Review Questions from Chapter 17 If the temperature of an object is increased from 0°C to 273°C, what happens to the power of the radiation emitted? If you mix 1 g of ice at 0°C with 1 g of water vapor at 100°C in an insulated container, what will be the equilibrium temperature of the mixture? How much ice at 0°C would you have to mix with 1 g of water vapor at 100°C in an insulated container to ensure that there is no water vapor left at equilibrium?

Thermodynamics Thermodynamics started out as a macroscopic theory. Max Plank pushed the idea that you could describe things both macroscopically and microscopically. What is heat? It was originally thought that there is a thing called caloric fluid that flows from hot to cold. This was disproved experimentally. Ex: cannon making Cannons got hotter without adding heat

For our purposes, unless otherwise specified, we will think that changes in internal energy come only from changes in thermal energy. This will be true of there are no chemical reactions, no nuclear reactions, and no changes in the magnetic or electric properties.

Recall that work is force times displacement (a special sort of “times” called the dot product).

Adding heat to the system increases internal energy The system doing work on its surroundings decreases internal energy

Which has more internal energy, solid water at 0C or liquid water at 0C? Liquid water, you add Q to melt ice. Example - blow on hand, then purse lips, compressed air. Air expands and cools after leaving your lips, doing work on the surrounding air, so the air hitting your hand feels cool. Which has more internal energy, solid water at 0C or liquid water at 0C? Liquid water, you add Q to melt ice. Example - blow on hand, then purse lips, compressed air. Air expands and cools after leaving your lips, doing work on the surrounding air, so the air hitting your hand feels cool.

Which has more internal energy, solid water at 0C or liquid water at 0C? Liquid water, you add Q to melt ice. Example - blow on hand, then purse lips, compressed air. Air expands and cools after leaving your lips, doing work on the surrounding air, so the air hitting your hand feels cool. Which has more internal energy, solid water at 0C or liquid water at 0C? Liquid water, you add Q to melt ice. Example - blow on hand, then purse lips, compressed air. Air expands and cools after leaving your lips, doing work on the surrounding air, so the air hitting your hand feels cool.

Why does air cool as it rises? What is an adiabatic process?

What happens to the temperature of compressed air when it is allowed to escape through an expansion nozzle into the atmosphere? What happens to the air temperature when an air mass carried by prevailing wind rises from a valley floor up a mountain?

First law of thermodynamics Relation of heat, work and internal energy As a specific case of conservation of energy Historical significance End of “caloric” theory of heat Examples of phenomena explained by the 1 st law Heating during rapid compression

Second law of thermodynamics The most basic way of saying the second law is that heat tends to flow from hot objects to cold objects We will have three other ways of saying this – you need to know all of them Concerns of the 2 nd law Irreversibility of processes The quality of energy (usefulness of energy)

There are several ways to express the 2nd Law: Macroscopic: 1. Heat flows from hot to cold 2. You can’t have a perfect heat engine 3. You can’t have a perfect refrigerator. Microscopic: 4. In a closed system, entropy increases as time goes by (or at least does not decrease). Entropy is a measure of disorder.

2 nd Law of Thermodynamics Experimental evidence suggests strongly that it is impossible to build a heat engine that converts heat completely to work hot W Qh device Cannot happen

2 nd Law of Thermodynamics Heat engine statement of 2 nd law - it is impossible to build a heat engine that converts heat completely to work, some heat must be dumped to a colder place hot W Qh device cold Qc Some heat must be dumped to a colder place W<Qh

There are several ways to express the 2nd Law: Macroscopic: 1. Heat flows from hot to cold 2. You can’t have a perfect heat engine 3. You can’t have a perfect refrigerator. Microscopic: 4. In a closed system, entropy increases as time goes by (or at least does not decrease). Entropy is a measure of disorder.

Thermodynamic Efficiency of a heat engine Efficiency of a heat engine is the ratio of output work to the heat from the hot reservoir (often the heat from burning fossil fuel) The heat dumped to a colder place causes W to be smaller than Qh, and therefore the efficiency to be less than 100%

Thermodynamic Efficiency of a heat engine The best possible efficiency of a heat engine (the ideal efficiency or Carnot efficiency) depends on the ratio of the cold temperature to the hot temperature, or more directly, the ratio of the temperature difference to the hot temperature. (the temperatures are in kelvin) Most power plants boil water with the heat from burning fossil fuel or from nuclear reactions, and this steam does work on a turbine. Why is high pressure useful for the steam that is used in a power plant?

