# Ch15 Thermodynamics

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27 Οκτ 2013 (πριν από 4 χρόνια και 7 μήνες)

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Ch15 Thermodynamics

Zeroth Law of Thermodynamics

If two systems are in thermal equilibrium with a third system, then
they are in thermal equilibrium with each other.

First Law of Thermodynamics

The Internal Energy of a closed system will be equal to the energy
added to the system by heating minus the work done by the system
its surrounding

Second Law of Thermodynamics

Heat flows out from hot objects to cold; heat does NOT
flow from cold to hot

Internal Energy

The sum of all the energy of all the molecules
in an object (thermal energy)

Internal Energy of an Ideal Gas

Ch15 Thermodynamics

Heat

Transfer of energy due to
Δ
T

Work

Transfer of energy NOT due to
Δ
T

Q

Heat

W

Work

W done on the system is negative (Giancoli)

W done by the system is positive (Giancoli)

Δ
U

Change in energy

Ch15 Thermodynamics

First Law of Thermodynamics

Law of Conservation of Energy

AP equation sheet

Heat lost is
-

Work on system is

Work by system is +

Ch15 Thermodynamics

The distinction between work done on the gas and
work done by the gas is one that is often made on the
AP Exam

The area under the
P
-
V

curve will always be the work done
by the gas during the process

First Law of Thermodynamics

2500J of heat is added to a system, and 1800J of work is
done on the system. What is the changed in internal
energy of the system?

(Q) 2500J of heat will increase the Internal Energy

(W) 1800J of work done ON the system will …

Is the work positive or negative? Why?

Did the temperature increase or decrease?

Ch15 Thermodynamics

Isothermal process: Constant temperature

The system is in contact with a heat reservoir

Change of phase

The work done by the gas in an
isothermal process equals the heat

Isothermal process:

Constant temperature, i.e. PV is constant

Which Isothermal process is
at a higher Temperature?

Which Isothermal process
does more work?

Adiabatic Process: No heat in or out of the system

Well insulated (like a thermos)

The process happens very quickly (firing of a car
cylinder

Work

Given the following two processes: Isothermal and Adiabatic.

Both processes start at 10Pa and end with a volume of 10m
3

During which process is more work done?

Estimate the work done in each process.

Ch15 Thermodynamics

Isovolumetric: (Isochoric) No change in volume

Inside a ridged container

Ch15 Thermodynamics

Isobaric: No change in pressure

Movable piston

Internal Energy
Δ
U

1 mole of an ideal gas is brought from point a to point c
by 3 different process paths. Which path has the
highest change in internal energy?

1)

2)

3)

4) All the same

5) Unknown

10

20

30

40

50

60

100

400

300

200

a

b

c

d

Pressure (Pa)

Volume (m
3
)

Work (W)

1 mole of an ideal gas is brought from point a to point c
by 3 different process paths. During which path did
the
gas

do the most work?

1)

2)

3)

4) All the same

5) Unknown

10

20

30

40

50

60

100

400

300

200

a

b

c

d

Pressure (Pa)

Volume (m
3
)

Heat (Q)

1 mole of an ideal gas is brought from point a to point c
by 3 different process paths. During which path was

1)

2)

3)

4) All the same

5) Unknown

10

20

30

40

50

60

100

400

300

200

a

b

c

d

Pressure (Pa)

Volume (m
3
)

One mole of monatomic ideal gas is enclosed under a
frictionless piston. A series of processes occur, and
eventually the state of the gas returns to its initial state with
a P
-
V diagram as shown below. Answer the following in
terms of
P0
,
V0
, and
R
.

Find the temperature at each vertex.

Find the change in internal energy for each
process.

Find the work by the gas done for each
process.

2

Volume m
3

10

An ideal gas is slowly compress at constant pressure (2.0 ATM) from
10.0L to 2.0L

Heat is then added to the gas holding the volume constant and the
pressure and temperate are allowed to rise until the temperature reaches
its original value.

a)
Calculate the total work done by the gas

b)
Calculate the total heat flow into the gas

Pressure Pa

In an engine 0.25 moles of an ideal gas in the cylinder expands rapidly
and adiabatically against the piston. In this process, the temperature of
the gas drops from 1150K to 400K.

a)
What type of process is this?

b) How much work does the gas do?

Is the work done by the gas positive or
negative?

Efficiency

Efficiency (e): the ratio of work W done by the
system to the input heat Q
H

An automobile engine has an efficiency of 20% and produces an average
of 23,000J of mechanical work per second.

a)

How much input heat is required?

b)

How much heat is discharged as wasted per second?

a)

b)

Is the car’s efficiency higher or lower as
the car warms up?