First Law of Thermodynamics: The gas in a perfectly insulated but flexible container does work at a rate of 13 W. At what rate is the internal (thermal) energy of the gas changing?
A. -13 W
B. 13 W
C. 0 W
D. 6.5 W
Answer: A
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A plane wall 15 cm thick has a thermal conductivity given by the relation k = 2.0 + 0.0005 T W/(m K) where T is in degrees Kelvin. If one surface of this wall is maintained at 150 °C and the other at 50 °C, determine the rate of heat transfer per square meter. Sketch the temperature distribution through the wall.
GIVEN
A plane wall
Thickness (L) = 15 cm = 0.15 m
Thermal conductivity (k) = 2.0 + 0.0005 T W/(m K) (with T in Kelvin)
Surface temperatures: Th = 150 °C Tc = 50 °C
FIND
(a) The rate of heat transfer per square meter (q/A)
(b) The temperature distribution through the wall
ASSUMPTIONS
The wall has reached steady state
Conduction occurs in one dimension
SKETCH
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Two billiard balls have velocities of 2.0 m/s and -1.0 m/s when they meet in an elastic head-on collision. What is the final velocity of the second ball after collision?
a. +2.0 m/s c. -0.5 m/s b. -1.0 m/s d. +1.0 m/s
Which of these statements is true for the image formed by an object placed in front of a thin lens?
A. convex lenses never produce virtual images B. real images are always enlarged C. real images are always inverted D. virtual images are always inverted