Chapter 11: The 1 st Law of Thermodynamics

Transcription

1 Chapter 11: The 1 st Law of Thermodynamics Thermodynamics: the study of processes in which energy is transformed as heat and as work. ( movement of heat ) Recall: Work is done when energy is transferred from one body to another by mechanical means. (Ex: compressing a gas.) Heat is a transfer of energy due to a difference in temperature. (Ex: Heating a gas.)

2 Lets start with what you know In this video, what is energy being transferred to the wheel from? What needed to exist prior to the wheel spinning? kn9hac AZeg34

3 Heat Engines Defined A heat engine is any device that changes thermal energy (heat) into mechanical energy (work), such as steam engines and automobile engines. Any device that uses heat to do work. Note: it is easier to produce heat (thermal nrg) from work (rub your hands together) than it is to produce work from thermal nrg.

4 Heat Engine Process Heat is supplied to the engine at relatively high temperatures from a place called the hot reservoir (the hot fuel). Part of the heat input is used to perform work via the working substance of the engine which is the material w/in the engine that actually does the work. (Ex: gas/air mixture in combustion chamber.) The remainder of the input heat is rejected at a temperature lower than the input temperature to a place called the cold reservoir (or heat sink). (Ex: radiator or exhaust). This is waste heat!

5 Schematic of Heat Engine, Fig This schematic representation of a heat engine shows the input heat (Q H ) that originates from the hot reservoir, the work (W) that the engine does, and the heat (Q C ) that the engine rejects to the cold reservoir. Work is only done when heat is transferred from high temp. to low temp. Figure 15.12

6 Power plants: one example of a Heat Engine Power plants use the heat from the burning of coal, oil, gas, or heat from nuclear fission to produce energy that does work by turning electric generators. Waste heat is also produced, sometimes referred to heat pollution because it pollutes the environment. Waste heat discharged into waterways increases the temperature of that waterway. Waste heat discharged into air can contribute to weather changes.

7 Efficiency (e) of a Heat Engine Efficiency = ratio of work the heat engine does to the heat input at high temp. e = (work done) / (Input heat) = W / Q H can multiply this value by 100 to get percent efficiency The less heat needed to do the work = greater efficiency An engine that is 100% efficient would have an e value of 1 (or 100%), meaning that Q H = W (non-existent)

8 Heat engines must obey Conservation of Energy Law (1 st Law of Thermodynamics), so: Q H = W + Q C (1 st Law) W = Q H Q C Substitute into efficiency equation: e=w/q H = (Q H Q C )/Q H e = 1 (Q C / Q H ) Or e = 1 (Tc / Th ) The smaller Q C is, the greater the efficiency (less Figure 15.12

9 Operation of a Heat Engine Slide 11-26

10 Answer Consider your body as a system. Your body is burning energy in food, but staying at a constant temperature. This means that, for your body, A. Q > 0. B. Q = 0. C. Q < 0. Slide 11-40

11 Additional Questions The following pairs of temperatures represent the temperatures of hot and cold reservoirs for heat engines. Which heat engine has the highest possible efficiency? A. 300 C 30 C B. 250 C 30 C C. 200 C 20 C D. 100 C 10 C E. 90 C 0 C Slide 11-41

12 Answer The following pairs of temperatures represent the temperatures of hot and cold reservoirs for heat engines. Which heat engine has the highest possible efficiency? A. 300 C 30 C B. 250 C 30 C C. 200 C 20 C D. 100 C 10 C E. 90 C 0 C Slide 11-42

13 4 stroke engine cycle Internal Combustion Engine

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15 Refrigerators, Air Conditioners & If work is done on a system, heat can be forced to flow from cold to hot! In general, this is called a refrigeration process. Work (W) is used to remove heat (Q C ) from cold reservoir and deposit heat into hot reservoir (Q H ) Heat Pumps

16 Operation of a Heat Pump Slide 11-29

17 Refrigerator and Air Conditioner

18 Checking Understanding: Increasing Efficiency of a Heat Pump Which of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen. A. All of the above B. 1 and 4 C. 2 and 3 Slide 11-31

19 Answer Which of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen. A. All of the above B. 1 and 4 C. 2 and 3 Slide 11-32

20 The Theoretical Maximum Efficiency of a Heat Engine Note: This equation does not depend on the type of fuel used in the heat engine nor is that important. Slide 11-27

21 Slide 11-23

22 Closed system: ΔU = Q + W (Only nrg exchanged) Open system: ΔU = Q + W (but need to take into account the change in internal nrg due to the increase/decrease in the amount of matter) Isolated system: ΔU = Q + W = 0 (since no nrg in any form can leave or enter the system i.e., ΔU=0.)

