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Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley James Joule and the mechanical equivalent of heat Joule knew a mass above.

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Presentation on theme: "Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley James Joule and the mechanical equivalent of heat Joule knew a mass above."— Presentation transcript:

1 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley James Joule and the mechanical equivalent of heat Joule knew a mass above the ground had potential energy. He dropped an object on a cord, turning a paddle in water monitored by a very accurate thermometer. His conclusion was to connect energy conservation (potential and kinetic) to heat as a third form observed.

2 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Specific heat A specific heat value reveals how much temperature will change when a given amount of a substance absorbs a given amount of heat. Water is a “benchmark” as one ml of water will absorb 1 cal of heat to raise its temperature by 1 o C.

3 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Specific heat values

4 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley You put 1 kg of the following materials on a bunsen burner. Which one’s temperature rises the least? A. Aluminum, c = 910 J/kg K B. Berillium, c = 1970 J/kg K C. Copper, c = 390 J/kg K D. Water, c = 4190 J/kg K Q17.3.5

5 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley A. Aluminum, c = 910 J/kg K B. Berillium, c = 1970 J/kg K C. Copper, c = 390 J/kg K D. Water, c = 4190 J/kg K A You put 1 kg of the following materials on a bunsen burner. Which one’s temperature rises the least?

6 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley A 500W heater dumps all its energy into heating 1kg of water in a teapot. How long does it take to heat the water to boiling if the water started out at room temperature? How many moles of water is this? c water = 4190 J/kg K M water = kg/mol Water in a teapot

7 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Phase changes and temperature behavior A solid will absorb heat according to its heat capacity, becoming a hotter solid. At the melting point, a solid will absorb its heat of fusion and become a liquid. An equilibrium mixture of a substance in both its liquid and solid phases will have a constant temperature. A cold liquid will absorb heat according to its heat capacity to become a hotter liquid. At the boiling point, a liquid will absorb its heat of vaporization and become a gas. An equilibrium mixture of liquid and gas will have a constant temperature. A cold gas can absorb heat according to its heat capacity and become a hotter gas.

8 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley You wish to increase the temperature of a 1.00-kg block of a certain solid substance from 20°C to 25°C. (The block remains solid as its temperature increases.) To calculate the amount of heat required to do this, you need to know A. the specific heat of the substance. B. the molar heat capacity of the substance. C. the heat of fusion of the substance. D. the thermal conductivity of the substance. E. more than one of the above Q17.4

9 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley A. the specific heat of the substance. B. the molar heat capacity of the substance. C. the heat of fusion of the substance. D. the thermal conductivity of the substance. E. more than one of the above A17.4 You wish to increase the temperature of a 1.00-kg block of a certain solid substance from 20°C to 25°C. (The block remains solid as its temperature increases.) To calculate the amount of heat required to do this, you need to know

10 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Hot pot A heavy copper pot of mass 2 kg is at temperature of 150C. You pour 0.1 kg of water at 25C into the pot then quickly close the lid so no steam can escape. Find the final temperature of the pot and its contents and determine the phase (liquid or gas) of the water. Assume no heat is lost to the surroundings. c water = 4190 J/kg K, c copper = 390 J/kg K L water = 2256 x 10 3 J/kg

11 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Why, and how well, do materials transfer heat? Conduction: heat transfer within a body or between two bodies in contact. Convection: heat transfer through movement of mass from one place to another Radiation: heat transfer by electromagnetic radiation

12 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Conduction of heat I You bring a cooler to the beach to keep some tasty beverages cold. The cooler has a total wall area of 0.8 m 2 and a wall thickness of 2.0 cm. It is filled with ice, water and your tasty beverage at 0C. What is the rate of heat flow into the cooler if the outside wall is at 30C? How much ice melts in 8 hours?

13 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Convection of heat Heating by moving large amounts of hot fluid, usually water or air. Figure at right illustrates heat moving by convection.

