K)
• The ignition temperature of the wood (TI) = 427°C
• Density of the rod (?) = 800 kg/m3
• Thermal conductivity (k) = 0.173 W/(m K)
• Specific heat (c) = 2500 J/(kg K)
FIND
• The time between initial exposure and ignition of the wood
SKETCH
When the hydrogen isotope tritium-3 emits a beta particle, it becomes an isotope of
A) hydrogen. B) helium. C) lithium. D) carbon. E) none of the above
The ________ Space Telescope still gives us the highest resolution optical images
A) Chandra B) Kepler C) Hubble D) Fermi E) Einstein
The resistance of a wire can be increased by
a. making the wire longer. b. lowering the temperature of the wire. c. making the wire thinner. d. making the wire shorter.