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LPH 105 W15 CH 15: intro

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Richard Datwyler

on 13 July 2016

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Transcript of LPH 105 W15 CH 15: intro

thermodynamics
First law
Processes
isobaric
isochoric (isovolumetric)
isothermol
adiabatic (isentropic)
Work and Heat in a process
Heat Engines/ Refrigerators
efficiency
Ideal
Second law
entropy
1st Law
Internal energy is the motion of particles.

two things can happen.
Work (positive or negative)
Heat 'flow' (in or out)
Here Q is positive if heat is being added
TO the system

Q is negative if heat is being taken FROM
the system

Also W is positive if it is work done BY the
system on the environment

It is negative if the environment does
work ON it.
S
Processes
There are many ways this internal energy can change. Each doing a different amount of work, and heat flow
Often Ideal gas law helps with this.
We will look at cases where each term is held constant
Main processes
Isobaric
Isochoric
Isothermal
adiabatic
no heat flow
constant temperature
constant pressure
constant volume
Main Processes
It is helpful to do this pictorially on a Pressure Volume graph
Graph
Temperature increase up and right
Area under process is Work done by engine
Increases in volume are positive work
Decreases are negative work
Generally increases in temperature are positive Q,
decreases in temp. are negative Q. (adiabatic is the exception)
Best summed up with a table
This is not all inclusive.
but it is what you 'need' - also use Ideal gas law
example isobaric work:
An adiabatic process is one where :
A. Pressure is constant
B. Volume is constant
C. Temperature is constant
D. No heat is exchanged.

In Isochoric processes:
A. No heat is exchanged
B. The temperature is constant
C. No work is done
D. The pressure is constant

Thermodynamics
Linking thermodynamical processes via reversible/nonreversible processes will result in a some work being done for some heat used.
This is the idea of a heat pump, Engine, or Refrigerator.
(TABLE)
Diagram of 1st law
Cyclic - note the 'red' line returns cyclically to the same temperature, so no change in internal energy
Efficiency
We note that the Heat from the higher temperature
is not destroyed but transferred to work and heat low
Solving for efficiency, we want the most going to work
The most efficient cycle is a Carnot engine
It is a set of isothermal and adiabatic steps.
This is because no heat flows in an adiabatic
process, and in isothermal the heat gets
changed to work.
If 50 J of Heat is
taken from a hot source
and 20 J of work are produces
what is the efficiency of this
engine
A. 20%
B. 40 %
C. 60 %
D. 250 %

If the temperature range
that is available for an
engine to perform goes
from 300 - 400 degrees
what is the maximum efficiency
possible
A. 25 %
B. 33 %
C. 75 %
D. 133%

A Refrigerator is just an engine running backwards.

Instead of taking heat from a High source and changing it to work,
we put work in and take heat from a Low source effectively making it even lower.
Refrigerators also have efficiencies
The text uses a horrible term COP
for coefficient of performance to describe
this efficiency of refrigeration.
Ideally this is:
2nd law of thermodynamics
Total entropy of a system plus that of its environment increases as a result of any natural process
corollaries
heat flows from hot to cold spontaneously
any natural process leads to a greater state of disorder
Entropy is calculated by the Heat that flows
over a fixed temperature



This is in practice generally not feasible to use
in an equation, because when heat flows, the
temperature changes.
If looked over little tiny steps, the temperature
would be fixed, while the heat is flowing, and then
you would just add them all up.

In practice this is done by taking the average temperature
Say I raised the the temperature of 500 g of water from
22 - 24 degrees Celsius.

the average temperature is 23 or 296
and the heat flow is Q=mcT = 4186

This gives

Entropy is a state variable, yet it can be used to describe microscopic terms to.

Orderliness
or
probability

This then is another version of the 2nd law of
thermodynamics

Because entropy increase, we can say

Natural process tend to move toward a state of greater disorder.


This one might be the most familiar of the different
versions of the 2nd law of thermodynamics

Entropy always increases, disorder always increases, for natural processes.
"I need some clarity on what each law of thermodynamics entails."
"Why are work and heat not state variables?"
"Could you go over the processes of the first law of thermodynamics?"
"Can you go over state variables just a little review?"
"Can you explain the different thermodynamic processes? "
"I have trouble keeping the laws of thermodynamics straight and when to use them. Is there a way to remember when to best apply each one?"
"Can I get a breakdown of the equation to solve for entropy"
"What is an example of a process that is reversible? Irreversible?"
Full transcript