14 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Symbols, signs, and definitions for heat and work Hot coffee is poured into a room-temperature mug and over time, they reach thermal equilibrium. What is the sign of Q for the coffee? Sign of Q for mug? What is Q and the sign of work for a coffee mug, if it slides along a table with friction (might help to consider what happens to thermal energy of cup)?

15 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Does the path of the PV change matter? The start, the finish, and the shape of the curve are all significant.

16 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Compressed air A compressed air cartridge at a starting pressure of p 1 = 50 atm and starting volume V 1 = 5 cm 3 is put into an empty, sealed balloon. It pops and causes the balloon to expand to 10 times the volume of the cartridge, V 2. Assuming the air undergoes an isothermal expansion and behaves like an ideal gas, draw the pV diagram of this process. What is the final pressure of the balloon? Two other ways different than the first method 1) of inflating the balloon to the same final volume are 2) a constant pressure p 1 = 50 atm inflates the balloon from V 1 to V 2 or 3) a constant pressure p 2 (calculated above) inflates the balloon from V 1 to V 2. Draw pV diagrams and rank the work done by the expanding air for the three cases of isothermal expansion.

17 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Signs of heat and work on a system Is the work W, the heat Q, and the change in thermal energy  E th, positive (+), negative (-) or zero (0) for the following situations? Does the temperature increase (+), decrease (-), or stay the same (0) WQ  E th  T 1.You hit a nail with a hammer 2.You hold a nail over a Bunsen burner 3.You compress the air in a bicycle pump by pushing down on the handle very rapidly 4.You turn on a flame under a cylinder of gas, and the gas undergoes an isothermal expansion 5.A flame turns liquid water into steam 6.High pressure steam spins a turbine 7.Steam contacts a cold surface and condenses

18 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Expanding Helium Helium gas starts at a volume V i = 1L and P i = 1 atm. It expands linearly to V f = 3L and P f = 3 atm. 1.Draw the pV diagram for this process 2.What is the work done by the gas when it expands? 3.What is its increase in internal energy? 4.How much heat is gained or lost by the gas?

19 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Condensing steam 1671 cm 3 of steam condenses to form 1 gram of water (1 cm 3 ) when held at a constant pressure of 1 atm (1.013 x 10 5 Pa). The heat of vaporization at this pressure is L v = x 10 6 J/kg. 1.Draw the pV diagram for this process 2.What is the work done by the water when it condenses? 3.What is its increase in internal energy?

20 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Cyclic processes and isolated systems Consider Problem-Solving Strategy Follow Example 19.2.

21 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Study of thermodynamic processes The cyclic process shown proceeds counterclockwise from a in the pV diagram to b and back and the total work is W = -500J. Why is the work negative? Find the change in internal energy and the heat added during this process

22 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Note the definitions on page 657 Adiabatic Isochoric Isobaric Isothermal

23 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley The processes on a PV diagram Notice the subtle differences for each curve in Figure

24 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Adiabatic changes In an adiabatic process, no heat is transferred from system and surroundings.

25 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Cyclic process A cyclic thermodynamic process occurs as shown, where path c-b is isothermal. Draw isotherms to determine temperatures of states a, b, c. Predict the Q, W and  E th for each process: What changes if c-b is adiabatic? QW UU a-c c-b b-a Whole cycle

26 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Adiabatic and isothermal processes An ideal gas at P i = 1 atm and V i = 1m 3 doubles its volume A isothermally and B adiabatically Draw a PV diagram for both processes What is the final pressure for process A and B? Compute W,  E th and Q for each process.

27 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Measuring heat capacities Heat capacities may be measured at constant volume in a fairly complex process using a bomb calorimeter. Heat capacities may be measured at constant pressure using equipment as simple as a coffee cup.

28 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Relating heat capacities at constant volume and pressure

29 Copyright © 2008 Pearson Education Inc., publishing as Pearson Addison-Wesley Heat capacities tabulated for selected gasses


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