Review

An amount of heat energy – change in energy – is Q.

Heat capacity (C) is the amount of energy transferred via heating necessary to raise the temperature of a substance by one degree.

calorie (cal) is defined in terms of the SI unit of energy

1 cal = 4.184 J

(The US uses the Btu which is now defined as related to the joule and the calorie.)

1 Btu = 1.054 kJ = 252 cal

Specific heat (c) is the heat quantity of a substance per unit mass. (The higher the specific heat of a substance, the more slowly it changes temperatures. You must add more heat to water to change its temperature by one degree than to iron.)

Molar specific heat (c’ ) is the heat capacity per mole.

NOTE: Systems have amounts of internal energy – not heat or work.

During a quasi-static process, a material never moves far from an equilibrium state.

Experimentally, most solids have molar heat capacities approximately equal to 3R (c’ = 3R = 24.9 J/mol-K). This is known as the Dulong-Petit law.

Heat is transferred from one substance to another because of a temperature difference.

The internal energy of a system is a property of the state of the system, as is the pressure, volume, and temperature. Heat and work are NOT properties of state.

2nd law (Clasius statement): A process whose only net result is to transfer energy as heat from a cooler object to a hotter one is impossible.

2nd law (Kelvin statement): No system can take energy as heat from a single reservoir and convert it entirely into work without additional net changes in the system or its surroundings.

heat engine is a cyclic device whose purpose is to covert as much heat input into work as possible. Heat engines convert thermal energy into mechanical energy (heat into work). Heat engines contain a working substance (i.e., gasoline vapor).

2nd law (heat engine statement): It is impossible for a heat engine working in a cycle to produce only the effect of extracting heat form a single reservoir and performing an equivalent amount of work. Engines are never 100% efficient.

Refrigerator (freezer, air conditioner, etc.) is essentially a heat engine running in reverse.

2nd law (refrigerator statement): It is impossible for a refrigerator working in a cycle to produce only the effect of extracting heat form a cold object and reject the same amount of heat to a hot object.

2nd law (Carnot theorem): No irreversible engine working between 2 given heat reservoirs can be more efficient than a reversible engine working between those 2 reservoirs.

Conditions necessary for a process to be reversible:

1. No mechanical energy is transformed into thermal energy by friction, viscous forces, or other dissipative forces.

2. Energy transfer as heat can only occur between objects at the same temperature (or infinitesimally near an equilibrium state).

3. The process must be quasi-static so that the system is always in an equilibrium state (or infinitesimally near an equilibrium state).

Steps in a Carnot Cycle

1. A quasi-static isothermal absorption of heat from a hot reservoir.

2. A quasi-static adiabatic expansion to a lower temperature.

3. A quasi-static isothermal exhaustion of heat to a cold reservoir.

4. A quasi-static adiabatic compression back to the original state.

Heat pump – refrigerator with a different objective: to heat an object or region.

In an irreversible process, some of the energy becomes unavailable to do work.

Entropy is a measure of the disorder of a system. It is related to probability. A state of high order has a low probability; a state of low order has a high probability.

The universe moves from a state of low probability to one of high probability in an irreversible process.

Thermodynamics is applicable only to macroscopic systems.

In a reversible engine, the total entropy change is zero. For reversible heat engines, the amount of work done by the engine is W = efficiency X the amount of energy taken from the hot reservoir and the efficiency of such an engine is efficiency = 1 – [T (cold) / T (hot)].

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