Слайд 2Learning Objectives
understand and apply first law of thermodynamics
distinguish graphs of
adiabatic and isothermal processes
understand second law of thermodynamics
Слайд 3Terms to Remember!
Thermodynamics
Thermodynamic System
Surroundings
Heat
Work
Internal Energy
Слайд 4Thermodynamics
Thermodynamics is the macroscopic study of the behaviour of systems.
It was a mathematical theory developed before a detailed understanding
of the particulate nature of gases.
Слайд 5Thermodynamic System
Thermodynamic System is a macroscopic aspect of a problem
than can be considered as a separate whole. An ideal
gas for example can have energy flowing in or out of it from
the surroundings.
Слайд 6Surroundings
Surroundings is everything in the problem outside the
thermodynamic system.
Heat can flow from the system to the surroundings and
vice versa.
Слайд 12Heat
Heat (Q) is an amount of thermal energy transferred from
the surroundings to an ideal gas. It is a result
of a temperature difference.
Слайд 14Work
Work (W) is simply a macroscopic transfer of energy from
the gas to the surroundings.
Слайд 21Change in Internal Energy
Change in Internal Energy (∆U) is the
change in a gas’ energy due to the PE and
KE of the molecules.
It does not include external factors such as gravity.
∆U for an ideal gas will involve temperature changes (∆θ).
Слайд 22Change in Internal Energy
The change in internal energy (ΔU) of
a closed system will be equal to the energy added
to the system by heating minus the work done by the system on the surroundings.
ΔU = Q – W
Слайд 231st Law of Thermodynamics
The First Law of Thermodynamics states that
when heat Q is added to a system while the
system does work W, the internal energy U changes by an amount equal to Q – W.
ΔU = Q – W
Слайд 241st Law of Thermodynamics
Since Q and W represent energy transferred
into or out of the system , the internal energy
changes accordingly.
The First Law of Thermodynamics is great and broad statement of the law of conservation of energy.
Слайд 251st Law of Thermodynamics
The internal energy of any thermodynamic system
depends only on its state. The change in internal energy
in any process depends only on the initial and final states, not on the path.
The internal energy of an isolated system is constant.
Слайд 26State
State is defined as the physical condition of the system.
Слайд 27Important Note
A given system at any moment is in particular
state and can be said to have a certain amount
of internal energy.
But a system does NOT have a certain amount of heat or work.
Слайд 28Important Note
Rather, when work is done on a system or
when heat is added or removed from a system, the
state of the system changes.
Слайд 29Important Note
Thus, work and heat are involved in thermodynamic processes
that can change the system from one state to another;
they are not characteristic of the state itself.
Слайд 30State Variables
Quantities which describe the state of the system is
called state variables.
Internal Energy (U) Mass (m)
Pressure (P) Volume (V)
Temperature (T)
Number of Moles (n)
Слайд 31Sample Problem
2500 J of heat is added to a system,
and 1800 J of work is done on the system.
What is the change in internal energy of the system?
Слайд 32Sample Problem
2500 J of heat is added to a system,
and 1800 J of work is done by the system.
What is the change in internal energy of the system?
Слайд 33Isothermal Process and 1st Law of Thermodynamics
To analyze some thermodynamic
process in light of the 1st law of thermodynamics, consider
a fixed mass of an ideal gas enclosed in a container fitted with movable piston.
Слайд 34Isothermal Process and 1st Law of Thermodynamics
Consider an idealized process
that is carried out at constant temperature. Such process is
called isothermal process.
If an isothermal process is carried out on our ideal gas, then
PV = nRT becomes PV = constant.
Слайд 35Isothermal Process and 1st Law of Thermodynamics
The process follows a
curve like AB on the PV diagram, which is a
curve for PV = constant.
PV Diagram for an ideal gas undergoing isothermal process at two different temperatures
Слайд 36Isothermal Process and 1st Law of Thermodynamics
Each point on the
curve, such as point A, represents a state of the
system – that is, its pressure P and volume V at a given moment.
PV Diagram for an ideal gas undergoing isothermal process at two different temperatures
Слайд 37Isothermal Process and 1st Law of Thermodynamics
The curves shown are
referred to as isotherms.
PV Diagram for an ideal gas undergoing
isothermal process at two different temperatures
Слайд 38Work Done
In the process A to D, the gas does
no work since the volume does not change
(isochoric).
Слайд 39Work Done
Going from D to B, the gas does work
equal to
PB(VB – VA).
This is the total work done in
the process ADB.
