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HEAT

Introduction:
      Heat is energy in transfer other than as work or by transfer of matter. When there is a suitable physical pathway, heat flows from a hotter body to a colder one. The transfer results in a net increase in entropy. The pathway can be direct, as in conduction and radiation, or indirect, as in convective circulation.
     Heat refers to a process of transfer between two systems, the system of interest, and its surroundings considered as a system, not to a state or property of a single system. If heat transfer is slow and continuous, so that the temperature of the system of interest remains well defined, it can be described by a process function.
     If W is performed and heat produced is H then W/ H = J or W = JH
    Where J is a constant called Mechanical Equivalent of Heat. Its value is 4.186 Joule/ Calorie. It means if 4.186 joule of work is performed, 1 calorie of heat is consumed.

 

Units of Heat:
C.G.S unit: Calorie = it is the amount of heat required to raise the temperature of 1 g of pure water through 1o C.

 

International calorie: it is the amount of heat required to raise the temperature of 1 g of; pure water from 14.5o  C to 15. 5°o C.
F.P.S unit: B.Th.U (British Thermal Unit) = it is the amount of heat required to raise the temperature of 1 pound of pure water through 1o F.

 

Relation between different units:
    1 B.Th.U = 252 calorie 
    1 Calorie = 4.186 joule 
    1 Thermo = 105 B.Th.U
    1 calorie = 4.186 joule
    1 pound calorie = 453.6 calorie.

 

Temperature:
      Temperature is that physical cause which decides the direction of flow of heat from one body to other body. Heat energy always flows from body at higher temperature to body at lower temperature.
    A temperature is a comparative objective measure of hot and cold. It is measured, typically by a thermometer, through the bulk behavior of a thermometric material, detection of heat radiation, or by particle velocity or kinetic energy. It may be calibrated in any of various temperature scales, Celsius, Fahrenheit, Kelvin, etc.

 

Measurement of Temperature:
Thermometer: The device which measures the temperature of a body is called thermometer.

 
 

Total Radiation Pyrometer
      When a body is at high temperature, it glows brightly and the radiation emitted by the body is directly proportional to the fourth power of absolute temperature of the body. Radiation pyrometer measures the temperature of a body by measuring the radiation emitted by the body.
    This thermometer is not put in contact with the body. But it cannot measure temperature below 800o C because at low temperature emission of radiation is very small and cannot be detected.

 

Specific Heat Capacity:
      The specific heat is the amount of heat per unit mass required to raise the temperature by one degree Celsius. The relationship between heat and temperature change is usually expressed in the form where c is the specific heat. The relationship does not apply if a phase change is encountered, because the heat added or removed during a phase change does not change the temperature.
     The specific heat of water is 1 calorie/gram oC = 4.186 joule/gram oC which is higher than any other common substance. As a result, water plays a very important role in temperature regulation. The specific heat per gram for water is much higher than that for a metal, as described in the water-metal example. For most purposes, it is more meaningful to compare the molar specific heats of substances.

 

Specific Heat Capacities of different materials (J/kg K)         

Thermal Expansion:
      Thermal expansion is the tendency of matter to change in volume in response to a change in temperature, through heat transfer.
     Temperature is a monotonic function of the average molecular kinetic energy of a substance. When a substance is heated, the kinetic energy of its molecules increases. Thus, the molecules begin moving more and usually maintain a greater average separation. Materials which contract with increasing temperature are unusual; this effect is limited in size, and only occur within limited temperature ranges. The degree of expansion divided by the change in temperature is called the material's coefficient of thermal expansion and generally varies with temperature.

 

Transmission of Head:
    The transfer of heat from one place to other place is called transmission of heat. There are three modes of heat transfer
     i. Conduction
     ii. Convection
     iii. Radiation

 

Conduction:
     In this process, heat is transferred from one place to other place by the successive vibrations of the particles of the medium without bodily movement of the particles of the medium. In solids, heat transfer takes place by conduction.
     Heat energy can move through a substance by conduction. Metals are good conductors of heat, but non-metals and gases are usually poor conductors of heat. Poor conductors of heat are called insulators. Heat energy is conducted from the hot end of an object to the cold end.

 

Convection:
      In this process, heat is transferred by the actual movement of particles of the movement from one place to other place. Due to movement of particles, a current of particles set up which is called convection current.
     In liquids and gases, heat transfer takes place by convection.
     Earth’s atmosphere is heated by convection.

 

Radiation:
       In this method transfer of heat takes place with the speed of light without affecting the intervening medium.
      All objects give out and take in thermal radiation, which is also called infrared radiation. The hotter an object is, the more infrared radiation it emits.
    Infrared radiation is a type of electromagnetic radiation that involves waves. No particles are involved, unlike in the processes of conduction and convection, so radiation can even work through the vacuum of space. This is why we can still feel the heat of the Sun, although it is 150 million km away from the Earth.

