Laws of Thermodynamics, Heat Engines and Entropy

• 1st Law of Thermodynamics

• 2nd Law of Thermodynamics

• Heat Engines

• Thermal Efficiency

The internal energy U of a system is increased by the transfer of either heat or work into the system.

system by the doneWork -Heat Added =Energy Internal

micsThermodyna of LawFrist

WQU

What is INTERNAL ENERGY?

The internal energy of the system is the sum of the kinetic and potential energies of the atoms and molecules making up the system.

Steam issuing from the kettle makes the pinwheel turn in this simple steam turbine. Work could be done to lift a small weight with such an engine.

Heat released by burning gasoline in the cylinder of an automobile engine causes the piston to move, converting some of the heat to work.

The First Law of Thermodynamics:

The increase in the internal energy of a system is equal to the amount of heat added to the system, minus the amount of work done by the system.

U = Q - W

Q=heat add to system

W = work done by system

Work done by a movable piston

Work=F x d = F/A x d x A = P V

2nd Law of Thermodynamics

No engine, working in a continuous cycle, can take heat from a reservoir at a single temperature and convert that heat completely into work.

A restatement of the Second Law which turns out to be equivalent:

Heat will not flow from a colder body to a hotter body unless some other process (which does work) is also involved.

Another restatement to that is:

The entropy of an isolated system can only increase or remain constant. Its entropy cannot decrease.

What is a Heat Engine?

• Thermal heat QH is introduced into the engine.

• Some of this is converted into mechanical work, W.

• Some heat is released into the environment at a lower temperature, QC.

What is a Heat Engine?

•Heat Engines are device that transforms heat into work.•It requires two energy reservoirs at different temperatures•An energy reservoir is a part of the environment so large wrt the system that its temperature doesn’t change as the system exchanges heat with the reservoir.•All heat engines and refrigerators operate between two energy reservoirs at different temperatures TH and TC.

A schematic representation of a heat engine. Heat is

taken in at high temperatures, TH. Some

heat is converted to work, and the remainder is released at a lower

temperature, TC.

Examples of Heat engines.Car, truck, jet, and rocket engines are heat engines. So are steam engines and turbines

What is a Heat Engine?

The arrow widths depict the quantities of energy in the sample exercise in box 11.1.

What does the First Law tell us about heat engines?

U = Q - W = QH - QC -W

the internal energy U of a heat engine does not change from cycle to cycle, so U =0.

Hence, Q = W.

The net heat flowing into the engine equals the work done by the engine:

W = QH - QC

The efficiency of a heat engine is defined as

e = W/ QH .

Engines are more efficient if there is a large difference between the high temperature inside and the low temperature outside.

For an ideal heat engine,

ec = (TH - TC )/ TH

The Second Law of Thermodynamics

The Carnot engine is an ideal system which turns out to have the maximum possible efficiency:

If TH is the hottest temperature in the engine, and TC is temperature outside the engine (in Kelvin), then the efficiency is:

ec = (TH - TC )/ TH

=W/QH

This shows that it is not possible too have an efficiency of 100%. You always lose some energy into heating the environment.