Thermodynamics Ch 21 to 24

Regelation When water is frozen and pressure is applied to a certain area, that area will melt – then immediately refreeze upon removal of the pressure. This is called REGELATION. An example of regelation is making a snowball.

Space Shuttle The kinetic energy of spacecraft in motion is many times greater than the amount needed to vaporize the craft. Heat shields are tiles made of synthetic resin and plastic that dissipates the heat by melting and vaporizing.

Space Shuttle At altitudes of 25 to 40 km (pressure is a factor) almost all of the kinetic energy is dissipated within about a minute. The shield tiles heat to several thousand degrees Celsius. The shield tiles have very low conductivity – only a small portion of the heat of re-entry is absorbed by the craft.

Space Shuttle About a centimeter of the tile is consumed. It radiates about 80% of the heat to the surrounding air. If the trajectory of the shuttle is too steep (increased kinetic energy), the heat is too severe for the tiles to dissipate.

Space Shuttle If the trajectory is too flat, the spacecraft is in danger of “bouncing” off the atmosphere.

1 st Law of Thermodynamics Whenever heat is added to a system, it transforms to an equal amount of some other energy. Heat added = increase in internal energy = external work done by the system.

Adiabatic The expansion or compression of gas occurring without gain or loss of heat. This happens when the process is performed rapidly so heat has little time to enter or leave. This also occurs if the system is insulated.

Adiabatic So … when work is done on a gas by adiabatically compressing it, the gas gains internal energy and becomes warmer. Then it is adiabatically expanded.It does work on its surroundings and gives up internal energy, and thus becomes cooler.

Adiabatic Applied to our atmosphere means that a change in air temperature means a change in air pressure. Adiabatic processes in the air occur in large air masses called “blobs”. Because these blobs are so large mixing of different temperatures or pressures only occurs at the edges.

Adiabatic As air rises, the pressure decreases and the blob can expand and cool. Therefore if you decrease pressure, you decrease temperature. An increase of one kilometer in altitude equates to a drop of 10ºC

Chinook The Chinook is a great wind found in the Rockies all the way to the Great Plains. Cool air moves down the slopes of mountains. It is compressed by the atmosphere. More pressure = higher temperature. The Chinook is an indicator of higher temperatures to come.

2 nd Law of Thermodynamics Heat will never flow from cold to hot without external effort. HEAT ALWAYS FLOWS FROM HOT TO COLD!!!!!

Heat You can change work to heat. An example is rubbing your hands together. You can’t change heat to work. A heat engine is any device that changes internal energy into mechanical work. Mechanical work can only be obtained when heat flows from a higher temperature to a lower temperature.

Heat Engines Any device that changes internal energy into mechanical work. Mechanical work can be obtained only when heat flows from high temperature to low.

EVERY heat engine WILL… Absorb heat from a reservoir of higher temperature. This increases internal energy. Convert some of this energy into mechanical work. Expel the remaining energy as heat to some lower temperature reservoir. (This is called the sink.)

Heat Engines There is always heat exhaust. Sometimes we want it. Sometimes we don’t. If we don’t, its called thermal pollution.

Heat Engines Ideal Efficiency of a Heat Engine (also called Carnot Efficiency) Ideal efficiency = (T hot – T cold )/T hot Temperatures are expressed in Kelvins only.

Heat Engines If you increase the operating temperature of any heat engine (as opposed to exhaust temperature), you increase that engine’s efficiency. Friction always reduces the efficiency of an engine.

Entropy Natural systems tend to proceed toward a state of disorder. Ordered energy tends towards disorder. High grade energy always degrades.

Entropy Entropy is the measure of the amount of disorder. If disorder increases, then entropy increases. ENTROPY ALWAYS INCREASES!

Entropy & Thermodynamics

YOU CAN’T WIN You can never get more energy out of the system than you put into the system.

YOU CAN’T BREAK EVEN You can’t even get as much energy out of the system as you put in. Friction takes a portion.

YOU CAN’T QUIT Entropy in the universe is always increasing – quitting won’t stop it.

EXAMPLES

Example #1 Calculate the ideal efficiency of a heat engine that takes in energy at 800K and expels heat to a reservoir at 300K.

Solution Efficiency = (T hot – T cold )/T hot (800K – 300K)/800K 0.63 or 63%

Example #2 Calculate the ideal efficiency of a ship’s boiler when steam comes out at 530K, pushes through a steam turbine and exits into a condenser that is kept at 290K by circulating seawater.

Solution Ideal efficiency = (T hot – T cold )/T hot (530K – 290K)/530K 0.45 or 45%

Example #3 Calculate the ideal efficiency of a steam turbine that has a hot reservoir of 112ºC high pressure steam and a sink at 27ºC.

Solution Reservoir = = 385K Sink = = 300K Efficiency = (T hot – T cold )/T hot (385K – 300K)/385K 0.22 or 22%