 

Newton’s law of cooling:
     The rate of loss of heat by a body is directly proportional to the difference in temperature between the body and the surrounding.
     Temperature difference in any situation results from energy flow into a system or energy flow from a system to surroundings. The former leads to heating, whereas latter leads to cooling of an object. Newton’s Law of Cooling states that the rate of temperature of the body is proportional to the difference between the temperature of the body and that of the surrounding medium. This statement leads to the classic equation of exponential decline over time which can be applied to many phenomena in science and engineering, including the discharge of a capacitor and the decay in radioactivity.
     Newton's Law of Cooling is useful for studying water heating because it can tell us how fast the hot water in pipes cools off. A practical application is that it can tell us how fast a water heater cools down if you turn off the breaker when you go on vacation.
    Suppose that a body with initial temperature T1o C, is allowed to cool in air which is maintained at a constant at a constant temperature T2o C.
       Let the temperature of the body be TC at time t.
      Then by Newton’s Law of Cooling,

          

Kirchhoff’s law:
     According to Kirchhoff’s law, the ratio of emissive power to absorptive power is same for all surfaces at the same temperature and is equal to emissive power of black body at that temperature.
     Kirchhoff’s law signifies that good absorbers are good emitter.
    If a shinning metal ball with some black spot on its surface is heated to high temperature and seen in dark, the shinning ball becomes dull but the black spots shines brilliantly, because black spot absorbs radiation during heating and emit in dark.

 

Stefan’s law:
    The thermal energy radiated by a blackbody radiator per second per unit area is proportional to the fourth power of the absolute temperature and is given by
                              E = σ T4
    The Greek letter sigma (σ) representing the constant of proportionality, called the Stefan–Boltzmann constant. This constant has the value 5.6704 × 10-8 watt per metre2.K4. The law applies only to blackbodies, theoretical surfaces that absorb all incident heat radiation.

 

Change of State
     Any material can remain in any of its three states (solid, liquid and gas). To change the substance from one state to other state is called change of state. For this either substance is heated or heat is extracted from the substance.

                   
Fusion:

     Fusion or melting is the process by which a solid changes into the liquid state at certain fixed temperature by the absorption of heat energy.
     The fusion point or melting point is the fixed temperature at which a solid starts changing into the liquid state.

 

Freezing: The process by which a substance is changed from liquid state to solid state is called freezing. Freezing takes at a fixed temperature called freezing point (F.P)
     For a substance Melting point = Freezing point.
      Melting of a substance changes with the change in pressure. Melting point of substances which contracts in the processes of fusion decreases with the increase in pressure. Melting point of substance which expands in the process of fusion increases with the increase in pressure.

 

Vaporization:
      The process by which a substance is changed from liquid state to vapour state is called vapourisation.

 

 Vapourisation takes places by two methods
     i. Evaporation
     ii. Boiling or Ebullition

 

Evaporation:
     Evaporation is the process by which water changes from liquid to vapour below its boiling point.
     Evaporation is very important in the cooling of surfaces. If we put a drop of methylated spirit on our hand the methylated spirit will soon evaporate and our hand feels cold. The molecules of methylated spirit get the energy they need from our hand, leaving the molecules of the hand with less energy and so colder. The same thing happens when you sweat, only in this case you are losing water from your body by evaporation. The evaporation of perfume is why you can smell it – no evaporation means no perfume molecules in the air and so no smell.

 

Boiling:
     When a liquid is heated, it eventually reaches a temperature at which the vapor pressure is large enough that bubbles form inside the body of the liquid. This temperature is called the boiling point. Once the liquid starts to boil, the temperature remains constant until all of the liquid has been converted to a gas.
      The normal boiling point of water is 100oC. But if you try to cook an egg in boiling water while camping in the Rocky Mountains at an elevation of 10,000 feet, you will find that it takes longer for the egg to cook because water boils at only 90ºC at this elevation.

 

Condensation:
     Condensation is the change of water from its gaseous form (water vapor) into liquid water. Condensation generally occurs in the atmosphere when warm air rises, cools and loses its capacity to hold water vapor. As a result, excess water vapor condenses to form cloud droplets.
* Boiling point of a liquid increases with the increase in pressure.
* Boiling point of a liquid increases with the addition of impurity.

