Absorptive power

When heat is incident on the surface a body a part of it is absorbed and the remaining part is reflected.

The amount of heat absorbed by the body depends upon the nature of the body. A measure of heat radiation absorbed by the abody is called absorptive power.

Absorptive power of a body is defined at a given temperature and wavelength is defined as the ratio of the amount of heat energy absorbed to the amount of heat energy incident on it in a wavelength range.

Emissive power

The radiation process is different for different materials and also the radiation process is will be different for a body at different temperatures. Obviously, we can say that the radiation for a body depends upon the surface area of the body, its temperature, the temperature of the surrounding, and time for which the radiation takes place.

Emissive power of a body at temperature corresponding to a wavelength is defined as the energy radiated by the body per sec per unit area of the body for unit range of wavelength.

Thermal radiation

The main point other than conduction and convection in thermal conduction is that transmission of heat between two bodies at different temperatures without aid of any material medium. Obviously,
we say that the process of transfer of heat energy in the absence of any medium is known as thermal radiation.

So, we can say that, in general all bodies except those at absolute temperature emit heat energy at all temperatures by radiation. This is the quickest process than conduction and convection.

Transfer of heat energy from the sun is the best example of thermal radiation. In this process, the intervening medium will not get heated.

When the body is hot the energy of vibration of atoms and molecules is sent out in the form of radiant heat waves.

These waves are of long wavelength electromagnetic waves ranging from 800 nm to 4,00,000 nm
The following properties of the thermal radiation are
1. Thermal radiation travel in straight line
2. Thermal radiation can travel through the vaccum
3. Thermal radiation travel with speed of light
4. Thermal radiation follows the inverse square law i.e., the intensity of heat at a point varies inversely as the square of the distance of the point from the source.
5. When the thermal radiation falls on a body, its temperature rises due to absorption of radiant heat

Prevost contributed to this concept in explanation way by developing the theory. According to this theory, the rate of emission of heat increases with an increase in the temperature of the body but is independent of the presence of the neighbouring objects.
This theory mainly explains the reason for sensation of cold and hot.
According to this theory, two bodies at different temperatures, the body at high temperature looses more heat due to radiation and gains less heat due to absorption and the other body at low temperature gains more heat due to absorption and looses less heat due to radiation. Even the two bodies at equal temperature, there is exchange of heat in the form of radiation between these two bodies, i.e., the bodies emit and receive equal amount of heat radiation.

Types of convection

There are two types of convection i.e., one is natural or free convection and forced convection.

Free or natural convection takes place in the still fluid, in this process the motion of the fluid particles will be due to their getting heated by the hot body.

In general natural or free convection is a consequence of gravity and always takes place vertically carrying heat upwards.

Forced convection takes place when steady flow of air is sent into the hot air
For example, cool air from open window enters the room and sends the hot air to outside through ventilators.
And more example is the air is caused to move by the action of pump or fan

Convection

The transmission of heat from one part to another part due transfer of particles of matter is known
as convection.

Convection process is mainly identified in liquids. This process can be understood by taking the water in a beaker and few pieces of potassium permagnete is added to the water. On heating the beaker, it is found that the particles at the bottom get heated and the water at the bottom of the beaker becomes less dense so that the particles at bottom of the beaker comes upward, the particles at the upper region being dense flows down to reach the bottom of the beaker and in turn those particles get heated becomes less dense and againg these particles comes upwards. Like this, the water in the beaker gets heated due to convection process. How can recognise this process? For that purpose we added pottasium permagnete to the water. The movement of the particles of pottasium permagnete with water particles moves ups and down confirm the process of convection.

But convection process is quicker than conduction process.
Still air cooling in the closed room, ventilation cooling by the constant current of air flow into the room, through an open window,land and sea breezes are examples of heat transfer by convection.

Important of thermal conductivity and its explanation

As conduction is different in different solids so that to identify the different solids with respect to conduction, the concept found is thermal conductivity. Thermal conductivity is nothing but ability to conduct heat in solids. For example, silver has the highest thermal conductivity, next comes copper. Liquids except mercury, are the poor conductors than solids, while gases are much poorer than liquids.

In order to understand the property of thermal conduction of a solid, we have to find the coefficient of thermal conductivity for that particular solid









Here the coefficient of thermal conductivity K depends upon the nature of the material of the slab, and it is defined as the amount of heat conducted through the material of unit length per second through unit area of cross section, per unit temperature difference.

Conduction of heat

Conduction of heat is nothing but the transmission of heat from hotter to colder part of a body without transfer of particles of the material medium.

If we put a spoon in vessel which is heating, after sometime the spoon gets heated, i.e., the heat is travelled from the vessel to spoon, i.e., from hotter body to colder body. But this process takes in moderately slow and gradual manner. So this process of transmission of heat is called thermal conduction

If one end of a metal rod is placed in a flame, then the atoms at the end of the rod gets heat energy and gets vibrated, this vibrational atoms with increased amplitude collide with the adjacent atoms, then they also get vibrated. Like that this vibration causes the adjacent atoms to be vibrated. Like this, the total atoms in the material medium gets vibrated and heated, but atoms position is not changed,i.e., they are in equilbrium position. As the atoms in the solid are in arrangement manner and adjusted so that conduction is possible only in solids. Conduction is different in different solids.