23 Refrigeration Process: Energy is conserved in this process: Q H = W + Q C This process is reversible. The inside of a refrigerator is the cold reservoir, the outside is the hot reservoir. The warm air you feel blowing out of your frig is equal to the heat removed from inside your frig plus the work done in removing it. Q H = W + Q C The same holds for air conditioner: house=inside of frig, etc.

24 So, is it possible to cool your kitchen by leaving your frig door open (or placing an air conditioning unit on the floor in the middle of the room)? NO! Heat is removed from inside frig and exhausted into kitchen, plus the work to remove this heat is also exhausted result? Your kitchen gets warmer when you open the frig door! Heat is removed from room by A/C unit, but exhausted back into room, in addition to the work to remove this heat is also exhausted! Doesn t cool the room; heats it up and wastes electricity!!

25 Reading Quiz 3. A refrigerator is an example of a A. reversible process. B. heat pump. C. cold reservoir. D. heat engine. E. hot reservoir. Slide 11-10

26 Answer 3. A refrigerator is an example of a A. reversible process. B. heat pump. C. cold reservoir. D. heat engine. E. hot reservoir. Slide 11-11

27 Checking Understanding: Increasing Efficiency of a Heat Pump Which of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen. A. All of the above B. 1 and 4 C. 2 and 3 Slide 11-31

28 Answer Which of the following changes would allow your refrigerator to use less energy to run? (1) Increasing the temperature inside the refrigerator; (2) increasing the temperature of the kitchen; (3) decreasing the temperature inside the refrigerator; (4) decreasing the temperature of the kitchen. A. All of the above B. 1 and 4 C. 2 and 3 Slide 11-32

29 Refrigeration Cycle how does a refrigerator or A/C work anyway? 1. Low pressure gas enters compressor where it becomes high pressure, hot gas. 2. The gas goes into condenser coil (behind or under the frig) where heat* is removed (by coil) and cold (high pressure) fluid comes out (gas liquid: releasing heat in process). *Part of this heat is from what was extracted from inside frig. 3. The high pressure fluid goes through an expansion valve which allows the pressure (& temp.) to drop. 4. The low pressure (cold) fluid goes into evaporator coils in frig which turns into a gas as energy is absorbed from inside frig. 5. Back to the compressor step 1.

30 Refrigeration Cycle

31 Heat Pumps The air conditioner and heat pump do closely related jobs. The air conditioner refrigerates the room and heats up the outdoors, while the heat pump refrigerates the outdoors and heats up the inside room. A heat pump can serve in a dual capacity equipped w/a switch that converts them from A/C units in summer to heaters in winter cool!

32 Heat Pumps A heat pump takes heat from Q C and with the addition of work, places it in Q H. Q H = Q C + W Heat is removed from inside frig (or outside of house where it s cold) and exhausted into kitchen (or inside your house), plus the work to remove this heat is also exhausted result? Your kitchen (inside of house) gets warmer (Remember opening the frig door causes the kitchen to heat up!)

33 What s more efficient, a heat pump or conventional electric heating system? A heat pump is very efficient b/c if 1000J is used to do work of moving Q C from outside to inside, you get heat totaling Q H =Q C +W, or Q H =Q C +1000J inside! Your heat pumps 1000J + the Q c taken from the outdoors! Versus, if you have an electric heater and 1000J of electric nrg is being delivered, then all you get is 1000J of heat, Q H. conventional electric heating system

34 Does it take more or less work the greater the temperature difference when heating your house with a heat pump? More it takes more work to pump heat uphill from cold reservoir (outside) to hot reservoir (inside). Remember, heat naturally flows from hot to cold, so going from cold to hot is an uphill climb for the heat. m/mytopic=12610