 

Latent Heat or heat of transformation:
      Latent heat, characteristic amount of energy absorbed or released by a substance during a change in its physical state that occurs without changing its temperature.
     The latent heat associated with melting a solid or freezing a liquid is called the heat of fusion; that associated with vaporizing a liquid or a solid or condensing a vapour is called the heat of vaporization. The latent heat is normally expressed as the amount of heat (in units of joules or calories) per mole or unit mass of the substance undergoing a change of state.
     For example, when a pot of water is kept boiling, the temperature remains at 100oC (212oF) until the last drop evaporates, because all the heat being added to the liquid is absorbed as latent heat of vaporization and carried away by the escaping vapour molecules. Similarly, while ice melts, it remains at 0oC (32oF), and the liquid water that is formed with the latent heat of fusion is also at 0oC. The heat of fusion for water at 0oC is approximately 334 joules (80 calories) per gram, and the heat of vaporization at 100oC is about 2,260 joules (540 calories) per gram. Because the heat of vaporization is so large, steam carries a great deal of thermal energy that is released when it condenses, making water an excellent working fluid for heat engines.

 

Sublimation:
      Sublimation is the term for when matter undergoes a phase transition directly from a solid to gaseous form, or vapor, without passing through the more common liquid phase between the two. It is a specific case of vaporization.
     The most well known example of a material that undergoes sublimation is dry ice, or frozen carbon dioxide.

 

Hoar Frost:
    Hoar frost is just the reverse process of sublimation i.e. it is the process of direct conversion of vapour into solid.
    Steam produces more severe burn than water at same temperature because internal energy of steam is more than that of water at same temperature.

 

Relative Humidity:
     Relative humidity is defined as the ratio of amount of water vapour present in a given volume of atmosphere to the amount of water vapour required to saturate the same volume at same temperature.
The ratio is multiplied by 100 to express the relative humidity in percentage.
* Relative humidity is measured by Hygrometer.
* Relative humidity increases with the increase of temperature.

 

Air conditioning:
    For healthy and favorable atmosphere of human being, the conditions are as follows:
      i. Temperature: from 23ºoC to 25oC.
      ii. Relative humidity: from 60% to 65%.
      iii. Speed of air: from 0.75 meter/minute to 2.5 meter/minute.

 

Sample Thermodynamics:
First law of thermodynamics:
     Heat energy given to a system is used in the following two ways.
   i. In increasing the temperature and hence internal energy of the system.
   ii. In doing work by the system.
      ΔQ = heat energy given to the system.
      ΔU= Increase in the internal energy of the system.
      ΔW = work done by the system.
   Then, ΔQ = ΔU + ΔW is the mathematical statement of first law of thermodynamics.
* First law of thermodynamics is equivalent to principle of conservation of energy.

 

Isothermal Process:
     If the changes are taking place in a system in such a way that temperature of the system remains constant throughout the change, then the process is said to be an isothermal.
Adiabatic process:
    If the changes are taking place in a system in such a way that there is no exchange of heat energy between the system and the surrounding, then the process is said to be an adiabatic process.
* If carbon dioxide is suddenly expanded, it is changed into dry ice. This is an example of adiabatic process.

 

Second law of Thermodynamics:
   The second law of thermodynamics states that in a natural thermodynamic process, there is an increase in the sum of the entropies of the participating systems.
   The second law is an empirical finding that has been accepted as an axiom of thermodynamic theory.
    The law defines the concept of thermodynamic entropy for a thermodynamic system in its own state of internal thermodynamic equilibrium. It considers a process in which that state changes, with increases in entropy due to dissipation of energy and to dispersal of matter and energy.
     The law envisages a compound thermodynamic system that initially has interior walls that constrain transfers within it. The law then envisages a process that is initiated by a thermodynamic operation that changes those constraints, and isolates the compound system from its surroundings.

 

Kelvin’s Statement:
    Whole of the heat can never be converted into work.

 

    Clausius statement: heat by itself cannot flow form a colder body to a hotter body.
 

Heat engine:
    Heat engine is a system that converts heat or thermal energy to mechanical energy, which can then be used to do mechanical work. It does this by bringing a working substance from a higher state temperature to a lower state temperature. A heat "source" generates thermal energy that brings the working substance to the high temperature state. The working substance generates work in the "working body" of the engine while transferring heat to the colder "sink" until it reaches a low temperature state. During this process some of the thermal energy is converted into work by exploiting the properties of the working substance. The working substance can be any system with a non-zero heat capacity, but it usually is a gas or liquid.

 

Heat engine may be divided into two types:
i. Internal combustion Engine: In this engine, heat is produced in the engine itself.
Example: Otto engine or petrol engine (efficiency = 52%), Diesel engine (efficiency = 64%).
ii. External Combustion Engine: In this engine heat is produced outside the engine. Steam engine is an example of external combustion engine. (Efficiency = 20%).
Refrigerator Heat Pump: A refrigerator is an apparatus which transfers heat energy from cold to a hot body at the expanse of energy supplied by an external agent. The working substance is called refrigerant.
     In actual refrigerator, vapours of Freon (CCI2F2) acts as refrigerant.

Posted Date : 03-02-2021

 

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