# handout in physics 1st and 2nd laws of thermodynamics.doc by FaisalKhattak2

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```									                   HAND OUT NO.14 IN PHYSICS: 1st and 2nd laws of Thermodynamics
First law of thermodynamics
When heat flows to or from a system, the system gains or loses an amount of energy equal
to the amount of heat transferred.

If we add heat to the steam in a steam engine, to the Earth's atmosphere, or to the body
of a living creature, we are adding energy to the system. The system can "use" this heat
to increase its own internal energy or to do work on its surroundings. So adding heat does
one or both of two things:
(1) it increases the internal energy of the system, if it remains in the system, or
(2) it does work on things external to the system, if it leaves the system.
More specifically, the first law states:
Heat added to a system = increase in internal energy + external work done by the
system.
ΔQ (Heat) = ΔU (internal energy) + W(work) 0r             ΔU = Q - W
Put an airtight can of air on a hot stove and heat it up. Let's define the air within the can
as the "system." Because the can is of fixed volume, the air can't do any work on it
(work involves movement by a force). All the heat that goes into the can increases the
internal energy of the enclosed air, so its temperature rises. If the can is fitted with a
movable piston, then the heated air can do work as it expands and pushes the piston
outward.
If heat is added to a system that does no external work, then the amount of heat added
equals the internal energy increase of the system. If the system does external work, then
the increase in internal energy is correspondingly less.
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I . If 100 J of heat is added to a system that does no external work, by how much is the
internal energy of that system raised?
2. If 100 J of heat is added to a system that does 40 J of external work, by how much is
the internal energy of that system raised?

We always talk about energy transfer to or from some specific system. The system might
be a mechanical device, a biological organism, or a specified quantity of material, such as
the refrigerant in an air conditioner or steam expanding in a turbine. In general, a
thermodynamic system is any collection of objects that is convenient to regard as a
unit, and that may have the potential to exchange energy with its surroundings. A
familiar example is a quantity of popcorn kernels in a pot with a lid. When the pot is
placed on a stove, energy is added to the popcorn by conduction of heat. As the popcorn
pops and expands, it does work as it exerts an upward force on the lid and moves it
through a displacement.

We describe the energy relationships in any thermodynamic process in terms of the
quantity of heat Q added to the system and the work W done by the system. Both Q and W
may be positive, negative, or zero. A positive value of Q represents heat flow into the
system, with a corresponding input of energy to it; negative Q represents heat flow out of
the system. A positive value of W represents work done by the system against its
surroundings, such as work done by an expanding gas, and hence corresponds to energy
leaving the system. Negative W, such as work done during compression of a gas in which
work is done on the gas by its surroundings, represents
energy entering the system.

If the pressure p remains constant while the volume
changes from V 1 to V2, the work done by the system is:

In any process in which the volume is constant, the system
does no work because there is no displacement.
One gram of water (1 cm3) becomes 1671 cm3 of steam
when boiled at a constant pressure of 1 atm (1.013 X 105
Pa). The heat of vaporization at this pressure is Lv =
2.256 X 106 J /kg. Compute (a) the work done by the
water when it vaporizes and (b) its increase in internal
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HAND OUT NO.14 IN PHYSICS: 1st and 2nd laws of Thermodynamics

