heat engines introduction section 0 lecture 1 slide 1 lecture 25 slide 1 introduction to modern...

Heat Engines

Introduction Section 0 Lecture 1 Slide 1

Lecture 25 Slide 1

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Physics of Technology

PHYS 1800

Lecture 25

Heat Engines and the

2nd Law of Thermodynamics

Heat Engines


Lecture 25 Slide 2


Fall 2004


Spring 2009

PHYSICS OF TECHNOLOGY Spring 2009 Assignment Sheet

*Homework Handout

PHYSICS OF TECHNOLOGY - PHYS 1800 ASSIGNMENT SHEET

Spring 2009 Date Day Lecture Chapter Homework Due Feb 16 17 18 19 20

M Tu W H F*

Presidents Day Angular Momentum (Virtual Monday) Review Test 2 Static Fluids, Pressure

No Class 8 5-8 5-8 9

-

Feb 23 25 27

M W F*

Flotation Fluids in Motion Temperature and Heat

9 9 10

6

Mar 2 4 6

M W F*

First Law of Thermodynamics Heat flow and Greenhouse Effect Climate Change

10 10 -

7

Mar 9-13 M-F Spring Break No Classes Mar 16 18 20

M W F*

Heat Engines Power and Refrigeration Electric Charge

11 11 12

8

Mar 23 25 26 27

M W H F*

Electric Fields and Electric Potential Review Test 3 Electric Circuits

12 13 9-12 13

-

Mar 30 Apr 1 3

M W F

Magnetic Force Review Electromagnets Motors and Generators

14 9-12 14

9

Apr 6 8 10

M W F*

Making Waves Sound Waves E-M Waves, Light and Color

15 15 16

10

Apr 13 15 17

M W F*

Mirrors and Reflections Refraction and Lenses Telescopes and Microscopes

17 17 17

11

Apr 20 22 24

M W F

Review Seeing Atoms The really BIG & the really small

1-17 18 (not on test) 21 (not on test)

No test week 12

May 1 F Final Exam: 09:30-11:20am * = Homework Handout

Heat Engines


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Fall 2004


Spring 2009


PHYS 1800

Lecture 25



Review of Thermodynamics

Heat Engines


Lecture 25 Slide 4


Fall 2004


Spring 2009

Describing Motion and InteractionsPosition—where you are in space (L or meter)

Velocity—how fast position is changing with time (LT-1 or m/s)

Acceleration—how fast velocity is changing with time (LT-2 or m/s2)

Force— what is required to change to motion of a body (MLT-2 or kg-m/s2 or N)

Inertia (mass)— a measure of the force needed to change the motion of a body (M)

Energy—the potential for an object to do work. (ML2T-2 or kg m2/s2 or N-m or J)

Work is equal to the force applied times the distance moved. W = F dKinetic Energy is the energy associated with an object’s motion. KE=½ mv2

Potential Energy is the energy associated with an objects position.Gravitational potential energy PEgravity=mghSpring potential energy PEapring= -kx

Momentum— the potential of an object to induce motion in another object (MLT -1 or kg-m/s)

Angular Momentum and Rotational Energy— the equivalent constants of motion for rotation (MT-1 or kg/s) and (MLT-2 or kg m/s2 or N)

Pressure— force divided by the area over which the force is applied (ML -1T-1 or kg/m-s or N/m2 or Pa)

Heat Engines


Lecture 25 Slide 5


Fall 2004


Spring 2009

Dennison’s Laws Thermal Poker(or How to Get a Hot Hand in Physics)

• 0th Law: Full House beats Two Pairs

• 1st Law: We’re playing the same game (but with a wild card)

• 2nd Law: You can’t win in Vegas.

• 3rd Law: In fact, you always loose.

• 0th Law: Defines Temperature

• 1st Law: Conservation of Energy (with heat)

• 2nd Law: You can’t recover all heat losses (or defining entropy)

• 3rd Law: You can never get to absolute 0.

Heat Engines


Lecture 25 Slide 6


Fall 2004


Spring 2009

• What is heat?• What is the relationship between quantity of heat

and temperature?• What happens to a body (solid, liquid, gas) when

thermal energy is added or removed?

Thermal Energy

Heat

Solid: Atoms vibrating in all directions about their fixed equilibrium (lattice) positions. Atoms constantly colliding with each other.

Liquid: Atoms still oscillating and colliding with each other but they are free to move so that the long range order (shape) of body is lost.

