The "half-life" of a large collection of radioactive nuclei is
A. larger for heavy nuclei than for lighter nuclei.
B. the number of nuclei that remain after 1 second.
C. the time for half of the collection to decay.
D. the number of nuclei that decay in 1 second.
Answer: C
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As measured in Earth's rest frame, a spaceship traveling at takes to travel between planets. How long does the trip take as measured by someone on the spaceship?
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On a cold winter day, the outside wall of a home is exposed to air temperature of –2°C when the inside temperature of room is at 22°C. As a result of this temperature gradient, there is heat loss through the wall to the outside. Consider the convective heat transfer coefficients for the air inside the room and at the outside wall surface to be respectively 12.0 and 28.0 W/(m2 K). If the composite room wall is modelled as a plane wall with thermal resistance per unit area of 0.5 m2 K/W, determine the temperature at the outer surface of the wall as well as the rate of heat flow through the wall per unit area. If the homeowner were to consider using a fiberglass insulation on the inside wall surface for reducing this heat loss by 50 %, what is the required thickness of this layer and the
outside wall temperature for this case?
GIVEN
• Heat transfer through a plane wall
• Air temperature
? Inside wall (Ti) = 22°C
? outside wall (To) = –2°C
• Heat transfer coefficient
? Inside wall ( ci h ) = 12 W/(m2 K)
? Outside wall ( co h ) = 28 W/(m2 K)
• Thermal resistance of a unit area (A Rw) = 0.5 (m2 K)/W FIND (a) Temperature of the outer surface of the wall (Two) (b) Rate of heat flow through the wall per unit area (q/A) (c) Thickness of fiberglass insulation inside wall surface (d) Outside wall temperature with fiberglass insulation
ASSUMPTIONS
• One dimensional heat flow
• Steady state has been reached
PROPERTIES AND CONSTANTS
Thermal conductivity
Fiber glass (k) =0.035 W/m K
SKETCH
