Chapter 24 Thermodynamics

Chapter 24 Thermodynamics

•The study of heat and its transformation into mechanical work.

•If we increase the thermal motion of atoms what happens?

Temperature increases.

There seems to be no upper limit of temperature.

However, there is a lower limit.

Absolute Zero•The point at which all molecular motion stops.

•No more energy can be extracted from a substance and the temperature can not be lowered any further.

•Kelvin: 0 K

•Celsius: -273 oC

•Fahrenheit: -459 oF

First Law of Thermodynamics

:The internal energy of an isolated system is

constant.

The increase in the internal energy of a system is equal to the amount of energy added by heating the system, minus the amount lost as a result of the work done by the system on its surroundings.

Signs (+/-) will tell you if energy is entering or

leaving a system.+ indicates energy enters a system

.- indicates energy leaves a system.

First Law of Thermodynamics

thermo:     heat       dynamics:  force------------------------------------------------------Work done on water by paddle-wheelis converted to thermal energy.------------------------------------------------------First Law:

Heat added = Work done

                   or

Heat added = Thermal energy increase                                     +                   = Work done by system

•Heat can be used to do work.

•Heat and work are energy transferred to or from a system.

•Work done on or by a gas is the pressure times the change in volume.

W = PV

Work is the transfer of energy that takes place when an object is moved against an opposing force

i.e. a system does work when it expands against an external pressure

Car engine: petrol burns & produces gases which push out pistons in the engine and transfer energy to the

wheels of car

•Work stimulates uniform motion

• Heat and work can be considered as energy in transit

Ex: An engine cylinder compresses a volume of gas by 0.04 m3. How much work is done by the cylinder if it exerts a constant pressure of 7.5 x 105 Pa?

G: P = 7.5 x 105 Pa, V = 4 x 10-2 m3

U: W = ?

E: W = PV

S: W =(7.5 x 105)(4 x 10-2)

S: W = 30,000 J

U = Q + W

• Q = quantity of heat that enters or leaves a system.

U = the change in internal energy.

W = work done on (+) or by the system (-).

Ex: When 5000 J of heat is added to an engine, the

engine does 1250 J of work. What is the change in the internal energy of

engine?

Q = 5000 J

W = 1250 J

U = ?

U = Q + W

U = Q – W

U = 5000 J–1250 J

U = 3750 J

Adiabatic•The process of compression or expansion of a gas so that no heat enters or leaves a system.

•Read Ch 24.3 in CP book.

•Examples combustion engine, bike pump, weather conditions.

Starting position, intake stroke, and compression stroke.

Ignition of fuel, power stroke, and exhaust stroke.

1. intake/induction stroke

2. compression stroke

3. power stroke4. exhaust stroke

2nd Law of Thermodynamics

Heat flows spontaneously from a body of a higher

temperature to a one at a lower temperature.

Heat EngineAny device that changes internal

energy into mechanical work.

To reverse the flow work must be done.

•Ex: steam engine, internal combustion engine, jet engine, etc.

•The basic idea of a heat engine is that mechanical work can be obtained only when heat flows from hot to cold temperatures.

•Not all heat is transformed into work.

• "Typical engines are only 15 to 20 percent efficient in consuming oil, as they lose a lot of energy when they emit hot exhaust gas into the air."

Every heat engine will:

1.Absorb internal energy from a reservoir of higher temperature.

2. Convert some of this energy into mechanical work.

3. Expel the remaining energy to some lower temperature reservoir, usually called a “sink”.

•When undesirable heat is expelled, its called thermal pollution.

Power plants and industrial factories are among the major contributors to the problem of thermal pollution. These facilities draw water from nearby lakes and streams, which they use to cool their machinery and steam-driven equipment. Although many such facilities now take care not to contaminate the water with chemical pollutants, few return the heated water to its original temperature before dumping it back into the lakes and streams from which it came. The heated water warms local bodies of water by as much as 10° C (18° F), making the water uninhabitable for fish and other organisms.

Carnot EfficiencyThe maximum amount of energy that can be converted to work in a heat engine.

•This gives us the ‘ideal efficiency’.

•This gives us the ‘ideal efficiency’.

hot

coldhot

T

TTeffideal

h

ch

Q

QQeff

Qh = energy added as heat

Qc = energy removed as heat

3rd Law of Thermodynamics

There is no process by which a body may be

cooled to a temperature of 0 K (Absolute Zero).

Entropy•The measure of the amount of disorder.