What happens when we place a hot bowl of soup in a cool room? ice water at 5 o C air trapped from outside at 25 0 C water at 15 0 C air at 15 0 C Why the difference in the two scenarios? Different number of molecules involved in energy exchange

Once the temperature equalizes there is no further NET transfer of thermal energy. This condition (no NET transfer) is called Thermal Equilibrium something warm something cool two some things at an “intermediate” temperature make thermal contact Thermal Equilibrium air = 25 C soup = 95 C What happens over a period of time? There is a net transfer of thermal from the soup to the air. As a result, the temperature of the soup and the temperature of the air become the same. (i.e. reach Thermal Equilibrium) air soup more KinE more momentum more KinE more momentum some KinE lost some momentum lost what can we say about the average kE of the soup molecules compared to the air molecules?..how is kE related to temp?

The zero th law of Thermodynamics: (i.e. the transitive property) If A is in thermal equilibrium with B (Temp A = Temp B) And B is in thermal equilibrium with C (Temp B = Temp C) Then A is also in thermal equilibrium with C (Temp A = Temp C) ABC ABC Thermal equilibrium

Consider once again the Bowl of Soup example…….. At first less average thermal energy more average thermal energy same average thermal energy everywhere In which situation is the energy of the system (bowl of soup + air in room) more organized ? Later that day At First you could easily select a molecule with larger thermal (kE) energy……. just scoop out some of the “hot” soup Later that day molecules of various energies evenly distributed throughout the soup and air…who knows where to find one with a lot of energy

As a general rule, the disorganization of the energy of the Universe NEVER DECREASES as a result of a process. And, the disorganization of the energy of the Universe could only stay the same as a result of an IDEAL process (which never actually occurs) NOTE: Over time, things run down…but the universe does not lose energy!! Remember the track and ball example: “later”“start” hAhA hEhE “IDEAL CASE” “later”“start” hAhA h E < h A all organized thermal  disorganized “REAL CASE” At the “start” the energy is in an organized state, at point E the energy is still all in just as organized a state not all organized

We use the word Entropy to describe the quantitative measure of disorganization of energy. 1. The soup and the air have higher entropy after coming to the same temperature than before 2. The ball and the track along with the surroundings have more entropy when the ball is at point E than at point A in the “REAL CASE” The 4 laws of Thermodynamics: zero: If two systems are in thermal equilibrium with a third system, then then they are also in thermal equilibrium with one another one: energy can neither be created nor destroyed, only transformed into another type of Energy two: Entropy NEVER decreases and only stays the same in IDEAL processes three: there is a lowest possible temperature (= -273 C) it is not attainable due to a number of irreversible energy transfer play

T in T ex measure these in Kelvin, K, K = C somewhat disorganized organized highly disorganized “exhaust” “waste” 2 nd Law: Any process that uses thermal energy to do work must also have a thermal energy output or exhaust. In other words, heat engines are always less than 100% efficient. 3 rd Law: It is not possible for T ex to be at or lower than absolute zero (0 K) Flow of thermal energy is essential to heat engines.

Summary: 1. Every heat engine has at least some thermal energy as output. 2. even an “ideal” heat engine (which does not exist) is less 100% efficient The Ideal Efficiency (IE) of a heat engine is computed using: Note: T in and T ex must be expressed in Kelvin (K) For any real heat engine, the Actual Efficiency is less than the Ideal Efficiency:AE < IE

Exponential Growth thermodynamics: 1. Energy quality runs down as time proceeds 2. Useful energy is needed to do work and thus support human civilization history: Human populations grow at nearly an exponential rate problem: Will we run out of high quality energy resources? Solution: 1. Renewable energy resources? 2. Slow down the exponential population growth?

Linear Growth Start a new job and work for free the 1 st day with the agreement that you get a fixed $10,000 raise each day. Exponential Growth Start a new job and work for $1 the 1 st day with the agreement that you get a fixed 1% raise each day. Which job would you take (assuming each involves the same set of responsibilities? A. job #1 – Linear B. job #2 – Exponential Does the length of the contract affect your decision?

For exponential growth at rate P%, the population DOUBLES in a time: If P% is P%/year then T is in years If P% is P%/day then T is in days…etc… NOTE: this approximation only works for P% < 10%!!