© Shannon W. Helzer. All Rights Reserved. Unit 9 Temperature and Heat

© Shannon W. Helzer. All Rights Reserved. Warming & Cooling  Suppose we have two beakers containing water.  What would happen to the temperature of the water if we added some ice?  What would happen if we turned on the hotplate?  Why? By what means? 9-1

© Shannon W. Helzer. All Rights Reserved. Heat  What is heat?  Heat is the energy that flows from one object to another where there is a temperature difference between them.  Temperature difference means that one is “hotter” and one is “colder.”  Relative temperature between two bodies allows us to tell in which direction heat will flow.  Heat flows from hot to cold always.  Heat and temperature are NOT the same thing. 9-2

© Shannon W. Helzer. All Rights Reserved. Temperature  The sum of the average kinetic energies of the atoms or molecules of a body.  The less kinetic energy they have, the lower the body’s temperatures.  The more kinetic energy they have, the higher the body’s temperatures.  Temperature always exist.  Heat only exist when the bodies in thermal contact have different temperatures. 9-3

© Shannon W. Helzer. All Rights Reserved. Heat Flow  Let us take a block from the refrigerator and one from a hotplate.  Place them in thermal contact side-by-side.  In which direction does the heat flow?  In what state are the blocks in once they have the same temperature? 9-4 Heat Flow Thermal Equilibrium

© Shannon W. Helzer. All Rights Reserved. Cooling  Let us put two beakers on hot plates and heat the water to the same temperatures.  Now suppose we want to cool the water in the beakers.  Which would cool the water fastest, g of ice or g of steel ball bearings? 9-5  Heat flows from the “hotter” water to the “colder” ice or steel.  Therefore, they are “warmed up.”

© Shannon W. Helzer. All Rights Reserved. Specific Heat  Recall that heat is the ENERGY that flows from one object to another where there is a temperature difference between them.  In which direction does heat flow?  On the previous slide, we saw that g of ice cooled the water more than g of iron ball bearings.  This faster cooling occurs because ice has a higher specific heat capacity than steel.  Specific heat capacity (c) of a material is the quantity of heat needed to change a unit mass of that material by a unit amount in temperature. 9-6

© Shannon W. Helzer. All Rights Reserved. Specific Heat Equation  For ice, the specific heat capacity is 2060 J/kg C .  For iron, the specific heat capacity is 450 J/kg C ..  It takes more heat transfer from the “hot” water to the ice to warm the ice; therefore, the “hot” water cools more for the ice than it does for the steel.  The equation for heat transfer is below where T is temperature in Celsius (C  ), Q is heat in Joules (J), m is mass in kilograms (kg), and c is specific heat. 9-7

© Shannon W. Helzer. All Rights Reserved. WS 43 #7 & 8  A iron pan full of water is placed on a stove as shown in the figure to the right. The mass of the water is 3.5 kg (C = 4180 J/kg C), and the mass of the iron pan is 3.5 kg (C = J/kgC). The initial temperature of both the pot and the water is 18  C. They are heated to 90  C.  How much heat transferred from the burner to the water in the pan?  How much heat transferred from the burner to iron pan? 9-8

© Shannon W. Helzer. All Rights Reserved. Conduction  Conduction – heat flow through a solid material placed between two other objects that are at different temperatures.  What is an example of conduction you experience every day? 9-9

© Shannon W. Helzer. All Rights Reserved. Convection  Convection – the transfer of heat by the motion of a liquid or fluid containing thermal energy.  What is an example of convection you experience every day? 9-10

© Shannon W. Helzer. All Rights Reserved. Radiation  Energy can also be transferred in the absence of a solid or liquid medium.  The energy is said to radiate and is called radiation.  Consider a common thermos bottle.  The thermos not only has insulation but also an evacuated area that prevents heat transfer via either convection or conduction.  However, the contents of the thermos will eventually reach the environment’s temperature.  By what mechanism does this energy transfer take place? Glass VacuumInsulation Radiation Infrared 9-11

© Shannon W. Helzer. All Rights Reserved. Calorimetry  Calorimeter Cup – a device used to minimize energy loss or gain with the outside environment.  The dead air space ensures that energy is exchanged primarily between the bodies inside the cup A B C D E F G

© Shannon W. Helzer. All Rights Reserved. WS 42 # 1 - Calorimetry  A calorimeter contains g of water (C = 4180 J/kg K) at 73.8  F g of copper (C = 385 J/kg K) at 0.0  C is added to the water. Recalling that heat lost is equal to the heat gained in a system, answer the following questions.  a. Given that a calorimeter cup prevents the loss of energy to the outside environment, what body(ies) make up the “system” in this problem?  b. Which body looses energy? Why?  c. Which body gains energy? Why?  d. What is the final temperature of the system? 9-13

© Shannon W. Helzer. All Rights Reserved. WS 42 # 2  In this problem there is not calorimeter cup.  As a result, energy is not only gained by the coffee can, it is also lost to the outside environment.  g of water (C = 4180 J/kg K) at 99.0  C is dumped into an aluminum coffee can (m = g, C = J/kg K) at room temperature (37  C). A g lead weight (C = J/kg K) at 10.0  C is also dropped into the water.  Given that a J of energy is lost to the outside environment, what is the final temperature of the system? 9-14 Heat Flow

© Shannon W. Helzer. All Rights Reserved. WS 45 #1  Hot coffee at  C (m = g) is poured into a glass coffee cup at 44.0  C (m = g). The coffee is stirred by a silver spoon taken from the refrigerator at 5.2  C (m = 15.0 g). Given that coffee is mostly water, determine the final temperature of the mixture. 9-15

© Shannon W. Helzer. All Rights Reserved. WS 45 #2  g of water at 98.6  C and g of water at 0.6  C are dumped into a 2.5 kg iron skillet at 43.3  F.  Determine the final temperature of the system °C0.6 °C43.3 °C

© Shannon W. Helzer. All Rights Reserved. WS 46 #2  A large Iron nut (m = 0.25 kg, T1 = 67.2  C) is dropped into an insulated container containing water (m = 2.2 kg, T1 = 55.0  C).  What is the final temperature of the water in the insulated container? 9-17

© Shannon W. Helzer. All Rights Reserved. WS 46 #3  A rare and very valuable silver coin (m = 0.30 kg, T1 = 12.2  C) is gripped by a pair of iron tweezers (m = 0.10 kg, T1 = 45.0  C).  J of energy is lost to the outside environment.  Calculate the final temperature of the most precious coin. 9-18

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