VIEWS: 76 PAGES: 16 POSTED ON: 7/9/2011
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 No Calculator **30 seconds** No Calculator **30 seconds** 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 = ? No Calculator **30 seconds** 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 No Calculator **1 minute** Calculator **7 minutes** 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? Calculator **7 minutes** 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. Calculator **10 minutes** 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.