(isobaric)
Слайд 40Work Done
If the pressure varies during a process, such as
for the isothermal process A,
W = PΔV
cannot be used directly
to determine the work.
Слайд 41Work Done
The calculation of work done (WAB) in this case
can be carried out using calculus, or by estimating the
area under the curve.
Слайд 42Adiabatic Process and 1st Law of Thermodynamics
An adiabatic process is
one in which no heat is allowed to flow into
or out of the system: Q = 0.
This situation can occur if the system is extremely well insulated, or the process happens so quickly that heat – which flows slowly – has no time to flow in or out.
Слайд 43Adiabatic Process and 1st Law of Thermodynamics
The very rapid expansion
of gases in an internal combustion engine is one example
of a process that is very nearly adiabatic.
Слайд 44Adiabatic Process and 1st Law of Thermodynamics
A slow adiabatic expansion
of an ideal gas follows a curve like that labelled
AC in the PV diagram shown.
Слайд 45Adiabatic Process and 1st Law of Thermodynamics
Since Q = 0,
then ΔU = –W.
That is, the internal energy decreases if
the gas expands; hence the temperature decreases as well because
Слайд 46Adiabatic Process and 1st Law of Thermodynamics
In the reverse operation,
an adiabatic compression, work is done on the gas, and
hence the internal energy increases and the temperature rises.
Слайд 47Thermodynamic Process and 1st Law of Thermodynamics
Слайд 48Sample Problem:
An ideal gas is slowly compressed at a constant
pressure of 2.0 atm from 10.0 L to 2.0 L.
Calculate (a) the total work done by the gas in the process BDA, and (b) the total heat flow into the gas.
Слайд 50Sample Problem:
In an engine, 0.25 moles of an ideal monatomic
gas in the cylinder expands rapidly and adiabatically against the
piston. In the process, the temperature of the gas drops from 1150 K to 400 K. How much work does the gas do?
Слайд 52Sample Problem:
A gas in a container with a piston expands
isothermally. If thermal energy Q = 105 is given to
the gas, what is the work done by the gas?
Слайд 54Sample Problem:
A gas expands adiabatically. Will its temperature increase or
decrease?
Слайд 56Sample Problem:
A monatomic gas is kept at constant pressure 3.00
× 106 Pa, initial volume 0.100 m3 and temperature 300
K. If the gas is compressed at constant pressure down a volume of 0.800 m3, find:
(a) the work done on the gas; (b) the thermal energy taken out of the gas.
Слайд 582nd Law of Thermodynamics
There are many processes in thermodynamics that
are consistent with the first law but are nonetheless impossible.
Слайд 592nd Law of Thermodynamics
If you put a layer of salt
in a jar and cover it with a layer of
similar-sized grains of pepper, when you shake it you get a thorough mixture. But no matter how long you shake it, the mixture does not separate into layers again.
Слайд 602nd Law of Thermodynamics
Coffee cups and glasses break spontaneously if
you drop them. But they don’t go back together spontaneously.
The
spontaneous (without the action of another agent) transfer of thermal energy from a cold body to hotter body
Слайд 612nd Law of Thermodynamics
The air in a room suddenly occupying
just one half of the room and leaving the other
half empty.
A glass of water at room temperature suddenly freezing, causing the temperature of the room to rise
Слайд 622nd Law of Thermodynamics
These processes do not open happen because
they are forbidden by a very special law of physics
– the second law of thermodynamics.
Слайд 632nd Law of Thermodynamics
The 1st law of thermodynamics (conservation of
energy) would not be violated any of these processes.
To explain
this lack of reversibility, scientists in the latter half of the 19th century formulated a new principle known as the 2nd law of thermodynamics.
Слайд 642nd Law of Thermodynamics
The second law of thermodynamics describes the
directionality of natural thermodynamic processes.
It can be stated in several
equivalent forms.
Слайд 652nd Law of Thermodynamics
According to Rudolf J.E. Clausius (1822 –
1888)
“heat can flow spontaneously from a hot object to a
cold object ; heat will not flow spontaneously from a cold object to a hot object”
Слайд 662nd Law of Thermodynamics
The engine statement is that “no cyclic
process can convert heat completely into work”.
Слайд 672nd Law of Thermodynamics
The refrigerator statement is that “no cyclic
process can transfer heat from a colder place to a
hotter place with no input of mechanical work”.
Слайд 682nd Law of Thermodynamics
The development of a general statement of
the second law of thermodynamics was based partly on the
study of heat engines.
Слайд 692nd Law of Thermodynamics
A heat engine is any device that
changes thermal energy into mechanical work, such as steam engines
and automobile engines.