Transmission of heat

Transmission of heat can take place in three ways i.e., conduction, conviction and radiation, in which radiation takes important role because medium will not be required for transmission of heat.
But for conduction and conviction, there should be material medium for passing heat.

According to law of conservation of energy, there is no dissipation of energy but it can transform to one place to another place. Another important point here to remember is that heat must be transformed from higher level of energy to lower level of energy. Like that, heat is transformed to hot body to cold body.

As energy is vector, this has direction and quantity, like that heat energy has the direction of transmission from hot body to cold body, this is told by second law of thermodynamics and it also tells the possibility of transmission of heat from cold body to heat body with aid of external agency.

irreversible process

A process that cannot be retraced back to its original position even after the system passes through the same intermediate states called irreversible process.

All the spontaneous and natural processes are irreversible processes.

Some more examples for irreversible process are
1. Work done against friction
2. Joule-heating-heat produced in a conductor by passing a current through it
3. Diffusion of gases
4. Magnetisation of a material.

Heat engines working between the given temperatures of source and sink, the engine working in reversible process have the highest efficiecy.

Reversible process

A process that can be retraced back to its original position even after the system passes through the same states as in the direct process, and finally the system and the surroundings reaches to its original states without any change in the universe is called reversible process.

This process is ideal process for which the following conditions should be required

1. The changes must take place infinitesimally slow rate.
2. The system must always be thermal equilbrium with the surroundings

Above two conditions tells the process is quasi-static process.

3. There should be no loss of energy due to conduction, conviction or dissipation of energy against any resistance such as friction, viscosity etc.
4. No amount of heat transformed in to magnetic or electric energy
Examples for reversible process are
A quasi-static isothermal expansion of an ideal gas in a cylinder fitted with a frictionless movable piston, Peltier effect and Seebeck effect and Fusion of ice and vaporisation of water.

Second law of thermodynamics

First law of thermodynamics does not give full information about the relation between mechanical work and heat, i.e., this law does not give idea about the condition under which conversion of heat into work or vice versa can take place and also it does not give the idea about the direction in which the transfer of heat take place.

The fundamental law in nature is heat always passes from hot body to cold body by itself. This can be stated in number of ways.
Classius states that it is impossible for a self acting machine unaided by any external agency to transfer heat from cold body to hot body i.e., from a body at lower temperature to a body at higher temperature.
Kelvins states this law in another way that it is impossible to continuously extract of work from cool body to temperature lower than that of the coldest of its surroundings. Therefore, no engine can convert the whole of the heat energy supplied to it into useful work. Under this concept, Kelvin stated the second law of thermodynamics that it is impossible to construct a heat engine which converts heat energy completely into work without any change in the working system.
Can you know what are the tremendous changes happen if second law of thermodynamics is not valid? if second law of thermodynamics is not valid, then every substance in the universe becomes ready to give out heat energy completely to do work whatever heat energy absorbed by it. Then endless source of energy available in the universe. But this is impossible.

Heat engines and refrigerators

Heat engine is a device which is used to convert of heat into work or mechanical energy
Refrigerator is a device which is the reverse of heat engine.
Heat engine represents the following figure
In the above figure, source indicates the hot reservoir and sink indicates the cold reservoir. A body contains the engine is called working substance. Engine extracts heat from the source and some process done at it, after certain amount of heat is released to the sink. Again source produces heat to engine, engine use it and it do some work, and then certain amount of heat released to the sink. Like this, heat engine undergoes cyclic process.
Whatever heat extracted by the engine is used to work done by the engine transferred to the environment via some arrangement to drive vehicles.
The efficiency of heat engine is defined as the amount of heat turns to work done by the working substance per given heat to the engine or the ratio of work done by the engine to the amount of heat absorbed by the engine, From the above figure
Here Q1>Q2, T1>T2
In case of the refrigerator indicates the following figure.
In a refrigerator the working substance extracts an amount of heat Q2 from the sink at lower temperature T2. An amount of external work done W on the working substance and then the amount of heat Q1 given to the source at a higher temperature T1.

Cyclic process

Before discussing about this concept, we take water for which in three states such as ice, water and vapour, after vapour state by decreasing temperature and pressure, again vapour state in turn to
solid state. This fact is explained in the matter triple point, i.e., water has cyclic process because it reaches to original state as ice after passing through different stages.
A process in which the system after passing through various stages returns to its initial state is called cyclic process. In a cyclic process, the change in the internal energy is zero.
According to first law of thermodynamics,
dQ=dU+dW
Since dU=0
dQ=dW

In cyclic process, the total heat absorbed by the system equals the work done by the system.
Obviously, the P-V diagram for a cyclic process will be closed curve.
The main application of cyclic process is that it is used in case of heat engines, refrigerators etc.

