Chapters Heat Temperature and Thermodynamics

Transcription

1 Chapters Heat Temperature and Thermodynamics Dr. Armen Kocharian

2 Second Law of Thermodynamics and You Can t Even Break Even

3 Energy Units Heating Heat Engines: Using Thermal Energy to Do Work Energy Quality: Things Run Down The Law of Entropy: Why You Can t Break Even The Automobile

4 Units Transportation Efficiency The Steam-Electric Power Plant Resource Use and Exponential Growth

5 Heating What happens if you put a cold object in contact with a warm object? The cold object warms up while the warm object cools off.

6 Heating This flow of thermal energy from a warmer object to a cooler one is called heating; it never spontaneously goes the other way. This is one way of stating the second law of thermodynamics: Thermal energy flows spontaneously from higher to lower temperature, but not from lower to higher temperature.

7 Heating So, what is temperature? Temperature is a quantitative measure of warmth. Most materials expand as they warm; this allows us to construct a thermometer. The Celsius temperature scale is in use throughout the world, except in the US. We use Fahrenheit.

8 Heat Engines: Using Thermal Energy to Do Work Creating thermal energy from other forms of energy is easy; all you have to do is rub your hands together. How can we create other forms of energy kinetic energy, say from thermal energy?

9 Heat Engines: Using Thermal Energy to Do Work One example is an automobile engine; it turns the thermal energy of burning fuel into kinetic energy of the car. Any device that turns thermal energy into work is called a heat engine. One feature of a heat engine is that not all of the thermal energy is used to do work. Just as with the hot exhaust coming out of a car s tailpipe, some thermal energy is always exhausted.

10 Heat Engines: Using Thermal Energy to Do Work The energy efficiency of a heat engine is its useful output divided by the total input: energy efficiency = (work output)/(thermal energy input)

11 Heat Engines: Using Thermal Energy to Do Work This leads to another way to state the second law of thermodynamics: Any cyclic process that uses thermal energy to do work must also have a thermal energy exhaust. In other words, heat engines are always less than 100% efficient at using thermal energy to do work.

12 Heat Engines: Using Thermal Energy to Do Work Heat engines depend on the spontaneous flow of thermal energy from hot to cold. The efficiency depends on both the hot input temperature and the cooler exhaust temperature.

13 Heat Engines: Using Thermal Energy to Do Work The best possible efficiency is without friction or other losses. If it is warm enough, the exhaust can be used for heating.

14 Energy Quality: Things Run Down The creation of thermal energy is irreversible you can never convert it all back to work. We see this every day, as friction causes things to slow down and stop. The energy is still there, but its quality is lost.

15 The Law of Entropy: Why You Can t Break Even This is a molecular view of a hot gas and a cold one, before and after they are put into contact. The difference between the two is gone; we say that the system is less organized.

16 The Law of Entropy: Why You Can t Break Even Molecular disorganization can be measured quantitatively; it is called entropy. 1 kg of ice has less entropy than 1 kg of water, as the ice is organized into crystals. This gives us yet another way to state the second law: The total entropy (or microscopic disorganization) of all the participants in any physical process cannot decrease during that process, but it can increase.

17 The Law of Entropy: Why You Can t Break Even The law of entropy means that most processes are irreversible; this is what makes it easy to tell if a movie is running backwards or forwards. This also gives us an insight into the ultimate fate of the universe its disorganization will continue to increase until no further interactions are possible. This is called the heat death of the universe.

18 The Law of Entropy: Why You Can t Break Even At first glance, biological systems might appear to be an exception to the rule; they certainly increase their own entropy, so what gives? What gives is that they can t do this without an external energy source; if you include that, the total entropy increases as it should.

19 The Automobile Automobiles are essential to our society; they also are substantial consumers of energy and contributors to pollution. This pie chart shows the fraction of the total U.S. energy consumed by each economic sector. Transportation, direct and indirect, uses 42%.

20 The Automobile This graph shows the fraction of petroleum consumed by each economic sector. Transportation, direct and indirect, uses 78%.

