Gas Behavior and The First Law - PowerPoint Presentation

Slide1

Gas Behavior and The First Law

Consider a gas in a cylinder with a movable piston. If the piston is pushed inward by an external force, work is done on the gas, adding energy to the system.

The force exerted on the piston by the gas equals the pressure of the gas times the area of the piston: F = PAThe work done equals the force exerted by the piston times the distance the piston moves: W = Fd = (PA)d = PV

Slide2

If the gas is being compressed, the change in volume is negative, and the work done is negative.

Work done on the system is negative.Negative work increases the energy of the system.If the gas is expanding, positive work is done by the gas on its surroundings, and the internal energy of the gas decreases.

Slide3

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.

Slide4

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 molecules

and

k

is Boltzmann’s constant.

Slide5

3/7/2011

Physics 214 Fall 2010

5

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

This system is analogous to the combustion cycle within a diesel engine or any fuel injected engine.

Can you guess the every-day application of this phenomenon

?

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

?

Slide6

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?

Slide7

The Flow of Heat

There are three basic processes for heat flow:Conduction

ConvectionRadiation

Slide8

In

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

Slide9

(Conduction Contd.)

The rate of heat flow depends on the temperature difference between the objects.It also depends on the

thermal conductivity of the materials, a measure of how well the materials conduct heat.For example, a metal block at room temperature will feel colder than a wood block of the exact same temperature.The metal block is a better thermal conductor, so heat flows more readily from your hand into the metal.Since contact with the metal cools your hand more rapidly, the metal feels colder.

Slide10

3B-04 Boiling Water in Cup

Slide11

3B-02 Safety Lamp

Slide12

3D-05 Solar Panel

3D-03 Radiation--Match

Slide13

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.

Slide14

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.

Slide15

The laws of thermodynamics

...are critical to making intelligent choices about energy in today’s global economy.

Slide16

How do heat engines work? What determines their efficiency?

What Is a Heat Engine?

Slide17

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.

Slide18

Heat Engines

The wheels push against the road.According to Newton’s third law, the road exerts a force on the tires, allowing the car to move forward.

Not all the heat from burning fuel is converted to work done in moving the car.The exhaust gases emerging from the tailpipe release heat into the environment.Unused heat is a general feature of heat engines.

Slide19

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.

Slide20

3E09, 3E10, 2E12 Engines

Steam Engine

Stirling

Engine

Stirling

Engine

Slide21

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.

Slide22

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?

33%

40%66%

Q

H

= 1200 J

W

= 400 J

e

= W

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

Slide23

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

33 J

400 J800 J1200 J

Q

C

=

Q

H

- W

= 1200 J - 400 J = 800 J

Slide24

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?In analogy to water wheels, Carnot reasoned that the greatest efficiency of a heat engine would be obtained by taking all the input heat at a single high temperature and releasing all the unused heat at a single low temperature.

Slide25

Carnot Engine and Carnot Cycle

Carnot also reasoned that the processes should occur without undue turbulence.The engine is completely reversible: it can be turned around and run the other way at any point in the cycle, because it is always near equilibrium.

This is Carnot’s ideal engine.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.

Slide26

Heat flows into cylinder at temperature

TH. The fluid expands isothermally

and does work on the piston.The fluid continues to expand, adiabatically.Work is done by the piston on the fluid, which undergoes an isothermal compression.The fluid returns to its initial condition by an adiabatic compression.

Slide27

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 TH and releasing heat to a reservoir at temperature TC.Even Carnot’s ideal engine is less than 100% efficient.

Slide28

A steam turbine takes in steam at a temperature of 400

C and releases steam to the condenser at a temperature of 120C. What is the Carnot efficiency for this engine?

30%41.6%58.4%70%TH = 400C = 673 KT

C

= 120C = 393 K

e

C

=

(T

H - TC ) / TH = (673 K - 393 K) / (673 K)

= 280 K / 673 K = 0.416 = 41.6%

Slide29

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?

0.83 J

16.64 kJ28 kJ208 kJQH = 500 kJe =

W

/

Q

H

,

so

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

208 kJ