Latent heat

It is the heat supplied to the matter of 1kg weight to change its state at constant temperature. If the solid can be changed into liquid at constant temperature by appyling the heat. This heat is called 'latent heat of melting or fusion'.
A liquid can be changed into vapour at constant temperature by applying the heat. This heat is called 'latent heat of vapourisation.
Some liquids have low latent heat of vapourisation called volatile liquids. Ether and petrol are the examples of volatile liquids because these liquids get vapourised quickly.

Triple point of water

The graph between the pressure and temperature of a substance describing the boundaries between the phases is called phase diagram. In this diagram the point at which solid, liquid and vapour can co-exist or will be in equilbrium is called triple point. This point represents particular pressure and temperature.
In order to understand this concept, we take the example water for which melting point is at zero degree centigrade and boiling point is at hundred degree centigrade. At melting point, boiling point and sublimation points, depending upon the state of matter, pressure can be rapidly changed but temperature is at constant value. Like this, at certain pressure, water is at solid state and also it reaches sublimation state at temperature zero degree centigrade. At that pressure and temperature,
solid, liquid and gaseous states are co-exist or equilbrium. If it indicates on the phase diagram,
the point at which pressure and temperature which indicates solid, liquid and gaseous states are equilbrium is called triple point. In the above diagram, ice line, steam line and sublimation line indicates line drawing between melting point and pressure, line between boiling point and pressure and line drawing between sublimation point and pressure respectively. Mainly, these lines indicate equilbrium situation between two states. When these three lines are extended, then they intersect at one point is called triple point.

Origin of Triple point of water

Before discussing about this topic, we should know that the matter undergoes three different states
known as phases. During the change of state, there is loss of heat or gain of heat. At constant temperature only, change in state occurs. At a definite pressure and temperature three phases occur. This property is applicable to all substances.
At constant temperature, a solid changes its state as liquid, then it is known as 'melting'. At constant temperature, a liquid changes its state as gas, then it is known as 'boiling'.
It is important point to remember that the heat supplied to the substance which causes to increase the inter-molecular distances.
Above diagram shows that how the water can undergo three different phases at definite temperatures.

adiabatic process

This the process in which the system undergoes pressure and volume changes while no heat enters the system and leaves the system.
In this process, temperature changes during this process, i.e., the system is not equilbrium with the surroundings of the system i.e., we can say that the system is thermally isolated.
The adiabatic process is more quicker than the isothermal process because it is the process different from the natural processes like melting and boiling. The adiabatic process is not open to the surroundings i.e., temperature changes occurs during the process. After adiabatic expansion, the temperature decreases and adiabatic compression the temperature increases.
According to first law of thermodynamics, dQ=dU+dW
In adiabatic process, there is no heat supplied to the system or lost by the system i.e., dQ=0
Due to temperature changes, Boyle's law does not hold good.
During this process, the work done by the gas

isothermal process

We have already known isothermal process is quasi-static process. It is the process in which the system undergoes physical changes while keep the temperature remains constant, i.e., the system is always thermal equilbrium with the surroundings.
For gases, at constant temperature, Boyle's law holds good. Therefore, we can apply Boyle's law to the system which undergoes isothermal process.
Alright, here is the question is how we can prepare a system(consider cylinder with the piston) which maintains isothermal process. For that, the following three requirements are needed.
1. The cylinder and the piston should be good conductors of heat
2. Sufficient time should be given to the system to compensate for the changes in temperature(for example, hot coffee takes time to get it cold, i.e., changes occurs in its temperature to get the temperature of the surroundings.)
3.Heat must be allowed to transfer from the system to the surroundings freely manner, i.e., until the system is in thermal equilbrium with surroundings there must be thermal communication between the system and the surroundings.
Under isothermal process, if the gas expands from the volume V1 to volume V2 at constant temperature T, then the workdone by the gas is given by

we can say the example of isothermal process is melting of solids at their melting points because solid is open to nature, so automatically there is thermal equilbrium between solid and nature.
We can say the another example is that vapourisation of liquids at their boiling points because when the liquid reaches to the boiling point then its surroundings gets reach to the same temperature. Obviously, there is no thermal communication between the liquid and surroundings i.e., these are two are in thermal equilbrium.

quasi-static process and its back ground

Let us take a container having some gas and to compress there is a piston to move up and down and the gas exerts some pressure on the piston. At that position, the temperature and the pressure of the gas becomes same as the surroundings of the container, so there is thermal equilbrium between the gas and surroundings. When the piston is moved down with external pressure, then the pressure of the gas is different from the pressure of surroundings and the temperature of the gas is different from the temperature of the surroundings because the gas undergoes sudden compression due to moving down the piston.
We stop the piston moved down, after some time again gas reaches the equilbrium with the surroundings of the container, i.e., gas takes some time to reach equilbrium.
In our hypothesis, if we take ideal gas and the piston is moved down infinitesimally slow, then each and every stage gas reaches the equilbrium with surroundings of the container. Like this, at every stage the temperature and the pressure are same as the surroundings. In general, it is not happened and it is difficult to moved down the piston with infinitesimally slow manner.
So, this hypothetical process is called quasi-static process.
Basing on this process, we can step up to know about isothermal, adiabatic processes.