# Lessons 3 and 4 Thermodynamics - MrSimonPorter

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27 Οκτ 2013 (πριν από 4 χρόνια και 6 μήνες)

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1

Thermodynamics

2

A few reminders

TEMPERATURE determines the
direction of flow of thermal energy
between two bodies in thermal
equilibrium

HOT

COLD

3

A few reminders

TEMPERATURE is also a measure
of the average kinetic energy of
particles in a substance

4

A few reminders

INTERNAL ENERGY is the sum of
the kinetic energy and potential
energies of particles in a
substance

K.E. + P.E.

5

Internal energy

The sum of the KE and PE of the
particles in a system

NOTE, THIS IS NOT THE SAME AS
THE TOTAL ENERGY.

6

A few reminders

In an
ideal gas
, the INTERNAL
ENERGY is all kinetic energy.

7

What is
THERMODYNAMICS?

A study of the connection between
thermal energy entering or leaving
a system and the work done on or
by the system.

8

A few words to consider

9

Thermodynamic system

The system/machine that we are
considering the flow of heat energy
in/out of and work done on/by the
system.

10

The surroundings

Everything else!

11

Heat

The quantity of heat/thermal energy
(transferred by a temperature
difference).

12

Work

The
energy transferred

(changed)

E.g. Work = Force x distance

or

Work = VIt

13

Example

Finding the

work done on or by a
gas when it expands at
constant
pressure

(i.e. a small change in
volume!)

(most of the systems we consider
will involve the compression or
expansion of gases under different
conditions)

14

Work done by a gas
(constant pressure)

Work = force x distance

Work = force x
Δ
x

(Pressure = F/A so F = PA)

Work = PA
Δ
x

(A
Δ
x =
Δ
V)

Work = p
Δ
V

P

Δ
x

A

P

15

The 1
st

law of
thermodynamics

Q =
Δ
U + W

16

The 1
st

law of
thermodynamics

Q =
Δ
U + W

Q = The
thermal energy

given to a system (if this is
negative, thermal energy is
leaving the system)

17

The 1
st

law of
thermodynamics

Q =
Δ
U + W

Δ
U = The increase in
internal energy

(if this is
negative the internal
energy is decreasing)

18

The 1
st

law of
thermodynamics

Q =
Δ
U + W

W = The work done
on the
surroundings

(if this is
negative the surroundings
are doing work on the
system)

19

The 1
st

law of
thermodynamics

Q =
Δ
U + W

This is really just
another form of the
principle of energy
conservation

20

Ideal gas processes

In most cases we will be
considering changes to an ideal
gas (this will be the “system)

21

pV diagrams and work
done

Changes that happen during a
thermodynamic process can
usefully be shown on a pV diagram

p

V

22

pV diagrams and work
done

The area under the graph represents
the work done

p

V

A

B

This area represents
the work done
by

the
gas (
on the
surroundings
) when it
expands

from state A
to state B

What happens if the gas is going from state B to A?

23

ISOCHORIC
(isovolumetric) processes

These take place at constant volume

V = constant, so p/T = constant

Q = negative

Δ
U = negative

W = zero

p

V

A

B

Isochoric
decrease in
pressure

24

ISOBARIC processes

These take place at constant
pressure

p = constant, so V/T = constant

Q = positive

Δ
U = positive

W = positive

p

V

A

B

Isobaric
expansion

25

ISOTHERMAL processes

These take place at constant
temperature

T = constant, so pV = constant

Q = positive

Δ
U = zero

W = positive

p

V

A

B

Isothermal
expansion

26

No thermal energy transfer with the
surroundings (approximately a
rapid expansion or contraction)

Q = zero

Δ
U = negative

W = positive

p

V

A

B

expansion

27

Heat engines and heat
pumps

A heat engine is any device that
uses a source of heat energy to do
work.

Examples include the internal
combustion engine of a car.

28

Heat engine

Below is a generalised diagram
showing the essential parts of any
heat engine.

Hot
reservoir

T
hot

Cold
reservoir

T
cold

Thermal
energy
Q
hot

Thermal
energy

Q
cold

Work done

Δ
W

Engine

“Reservoir”
implies a
constant heat
source

29

A simple example of using
an ideal gas in a heat
engine

p

V

Isobaric
expansion

Isovolumetric
decrease in
pressure

Isobaric
compression

Isovolumetric
increase in
pressure

Heat in

Heat out

Area = work
done by gas

Δ
U = (3/2)nR
Δ
T

Heat out

Heat in

A

B

C

D

30

Page 191 to 192 “An example of a
heat engine”

31

Heat
pump

Simply a heat engine run in
reverse! (Put work in, transfer heat
from cold reservoir to hot reservoir)

Hot
reservoir

T
hot

Cold
reservoir

T
cold

Thermal
energy
Q
hot

Thermal
energy

Q
cold

Input work

Δ
W

Engine

32

Heat pump

p

V

Isobaric
compression

Isovolumetric
increase in
pressure

Isobaric
expansion

Isovolumetric
decrease in
pressure

Heat out

Heat in

Area = work
done
on

gas

Heat in

Heat out

33

Questions

Page 193

Questions 1 to 5

Page 194

Questions 10

34

2
nd

Law of
Thermodynamics and
entropy

There are many ways of stating the
2
nd

law,
below is

the Kelvin
-
Planck
formulation

“No heat engine, operating over a
cycle, can take in heat from its
surroundings and totally convert it
totally
into work”

(some heat has to
be transferred to the cold reservoir)

This is possible
in a single
process
however

35

2
nd

Law of
Thermodynamics and
entropy

Other statements of the 2
nd

law
include

No heat pump can transfer thermal
energy from a low temperature to a
higher temperature reservoir
without work being done on it
(Clausius)

The entropy of the universe can
never decrease

36

Entropy

This is a measure of the
disorder of a system

Most systems, when left,
tend towards more disorder

from hot to cold.

Entropy can decrease in a
small part of a system

37

Entropy

T
hot

T
cold

Δ
Q

Decrease in
entropy =
Q/T
hot

Increase in entropy
=
Q/T
cold

38

1
st

and 2
nd

laws

These laws MUST apply in all
situations

A refrigerator does transfer heat from
cold to hot, but work must be done
(electricity supplied and some
converted into heat) to do this

A boat could use the temperature
difference between the sea and
atmosphere to run, but eventually the
two reservoirs would reach the same
temperature

39

the less useful it is. The heat
produced in the brakes of a car is
still energy, but not really in a
useful form. We call this energy

40

That’s it!

41

Now let’s try some
questions

Page 193

Questions 1 to 5

Page 194

Questions 10 to 13.

Let’s also have a
test on 4
th

November