In a certain chemical reaction, chemical bonds are broken. The reaction is
a. endothermic and liberates energy.
b. endothermic and absorbs energy.
c. exothermic and liberates energy.
d. exothermic and absorbs energy.
e. described by none of the above choices.
B
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A stainless steel cylindrical billet [k = 14.4 W/(m K), ? = 3.9 × 10–6 m2/s] is heated to 593°C preparatory to a forming process. If the minimum temperature permissible for forming is 482°C, how long can the billet be exposed to air at 38°C if the average heat transfer coefficient is 85 W/(m2 K)? The shape of the billet is shown in the sketch.
GIVEN
• A stainless steel cylindrical billet exposed to air
• Thermal conductivity (k) = 14.4 W/(m K)
• Thermal diffusivity (?) = 3.9 × 10–6 m2/s
• Initial temperature (To) = 593°C
• The minimum temperature permissible for forming is 482°C
• Air temperature (T?) = 38°C
• Average heat transfer coefficient ( )ch = 85 W/(m2 K)
FIND
• How long may the billet be exposed to the air?
ASSUMPTIONS
• End effects are negligible
• Constant heat transfer coefficient
• Conduction in the radial direction only
• Uniform thermal properties
SKETCH
An refrigerator keeps food in the freezer portion at 
and dumps excess heat into the kitchen, whose temperature is 
If the refrigerator has half the coefficient of performance of an ideal reversible refrigerator, and if its motor can supply 250W of power, what is the rate of heat flow out of the freezer portion of the refrigerator?
A. 340W B. 300W C. 1100W D. 625W
Which Jovian moon has gotten the most attention from exobiologists?
A) Miranda B) Europa C) Ariel D) Io E) Triton
You separate two magnets that are initially held together by magnetic forces. How does this affect the mass and energy of the system of magnets?
A) It creates additional mass, located within each magnet, but no additional energy. B) It creates additional mass, located within each magnet, and also additional energy. C) It creates neither additional energy nor additional mass. D) It creates additional mass, located in the space between the two magnets, and also additional energy. E) It creates additional energy, located in the space between the two magnets, but not additional mass.