21 The Automobile Finally, this chart shows the direct transportation energy consumption. This is 27% of the total energy consumption, and autos use 40% of that.

22 The Automobile Most transportation energy is consumed by heat engines using pistons.

23 The Automobile This diagram shows the energy flow in a typical car engine at highway speeds.

24 The Automobile The theoretical maximum efficiency of this engine is 30%; its actual efficiency is 25%. This second-law energy loss is the most significant one. Most of the lost energy is removed by the tailpipe and the radiator. The exhaust is carbon dioxide and water vapor, along with smaller amounts of toxics.

25 The Automobile Due to the increasing price of oil, concern about pollution, and the world political situation, there have been many efforts made to develop alternate fuels for vehicles.

26 The Automobile This is a solar-powered car. Electricity generated by the solar panels is stored in batteries. Solar cars are not street-legal.

27 The Automobile This is a battery-powered electric vehicle. It goes 200 km on one battery charge, and takes 6.5 hours to recharge.

28 The Automobile This was the first commercially available gas-electric hybrid in the US. Now many more models are available, and they are increasing in popularity.

29 The Automobile These are experimental fuel-cell vehicles. They convert the chemical energy of hydrogen and oxygen directly into electricity.

30 Transportation Efficiency One way of measuring the efficiency of automobiles is to compare gas mileage for different models.

31 Transportation Efficiency This does not give the whole picture, however. Mass transportation is much more efficient per person a city bus carrying 40 people uses much more gas than a hybrid car, but less than 40 hybrid cars. Other possibilities include trains, bicycles, and walking.

32 Transportation Efficiency Here we compare the relative efficiency per passenger for different modes of transportation. Bicycling is by far the most efficient, although it is not always practical.

33 Transportation Efficiency Here we do the same type of comparison for freight rather than passengers. Rail is the most efficient, due to its favorable aerodynamics and the relatively small energy loss of steel wheels on steel track.

34 Transportation Efficiency Combining all this information, we can see several ways to improve efficiency: New, higher mileage standards for cars Promote hybrids Promote carpooling Encourage mass transit, discourage cars Move freight by train Encourage walking, bicycling, and mass transit in cities, discourage driving

35 Transportation Efficiency So, how does the rest of the natural world fare compared to us? Humans on bicycles are still the most efficient, although if we look at unaided locomotion, walking humans are outclassed by salmon and horses.

36 The Steam-Electric Power Plant This is a diagram of a typical coal-fueled steam-electric power plant. Water is boiled and the steam used to turn a turbine. The steam is cooled back to water and the cycle repeats.

37 The Steam-Electric Power Plant This diagram shows the energy flow in the power plant. The plant has an overall efficiency of 40%.

38 Resource Use and Exponential Growth Compound interest is a good example of exponential growth. If you invest $100 at 10%, for example, the first year you get $10 (so now you have $110). The next year you get 10% of $110, or $11, and so forth.

39 Resource Use and Exponential Growth Any quantity whose growth is a percentage of the existing quantity will experience exponential growth. In the absence of predators or illness, populations grow exponentially. If you put one bacterium with a dividing time of one minute in a jar, and notice that the jar is full after an hour, when was the jar half full? Only a minute before!

40 Resource Use and Exponential Growth Resources can vanish quickly as exponential growth continues. The bacteria at 52 minutes didn t think they had a space problem, and could point to 52 minutes of history; 8 minutes later they had no space left.

41 Resource Use and Exponential Growth In the real world, exponential growth is usually slowed by many factors. This graph shows the electricity generated per year in the 20 th century U.S. It was approximately exponential from , but then slowed.

42 Resource Use and Exponential Growth A typical nonrenewable resource has a life cycle of exponential growth, followed by leveling off as the resource gets scarcer and more expensive, followed by decline as the resource is exhausted.

43 Resource Use and Exponential Growth A renewable resource has a life cycle of exponential growth, followed by leveling off to a steady rate of usage.

44 Resource Use and Exponential Growth The world s population is growing exponentially. Can this expansion be sustained, even with renewable resources?