Thermodynamic Processes
Compressing or expanding a gas while no heat enters or leaves the system is said to be an adiabatic process (from the
Greek for "impassable"). Adiabatic conditions can be achieved by thermally insulating a system from its surroundings
(with Styrofoam, for example) or by performing the process so rapidly that heat has no time to enter or leave. In an
adiabatic process, therefore, because no heat enters or leaves the system, the "heat added" part of the first law of
thermodynamics must be zero. Then, under adiabatic conditions, changes in internal energy are equal to the work
done on or by the system.
When there is no heat transfer, Δ Heat (Q) = 0.
So 0 = Δ internal energy (U) + work (W). Then we can say : -Work = Δ internal energy
For example, if we do work on a system by compressing it, its internal energy increases: We raise its temperature.
We notice this by the warmth of a bicycle pump when air is compressed. If work is done by the system, its internal
energy decreases: It cools. When a gas adiabatically expands, it does work on its surroundings, releasing internal
energy as it becomes cooler. Expanding air cools. You can demonstrate the cooling of air as it expands by blowing air
with your lips making a small opening.
Isochoric Process
An isochoric process is a constant-volume process. When the volume of a thermodynamic system is constant, it does
no work on its surroundings. Then W = 0 and U2 - U1 = ΔU = Q (isochoric process). In an isochoric process, all the
energy added as heat remains in the system as an increase in internal energy. Heating a gas in a closed constant-
volume container is an example of an isochoric process. Note that there are types of work that do not involve a
volume change. For example, we can do work on a fluid by stirring it. In some literature, ''isochoric'' is used to mean
that no work of any kind is done.
Isobaric Process
An isobaric process is a constant-pressure process. In general, none of the three quantities ΔU, Q, and W is zero in an
isobaric process, but calculating W is easy nonetheless. Most cooking involves isobaric processes. That's because the
air pressure above a saucepan or frying pan, or inside a microwave oven, remains essentially constant while the food
is being heated.
Isothermal Process
An isothermal process is a constant-temperature process. For a process to be isothermal, any heat flow into or out of
the system must occur slowly enough that thermal equilibrium is maintained. In general, none of the quantities ΔU,
Q, or W is zero in an isothermal process. In some special cases the internal energy of a system depends only on its
temperature, not on its pressure or volume. The most familiar system having this special property is an ideal gas. For
such systems, if the temperature is constant, the internal energy is also constant; ΔU = 0 and Q = W. That is, any
energy entering the system as heat Q must leave it again as work W done by the system. For most systems other than
ideal gases, the internal energy depends on pressure as well as temperature, so U may vary even when T is constant.
Thermodynamic Processes
Process                     Condition               Mathematical                           Example
Expression
Adiabatic            No heat enters or leaves the       Q=0, so ΔU = -W      Compression of the mixture of gasoline vapor
(no heat transfer)   system                                                  and air that occurs in the compression stroke
(internal combustion engine)
Isothermal            Temperature and internal         ΔT=0, then ΔU=0,       Thermos bottle. The insulation of the bottle
(constant temp.)      energy of system are constant    so Q=W                 keeps the temp. inside constant, & the
internal energy remains the same
Isochoric             Volume of the system does        ΔV=0, W=0, ΔU=Q        A rise in temp. due to transfer of heat into a
(constant vol.)       not change, no work done.                               subs. Inside a closed non-expanding
cylindrical container in a hot environment.
Isobaric              Pressure in the system does      ΔU≠0; Q≠0; W≠0;        Boiling water in an open container. Boiling
(constant pressure)   not change                       W= P(V2-V1)            occurs at constant atmospheric pressure.

METEOROLOGY AND THE FIRST LAW
Thermodynamics is useful to meteorologists when analyzing weather. Meteorologists express the first law of
thermodynamics in the following form:
Air temperature rises as heat is added or as pressure is increased.
The temperature of the air may be changed by adding or subtracting heat, by changing the pressure of the air (which
involves work), or by both. Heat is added by solar radiation, by long-wave Earth radiation, by moisture condensation,
or by contact with the warm ground. An increase in air temperature results. The atmosphere may lose heat by
radiation to space, by evaporation of rain falling through dry air, or by contact with cold surfaces. The result is a drop
in air temperature.

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HAND OUT NO.14 IN PHYSICS: 1st and 2nd laws of Thermodynamics
There are some atmospheric processes in which the amount of heat added or subtracted is very small-small enough
so that the process is nearly adiabatic.
Then we have the adiabatic form of the first law:
Air temperature rises (or falls) as pressure increases (or decreases).
Adiabatic processes in the atmosphere are characteristic of parts of the air, called parcels,
which have dimensions on the order of tens of meters to kilometers. These parcels are
large enough that outside air doesn't appreciably mix with the air inside them during the
minutes to hours that they exist. They behave as if they are enclosed in giant, tissue-light
garment bags. As a parcel flows up the side of a mountain, its pressure lessens, allowing it
to expand and cool. The reduced pressure results in reduced temperature. Measurements
show that the temperature of a parcel of dry air will decrease by 10°C for a decrease in
pressure that corresponds to an increase in altitude of 1 kilometer. So dry air cools 10°c for
each kilometer it rises. Air flowing over tall mountains or rising in thunderstorms or
cyclones may change elevation by several kilometers. Thus, if a parcel of dry air at ground
level with a comfortable temperature of 25°C were lifted to 6 kilometers, the temperature
would be a frigid -35°C. On the other hand, if air at a typical temperature of -20°C at an
altitude of 6 kilometers descends to the ground, its temperature would be a whopping
40°C A dramatic example of this adiabatic warming is the Chinook –a wind that blows
down from the Rocky Mountains across the Great Plains.

Cold air moving down the slopes of the mountains is compressed into a smaller volume and is appreciably
warmed. The effect of expansion or compression on gases is quite
impressive. A rising parcel cools as it expands. But the surrounding
air is cooler at increased elevations also. The parcel will continue to
rise as long as it is warmer (and therefore less dense) than the
surrounding air. If it gets cooler (denser) than its surroundings it
will sink. Under some conditions, large parcels of cold air sink and
remain at a low level with the result that the air above is warmer. When the upper regions of the atmosphere are
warmer than the lower regions, we have a temperature inversion. If any rising warm air is denser than this upper
layer of warm air it will rise no farther. It is common to see evidence of
this over a cold lake where visible gases and particles, such as smoke, spread out in a flat layer above the lake rather
than rise and dissipate higher in the atmosphere. Temperature inversions trap smog and other thermal pollutants.
The smog of Los Angeles is trapped by such an inversion, caused by low-level cold air from the ocean over which is a
layer of hot air that has moved over the mountains from the hotter Mojave Desert. The mountains aid in holding the
trapped air.

