Todays Topics: 1. 1st Law of Thermodynamics 2. Gases and Work Wed/Thurs Block Day February Todays Topics: 1. 1st Law of Thermodynamics *Equation 2. Gases and Work 3. Efficiency 4. Entropy 5. 2nd Law of Thermodynamics 6. Heat Death

1st Law of Thermodynamics and Work When it comes to changes in the internal energy of a system, the system may or may not have work done on it or by it, and it may not necessarily have heat added or lost.

∆U = Q + W 1st Law of Thermodynamics The change in the total internal energy (thermal energy) of a system is the sum of the work done on it or by it, and the quantity of heat that has been added to or removed from the system ∆U = Q + W ∆U = change in Total Internal Energy W = net work done on or by the system When it comes to changes in the internal energy of a system, the system may or may not have work done on it or by it, and it may not necessarily have heat added or lost. Q = net heat transferred (“gained” or “lost”)

Watch out for that sign!! Heat (Q) Work (W) Removed from the system, Q is negative Added to the system, Q is positive Work (W) Done by the system, W is negative Done on the system, W is positive

A beaker sitting on a hot plate receives 100 J of heat (and does no work). By how much does the internal energy of the water increase? ∆U = Q + W ∆U = (+100) + 0 = +100 J If 10 J of energy is added to a system that does 4 J of external work, by how much will the internal energy of that system be raised? ∆U = Q + W ∆U = (+10) + (- 4 ) = + 6 J 500 Joules of work are done on a system. In the process a small amount of heat, 25 J, is dissipated from the system. What is the change in the total internal energy of the system? ∆U = Q + W ∆U = (-25) + (+500) = + 475 J

A system initially contains 27 J of internal energy A system initially contains 27 J of internal energy. Heat is then added to the system. If the final internal energy is 34 J and the system does 26 J of work, how much heat was added to the system? ∆U = Q + W ∆U = (+100) + 0 = +100 J

A 200 g quantity of water is heated in a beaker and then is poured onto a thermos. The system’s internal energy increases by 8.3x103 J of energy is transferred to the surrounding air. How much work is done on the thermos?

Kinetic Molecular Theory Gases and Work Internal energy in a system can converted to mechanical energy (as well as other energy storage modes) to do work. HOW?? Particles in warmer bodies move faster than particles in cooler bodies higher temperature means more active particles (known as the ) Because of this, heating of a sealed pistol containing a gas will result in: particles moving faster volume of the gas increases pressure inside the sealed container increases Kinetic Molecular Theory

Gases and Work When the volume of a gas decreases, it is because work has been done on the gas by the surrounding system (-W) Gas is compressed Pressure increases When the volume of a gas increases, the gas can do work on the surrounding system (+W) Gas expands Pressure decreases The air/gas inside the cylinder is compressed. Once ignited, it expands rapidly to power the engine (does work).

W = P •ΔV Gases and Work W = net work done on or by the gas “change in volume” ΔV = Vf - Vi P = pressure inside of a closed system

Think about it: The carbon dioxide in a fire extinguisher is at room temperature. But when the carbon dioxide is expelled through the nozzle, it can get cold enough to chill the water vapor in the air to snow. How is this possible?

Efficiency % Efficiency = (Workout / Workin) x 100 It is impossible to convert all internal energy into purely mechanical energy & work Some heat (Q) is ALWAYS “lost” to the system, so as long as a machine has moving parts, it will never be 100% efficient. % Efficiency = (Workout / Workin) x 100

A Rube Goldberg design… In reality, this could never work!

Entropy & The 2nd law Of thermodynamics

What is Entropy? Entropy is a measure of disorder

Entropy One of the ideas involved in the concept of entropy is that nature tends from order to disorder in isolated systems. This tells us that the left hand box of molecules happened before the right. 

Entropy You can observe that nature, from the largest system to the smallest, tends to take things from order to disorder. It is a part of our common experience. Spend hours cleaning your desk, your basement, your attic, and it seems to spontaneously revert back to disorder and chaos before your eyes.

the 2nd law of thermodynamics States: Natural and spontaneous processes will always move towards a state of greater ENTROPY (disorder) Also, we observe that: Most Probable = Least Useful Thermal energy at equilibrium is an example of ”less useful” energy since the molecules are uniformly distributed in a high entropy state. If hot water is forcibly separated from cold water, heat will flow & ‘useful’ work can be done.

In order to prevent (or decrease) entropy, you must input energy into the system

Systems, work & entropy Staying ordered is difficult Easy to disorder the tower (knock it down) Environment is full of random things that can destroy the order of the tower Work and/or Energy are necessary to keep the tower from breaking down Systems need energy just to stay the same Even more energy input is needed to build systems

Many scientists believe that the ultimate fate of the universe is a “heat death” in which the whole universe is at one uniform temp. This would represent maximum entropy. No life could exist, since life requires energy uptake and expenditure. The Heat Death of the universe refers to a time in the distant future when the whole universe: Runs out of energy. Overheats Freezes None of the above.

The Heat Death refers to a time when the whole universe comes to the same temperature. It might be hot, or it might be cold, or it might be just right. It doesn’t matter what it is just as long as it is the same all over. The universe is not separated into hot and cold places. The starts are hot; the space between the stars is cold, but each day the stars get a little cooler as they radiate their energy—and each day the place the energy goes to gets warmer. Sooner or later the temperature difference must vanish. After heat death the universe will still have energy, but the energy will be all the same temperature, which takes away it’s ability to do work. It has the potential to do work only if a cooler place can be found.