Thermodynamics - Comcast.net

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27 Οκτ 2013 (πριν από 4 χρόνια και 12 μέρες)

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Chapter 12

Thermodynamics

Thermodynamics

Study of the transfer and actions of
heat


When can heat be exploited to do work?


How efficient can heat engines be?

Grew out of the efforts to build heat
engines


Steam engines


Industrial revolution

Thermodynamics

Important terms


System

definite quantity of matter
enclosed by boundaries (real or imagined)


Thermally isolated system

system in which it
is impossible to transfer heat into or out of


Heat reservoir

unlimited heat capacity


Heat can be taken from or given to the
system without changing its temperature
appreciably


Process

change in the state of a system


P, V, and T change in some way

Thermodynamics

Important terms


Equations of state

equations that
describe the conditions of thermodynamic
systems


State variables describe the conditions of
thermodynamics systems


Pressure, Volume, Temperature (most common)


Number of molecules (usually constant)

First Law of Thermodynamics

Conservation of energy, applied to
thermodynamics systems

Heat added to a system (Q) can . . .


+ Q means heat is added to the system


-

Q means heat is removed from the system


Do work (W)


+ W means work is done
by the system


-

W means work is done
on the system


Increase systems internal energy (

U)

First Law of Thermodynamics

Isobaric process

constant pressure


V/T must remain constant, so volume
increases as temperature (average internal
energy) increases


Work done on a piston


Work done = area enclosed by process on a
Pressure vs Volume graph

First Law of Thermodynamics

First Law of Thermodynamics

Isometric process

constant volume


No work is done



V = 0, W = 0

First Law of Thermodynamics

Isothermal process

constant temperature


Average KE doesn’t
change, so neither does
the total KE (

U = 0)


Heat added is turned
completely to work

First Law of Thermodynamics

Adiabatic process

no heat is added to
the system (Q = 0)

2
nd

Law of Thermodynamics

Hot object placed into cooler water


Heat flows from object to water


Final temperature is somewhere in
between both objects’ initial temperature


Heat lost by hotter object = heat gained
by colder object


Object could get hotter and water cooler
without violating conservation of energy
(1
st

Law), but we don’t see that happen

2
nd

Law of Thermodynamics

2
nd

law is empirical, and written in
several forms


Heat will not flow spontaneously from a
colder body to a warmer body


In a thermal cycle, heat energy cannot be
completely transformed into mechanical
work


It is impossible to construct an operational
perpetual motion machine

Net work done = area enclosed


+ if cycle is clockwise




if cycle is counterclockwise

2
nd

Law of Thermodynamics

Entropy


Measure of a system’s ability to do useful
work.


As it loses the ability, entropy increases


“Time’s arrow” distinguishing past events
from future ones


Measure of the disorder


Systems naturally move toward a state of
greater disorder

No philosophy today, lets just deal with
the mathematical definition

2
nd

Law of Thermodynamics

Total entropy of the universe increases
in every natural process


Entropy of an isolated system never
decreases


Can be constant (adiabatic process)

Thermal Cycles

Heat engines

devices that convert
heat energy into useful work


Take thermal energy from hot reservoirs,
do work, and transfer the rest to the
surroundings (cold reservoir)

Thermal pumps

devices that do work
to transfer heat from cold reservoirs to
hot reservoirs


Heat pumps (refrigerators, air
conditioners)

Thermal Cycles

Single processes can get work done

Continuous output necessitates
repetition

Thermal cycle

series of processes
that bring a system back to its original
condition

Otto cycle, page 404

How Car Engines Work

Otto cycle animation

Otto cycle explanation

Thermal Cycles

Thermal Efficiency(

th
)

ratio of useful work
to input energy (W
net
/Q
in
)


th

=
W
net
/Q
in


W = work done


Q = heat energy put into the system


Efficiency of heat engines will never be 100%


Minimize heat lost to cold reservoir to maximize
efficiency