Adiabatic parcels are not restricted to the atmosphere, and
changes in them do not necessarily happen quickly. Some
deep ocean currents take thousands of years for
circulation. The water masses are so huge and
conductivities are so low that no appreciable quantities of
heat are transferred to or from these
parcels over these long periods of
time. They are warmed or cooled adiabatically by changes in pressure. Changes in adiabatic
ocean convection, as evidenced by the recurring EI Nino, have a great effect on Earth's
climate. Ocean convection is influenced by the temperature of the ocean floor, which, in turn,
is influenced by convection currents in the molten material that lies beneath the Earth's crust.
Knowledge of the behavior of molten material in Earth's mantle is difficult to acquire.

Second law of Thermodynamics
Suppose you place a hot brick next to a cold brick in a thermally insulated region. You know
that the hot brick will cool as it gives heat to the cold brick, which will warm. They will arrive
at a common temperature: thermal equilibrium. No energy will be lost, in accordance with the
first law of thermodynamics.
But pretend the hot brick extracts heat from the cold brick and becomes hotter. Would this
violate the first law of thermodynamics? Not if the cold brick becomes correspondingly
colder so that the combined energy of both bricks remains the same. If this were to
happen, it would not violate the first law. But it would violate the second law of
thermodynamics. The second law identifies the direction of energy transformation in
natural processes.
The second law of thermodynamics can be stated in many ways, but, most simply, it is this:
Heat of itself never flows from a cold object to a hot object.
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HAND OUT NO.14 IN PHYSICS: 1st and 2nd laws of Thermodynamics
In winter, heat flows from inside a warm, heated home to the cold air outside. In summer, heat flows from the hot air
outside into the cooler interior. The direction of spontaneous heat flow is from hot to cold. Heat can be made to flow
the other way, but only by doing work on the system or by adding energy from another source-as occurs with heat
pumps and air conditioners, both of which cause heat to flow from cooler to warmer places.

Heat engine
A heat engine is any device that changes internal energy into mechanical work. The basic idea behind a heat engine,
whether a steam engine, internal combustion engine, or jet engine, is that mechanical work can be obtained only
when heat flows from a high temperature to a low temperature.In every heat engine, only some of the heat can be
transformed into work.
Heat flows out of a high-temperature reservoir and into a low-temperature one. Every heat
engine (1) gains heat from a reservoir of higher temperature, increasing the engine's
internal energy; (2) converts some of this energy into mechanical work; and (3) expels the
remaining energy as heat to some lower-temperature reservoir, usually called a sink.
In a gasoline engine, for example, (1) products of burning fuel in the combustion chamber
provide the high temperature reservoir, (2) hot gases do mechanical work on the piston,
and (3) heat is expelled to the environment via the cooling system and exhaust.
Applied to heat engines, the second law may be stated:
When work is done by a heat engine operating between two temperatures, Thot and Tcold,
only some of the input heat at Thot can be converted to work, and the rest is expelled at
TCold.
Heat Pumps
A heat pump is a device that transfers heat energy from a low-temperature reservoir to a high temperature reservoir.
Its function is basically the reverse of the heat engine. Example is the refrigerator and the air conditioner.

Thermal Efficiency
Before scientists understood the second law, many people thought that a very low-friction heat engine could convert
nearly all the input heat energy to useful work. But not so.
In 1824, the French engineer Nicolas Lkonard Sadi Carnot analyzed the functioning of a heat engine and made a
fundamental discovery. He showed that the greatest fraction of energy input that can be converted to useful work,
even under ideal conditions, depends on the temperature difference between the hot reservoir and the cold sink.
His equation is:
or      Eff = Wnet / QH = QH – Qc = 1 – (Qc /QH)
QH
Carnot's equation states the upper limit of efficiency for all heat engines, whether in an automobile, a nuclear-
powered ship, or a jet aircraft.
For example, when the hot reservoir in a steam turbine is 400 K (127 °C) and the sink is 300 K (27 °C), the ideal
efficiency is .25 . This means that, even under ideal conditions, only 25% of the heat provided by the steam can be
converted into work, while the remaining 75% is expelled as waste.

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HAND OUT NO.14 IN PHYSICS: 1st and 2nd laws of Thermodynamics
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