2 nd Law of Thermodynamics Refrigerator – one would like to be able to remove heat from a cold place (the inside of a refrigerator) and put it in a warm place (a room inside a house) hot Qh device cold Qc Cannot happen, work has to be done

There are several ways to express the 2nd Law: Macroscopic: 1. Heat flows from hot to cold 2. You can’t have a perfect heat engine 3. You can’t have a perfect refrigerator. Microscopic: 4. In a closed system, entropy increases as time goes by (or at least does not decrease). Entropy is a measure of disorder.

2 nd Law of Thermodynamics Refrigerator statement of 2 nd law – to move heat from a cold place to a warmer place, work must be done hot W Qh device cold Qc Need to do work to move heat from a cold place to a warmer place

Refrigerator Recall that another way to state the second law is that heat flows spontaneously from a hot place to a cold place, so the picture on the right is in violation of this

Inside refrigerator Room around refrigerator Try refrigerator in class? (hotter)

Couldn’t I just use my refrigerator to cool my home?

What if the heat was vented to the outside (the compressor and condenser were outside)?

Heat pump This is the same picture as the refrigerator. But if the device is a heat pump, the cold place is outside, and the hot place is inside the house or the building. Note that more energy goes into the house than the work that is done (in some sense, more than 100% efficient – but not in a thermodynamic sense). hot W Qh device cold Qc Need to do work to move heat from outside to the warmer inside

Second law of thermodynamics The 2 nd law in terms of organization (microscopic) Entropy – labeled S, a measure of disorder (microscopic disorder – may not be apparent macroscopically) In a closed system, the entropy tends to increase – it cannot decrease. For reversible processes, the entropy does not change Caution: 2 nd law applies to Systems, not processes Closed, not open systems

There are several ways to express the 2nd Law: Macroscopic: 1. Heat flows from hot to cold. 2. You can’t have a perfect heat engine 3. You can’t have a perfect refrigerator. Microscopic: 4. In a closed system, entropy never decreases

Suppose you drop an ice cube into a hot cup of tea. The ice melts and the water cools. Why doesn’t the reverse happen – the water warming up and the ice cube formed? Review question: pick up water and let it fall. How high must it be to change the temperature by 1 K when it hits the ground? use mgh = mCΔT

Suppose you drop an ice cube into a hot cup of tea. The ice melts and the water cools. Why doesn’t the reverse happen – the water warming up and the ice cube formed? Review question: pick up water and let it fall. How high must it be to change the temperature by 1 K when it hits the ground? use mgh = mCΔT

Rayleigh Benard convection – uniform temperature on bottom plate and on top plate

Rayleigh Benard convection – with enough temperature difference, convection begins and forms patterns that were not there in the system to begin with. Order increase

Rayleigh Benard convection videos 2-dimensional simulation – note that the small fluctuations are enhanced (positive feedback), but are opposed by viscosity and dissipation 3-dimensional simulation – note the stable pattern that develops CS/96-97/Masson-Nicole/rb.html (development of instability) CS/96-97/Masson-Nicole/rb.html

Conditions Needed for order to develop (within some portion of the system) Must be open (energy must flow through) Must be far from equilibrium Must have a feedback system If the Rayleigh Benard system is placed in an insulated box, so that it becomes a whole system, convection can start and order can develop. Is the entropy of the system increasing? That is, is the total order decreasing? Why or why not?

Spontaneous organization In which portion does the entropy change in which ways? Necessary conditions for occurrence Open system Far from equilibrium Description of process Order through fluctuations Dissipative structures Common misunderstandings of spontaneous organization – As a violation of the 2 nd law – Impossibly rare according to a naïve view of development by chance

Maxwell’s Demon

The drinking bird 1 st law of thermodynamics, 2 nd law of thermodynamics, evaporation as a cooling process, saturated vapor pressure changes rapidly with temperature (compared with single phase gas pressure)

If the cold place to which you are dumping heat is outside on a coolish day (10°C), how hot must the steam be in an ideal power plant be to get 50% efficiency? (ideal means that there is no waste whatsoever, and there is no friction in any of the moving parts) The vapor pressure of water is given in the table on the right (for interest) T (°C)P (torr)P (atm) , , , , , , , , , , , , ,