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Aim_ How can we solve energy transfer problems_ by malj

VIEWS: 76 PAGES: 16

									     Aim: How can we solve
    energy transfer problems?



Do Now:
Why does the temperature remain constant as
heat is added to ice causing it to melt?
1. James Joule did much to establish the value of the
(A)universal gravitational constant
(B)speed of light
(C)mechanical equivalent of heat
(D)charge of an electron
(E)specific heat capacity of helium




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2. Heat is added at a constant rate to a sample of a pure substance that is
   initially a solid at temperature To. The temperature of the sample as a
   function of time is shown in the graph above. From the graph, one can
   conclude that the
(A) substance sublimes directly from the solid phase to the vapor phase
(B) melting point is T2
(C) specific heat is greater for the liquid phase than for the solid phase
(D) heat of fusion and heat of vaporization are equal
(E) specific heat of the solid increases linearly with temperature

 More heat is required to raise the temp. of the liquid
3. A 2-kilogram block of metal with a specific heat of 100 Joules
   per kilogram·Kelvin falls from rest through a distance of 100
   meters to the Earth's surface. If half of the potential energy
   lost by the fallen block is converted to internal energy of the
   block, the temperature change of the block is most nearly
(A)1 K
(B)5 K          m = 2 kg
(C)10 K         c = 100 J/kg·K
(D)25 K
(E)45 K         vo = 0 m/s
                h = 100 m
                   ½U=Q
                  ΔT = ?

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4. A metal rod of length L and cross-sectional area A connects two
   thermal reservoirs of temperatures T1 and T2. The amount of
   heat transferred through the rod per unit time is directly
   proportional to
(A)A and L
(B)A and 1/L
(C)1/A and L
(D)1/A and 1/L
(E)1/A and L2




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5. A 0.020-kilogram sample of a material is initially a solid at a
temperature of 20 °C. Heat is added to the sample at a constant
rate of 100 Joules per second until the temperature increases to
60 °C. The graph above represents the temperature of the
sample as a function of time.
a. Calculate the specific heat of the solid sample in units of Joules
per kilogram·°C.
b. Calculate the latent heat of fusion of the sample at
its melting point in units of joules per kilogram.
c. Referring to the three intervals AB, BC, and CD
shown on the graph, select the interval or intervals on
the graph during which:
      i. the average kinetic energy of the molecules of
         the sample is increasing

           AB
           CD

     ii. the entropy of the sample is increasing

           AB
           BC          Entropy is always increasing
           CD
6. A ball thrown vertically downward strikes a horizontal
surface with a speed of 15 meters per second. It then
bounces, and reaches a maximum height of 5 meters.
Neglect air resistance on the ball.
a. What is the speed of the ball immediately after it
rebounds from the surface?




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b. What fraction of the ball's initial kinetic energy is
apparently lost during the bounce?
c. If the specific heat of the ball is 1,800 J/kg·°C, and if
all of the lost energy is absorbed by the molecules of the
ball, by how much does the temperature of the ball
increase?
7. A freezer contains 20 kilograms of food with a specific
heat of 2 x 103 J/kg·°C. The temperature inside the
freezer is initially -5 ºC. The freezer motor then operates
for 10 minutes, reducing the temperature to -8 °C.
a. How much heat is removed from the food during this
time? The freezer motor operates at 400 watts.




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b. How much energy is delivered to the freezer motor
during the 10-minute period?
c. During this time, how much total heat is ejected into
the room in which the freezer is located?
d. Determine the temperature change in the room if the
specific heat of air is 700 J/kg·°C Assume there are 80
kilograms of air in the room, the volume of the air is
constant, and there is no heat loss from the room.

								
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