The Flow of Heat• There are three basic processes for heat

flow:

–Conduction

–Convection

–Radiation

– In conduction, heat flows through a material when objects at different temperatures are placed in contact with one another.

3B-04 Boiling Water in Cup

3B-02 Safety Lamp

– In convection, heat is transferred by the motion of a fluid containing thermal energy.

• Convection is the main method of heating a house.• It is also the main method heat is lost from buildings.

3D-05 Solar Panel3D-03 Radiation--Match

– In radiation, heat energy is transferred by electromagnetic waves.

• The electromagnetic waves involved in the transfer of heat lie primarily in the infrared portion of the spectrum.

• Unlike conduction and convection, which both require a medium to travel through, radiation can take place across a vacuum.

• For example, the evacuated space in a thermos bottle.

• The radiation is reduced to a minimum by silvering the facing walls of the evacuated space.

• An ideal gas is a gas for which the forces between atoms are small enough to be ignored.– For an ideal gas, absolute

temperature is directly related to the average kinetic energy of the molecules of the system.

– Most gases behave approximately as ideal gases.

• If the process is adiabatic, no heat flows into or out of the gas.• Even though no heat is added, the temperature of a gas will increase

in an adiabatic compression, since the internal energy increases.

• In an isothermal process, the temperature does not change.– The internal energy must be constant.– The change in internal energy, U, is zero.– If an amount of heat Q is added to the gas, an equal amount of work W will be

done by the gas on its surroundings, from U = Q - W.

• In an isobaric process, the pressure of the gas remains constant.– The internal energy increases as the gas is heated, and so does the

temperature.– The gas also expands, removing some of the internal energy.

• Experiments determined that the pressure, volume, and absolute temperature of an ideal gas are related by the equation of state:

PV = NkT where N is the number of moleculesand k is Boltzmann’s constant.

3/8/2011 Physics 214 Fall 2010 10

3E-03 Fire Syringe

RAPID COMPRESSION IS ADIABATIC GIVING RAPID RISE OF AIR TEMPERATURE IN THE CHAMBER WHICH EXCEEDS THE IGNITION TEMPERATURE OF THE FLAMMABLE MATERIAL.

Compression and rise in air temperature

What will happen to the combustible material when the plunger is rapidly pushed down ?

• When gas is heated in a hot-air balloon, the pressure, not the temperature, remains constant.

• The gas undergoes an isobaric expansion.

• Since the gas has expanded, the density has decreased.

• The balloon experiences a buoyant force because the gas inside the balloon is less dense than the surrounding atmosphere.

What process makes a hot-air balloon rise?

Heat Engines• A gasoline engine is a form of a

heat engine. – Gasoline is mixed with air.– A spark ignites the mixture, which

burns rapidly.– Heat is released from the fuel as it

burns.– The heat causes the gases in the

cylinder to expand, doing work on the piston.

– The work done on the piston is transferred to the drive shaft and wheels.

3E09, 3E10, 2E12 Engines

Steam Engine

Stirling EngineStirling Engine

Heat Engines• All heat engines share these

main features of operation:– Thermal energy (heat) is

introduced into the engine.– Some of this energy is

converted to mechanical work.– Some heat (waste heat) is

released into the environment at a temperature lower than the input temperature.

QH W QC

Efficiency• Efficiency is the ratio of the

net work done by the engine to the amount of heat that must be supplied to accomplish this work.

e WQH

A heat engine takes in 1200 J of heat from the high-temperature heat source in each cycle, and does 400 J of work in each cycle. What is the

efficiency of this engine?a) 33%b) 40%c) 66%

QH = 1200 JW = 400 Je = W / QH

= (400 J) / (1200 J)= 1/3 = 0.33= 33%

How much heat is released into the environment in each cycle?

a) 33 Jb) 400 Jc) 800 Jd) 1200 J

QC = QH - W = 1200 J - 400 J= 800 J

Carnot Engine• The efficiency of a typical automobile engine is less

than 30%.– This seems to be wasting a lot of energy.– What is the best efficiency we could achieve?– What factors determine efficiency?

• The cycle devised by Carnot that an ideal engine would have to follow is called a Carnot cycle.

• An (ideal, not real) engine following this cycle is called a Carnot engine.

1. Heat flows into cylinder at temperature TH. The fluid expands isothermally and does work on the piston.

2. The fluid continues to expand, adiabatically.3. Work is done by the piston on the fluid, which

undergoes an isothermal compression.4. The fluid returns to its initial condition by an

adiabatic compression.

Carnot Efficiency• The efficiency of Carnot’s ideal engine is called the Carnot

efficiency and is given by:

• This is the maximum efficiency possible for any engine taking in heat from a reservoir at absolute temperature THand releasing heat to a reservoir at temperature TC.

• Even Carnot’s ideal engine is less than 100% efficient.

eC TH TC

TH

A steam turbine takes in steam at a temperature of 400C and releases steam to the condenser at a

temperature of 120C. What is the Carnot efficiency for this engine?

a) 30%b) 41.6%c) 58.4%d) 70%

TH = 400C = 673 KTC = 120C = 393 KeC = (TH - TC ) / TH

= (673 K - 393 K) / (673 K)= 280 K / 673 K= 0.416 = 41.6%

Quiz: If the turbine takes in 500 kJ of heat in each cycle, what is the maximum amount of work that could be generated by the turbine in each

cycle?a) 0.83 Jb) 16.64 kJc) 28 kJd) 208 kJ

QH = 500 kJe = W / QH ,

so W = e QH= (0.416)(500 kJ)= 208 kJ