Gas: No equilibrium position, no oscillations, atoms are free and move in perpetual high-speed “zig-zag” dance punctuated by collisions.

gas

liquid

solid

Heat Engines


Lecture 25 Slide 7


Fall 2004


Spring 2009

++

+

++

+

++

+

Heat

221 mvTkB

kB is Boltzmann’s constant

=1.38 10-23 J/K

Solid

Heat Engines


Lecture 25 Slide 8


Fall 2004


Spring 2009

• When two objects at different temperatures are placed in contact, heat will flow from the object with the higher temperature to the object with the lower temperature.

• Heat added increases temperature, and heat removed decreases temperature.

• Heat and temperature are not the same.

• Temperature is a quantity that tells us which direction the heat will flow.

Heat is a form of energy.(Here comes conservation of energy!!!)

Temperature and Heat

Heat Engines


Lecture 25 Slide 9


Fall 2004


Spring 2009

Joule’s Experiment and the First Law of Thermodynamics

• Joule’s experiments led to Kelvin’s statement of the first law of thermodynamics.– Both work and heat represent transfers of energy into or out of a

system.– If energy is added to a system either as work or heat, the internal

energy of the system increases accordingly.

• The increase in the internal energy of a system is equal to the amount of heat added to a system minus the amount of work done by the system. U = Q - W

Heat Engines


Lecture 25 Slide 10


Fall 2004


Spring 2009

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 = PA

• The work done equals the force exerted by the piston times the distance the piston moves:

W = Fd = (PA)d = PV

Heat Engines


Lecture 25 Slide 11


Fall 2004


Spring 2009


PHYS 1800

Lecture 25



Heat Engines

Heat Engines


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Fall 2004


Spring 2009

Heat Engines

It is a device that uses input heat to generate useful work.

From the 1st Law (Conservation of Energy)

In cyclic engines we return to the original state every cycle so

What is a heat engine?

Heat Engines


Lecture 25 Slide 13


Fall 2004


Spring 2009

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.

What is a heat engine?

Heat Engines


Lecture 25 Slide 14


Fall 2004


Spring 2009

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.

Or from the 1st Law

Heat Engines


Lecture 25 Slide 15


Fall 2004


Spring 2009

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%

Heat Engines


Lecture 25 Slide 16


Fall 2004


Spring 2009

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

Heat Engines


Lecture 25 Slide 17


Fall 2004


Spring 2009

Carnot Engine and Carnot Cycle

• Carnot considered the ideal (most efficient possible) engine for a give TH and TC.

• Carnot engine has negligible work lost to friction, turbulence, heat loss, etc.

• 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.

Heat Engines


Lecture 25 Slide 18


Fall 2004


Spring 2009

Carnot Efficiency

• The efficiency of Carnot’s ideal engine (one using an ideal gas with PV=NkBT) 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.

• This provides a useful limiting case.• Even Carnot’s ideal engine is less than 100% efficient.

Heat Engines


Lecture 25 Slide 19


Fall 2004


Spring 2009

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 (without heat loss).

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 Cycle

Heat Engines


Lecture 25 Slide 20


Fall 2004


Spring 2009

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%

Heat Engines


Lecture 25 Slide 21


Fall 2004


Spring 2009

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

Heat Engines


Lecture 25 Slide 22


Fall 2004


Spring 2009


PHYS 1800

Lecture 25



Second Law of Thermodynamics

Heat Engines


Lecture 25 Slide 23


Fall 2004


Spring 2009


• You can’t recover all heat losses .

• You can’t win in Vegas.

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

• Therefore, no engine can have a greater efficiency than a Carnot engine operating between the same two temperatures.

• Define entropy (something that measures randomness or disorder in an object) to take account of this.

Heat (random motion) is a special form of energy that cannot be fully (with complete efficiency) transformed to other forms of energy.

This leads to various forms of the Second Law of Thermodynamics.

Heat Engines


Lecture 25 Slide 24


Fall 2004


Spring 2009


• An engine with an efficiency greater than the Carnot engine would produce a greater amount of work than the Carnot engine, for the same amount of heat input QH.

• Some of this work could be used to run the Carnot engine in reverse, returning the heat released by the first engine to the higher-temperature reservoir.

Heat Engines


Lecture 25 Slide 25


Fall 2004


Spring 2009


• The remaining work Wexcess would be available for external use, and no heat would end up in the lower-temperature reservoir.

• The two engines would take a small quantity of heat from the higher-temperature reservoir and convert it completely to work.

• This would violate the second law of thermodynamics.

Heat Engines


Lecture 25 Slide 26


Fall 2004


Spring 2009


Next Lab/Demo: Fluid Dynamics TemperatureThursday 1:30-2:45

ESLC 46 Ch 9 and 10

Next Class: Wednesday 10:30-11:20

BUS 318 roomReview Ch 10

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