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CHAPTER 5

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Heat is a form of energy associatedwith the motion of atoms, molecules

and other particles which comprise

matter.

It can be created by chemicalreactions, nuclear reactions,electromagnetic dissipation, or

mechanical dissipation.

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Units of Heat

Joule (J)- The SI unit for energy named afterJames Prescott Joule, it is the workdone if a 1 Newton force act on a body

and causes a body to move by 1 meter.

Erg Unit of energy defined as the workdone, when a force of 1 dyne acts on a

body and causes a body to move

1 centimeter.

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Foot-pound (ft-lb)- Unit of energy defined asthe work done, when a force of 1

pound acts on a body and causes abody to move by 1 foot. For mostscientific work, the foot-pound equals1.356 joule.

Electron volt (eV)- A unit of energy equal tothe work done on an electrons movingit through a potential difference of

1 volt.

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Calorie (cal)- Is the quantity of heat thatraises the temperature of 1 gram of

pure water 1C. this unit is sometimescalled the small or gram calorie todistinguish it from the large calorie orkilocalorie.

Kilocalorie (kcal)- The amount of heatrequired to raise the temperature of 1kg of water through 1C. It is used to

measure large amount of heat. It isequal to 1000 cal, which is used innutrition studies.

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BTU (british thermal unit), Btu is the quantityof heat required to raise the

temperature of 1 lb of water 1F.Kilowatt-hour (kwh)- A commercial unit of

electrical energy equivalent to a power

consumption of 1000 watts for 1 hr.Dietary calorie (C)- the term dietary calorie,

written as C used in nutrition is the

amount of energy we could get fromfoods. One dietary calorie (C) is equalto one Kilocalorie.

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SOURCES OF HEAT

Natural

1. SUN

2. The interior ofthe Earth

Artificial

1. chemical action

2. mechanicalenergy

3. electrical

energy

4. nuclear energy

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Heat Capacity/ Specific heatcapacity

The specific heat of a body is thequantity of heat necessary to rise itstemperature by 1C. the unit for heat

capacity is J/K or J/C.

The specific heat capacity is theamount of heat necessary to raise the

temperature of one gram of a substanceby 1. The unit of specific heat is J/kgK.

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Specific heat capacities of commonsubstances

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Method of mixture

On cold days when we feel that thewater from the faucet or from the well istoo cold for our bath, we heat some water

and then mix this with the cold water in thetub where some of the heat is transferredfrom the hot water to the cold water. The

transfer of heat continue until both are atcommon temperature known as theequilibrium temperature.

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This equilibrium temperature is somewherebetween the initial temperature of the cold

and the hot water. It depends on the watermixed. The transfer of heat is always fromthe hot substance to the colder one.

During this process heat energy isconserved; hence, the heat lost by a hottersubstance is equal to the heat gained by acolder colder substance. This is theprinciple of the law of heat exchange.

The process of measuring the amount of heatinvolved in a chemical or physical change isknown as calorimetry.

Heat gained=Heat lostmcT=mcT

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Change of phase

A number of physical changes areassociated with the change of temperatureof a substance. Almost all substances

expand in volume when heated andcontract when cooled. The phase of asubstance refers to its occurrence as a

solid, liquid, or gas, and phase changes inpure substances occur at a definitetemperature and pressure

A phase change or a change ofstate occurs whenever the physical state

of a substance changes. Melting,freezing, evaporation, and condensationinvolve phase change.

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Sublimation- The process of changing fromsolid to gas without passing the liquid

state.Fusion- is the change of a substance from

the solid to the liquid state, usually by the

application of heat. The process is thesame as melting, but the term fusion isusually applied to substances such asmetals that become liquid at hightemperature, and to crystalline solids.

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Vaporization, is the change of water or otherliquids from a liquid to a gaseous state at a

temperature below its boiling point. Aliquid is made up of atoms or moleculesthat are in constant motion, traveling atdifferent speeds. The average speed of

these particles depends only on the liquidstemperature. If the particles have enoughenergy speed, and therefore enoughkinetic energy to cause the surface particle

to leave the liquid and become gas atomsor molecules.

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Boiling- is the vaporization of a liquid inbubbles in the body of the liquid as well as

the free surface. It is accompanied byagitation of the liquid as the bubbles rise,expand, and burst.

Condensation- is a process of reduction ofmatter into a denser form, as in theliquefaction of vapor steam. Condensationis the reduction of temperature by theremoval of latent heat of evaporation, theproduct being known is condensate.

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Solification or freezing- is the process ofchanging liquid to solid.

Deposition- is the process of changing gasto solid.

Melting point- is the temperature at which

any given substance changes from solid toliquid.

Boiling point- is the temperature at which a

substance changes from liquid to gas. Atemperature at which vapor pressure Iequal to the pressure exerted on the liquid.

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Heat of fusion- is the amount of heat neededto change a unit mass of a substance from

the solid to the liquid state without anychange in temperature. It is often calledthe latent heat of fusion.

Heat of solidification- is the amount of heatthat must be removed from a unit mass ofa substance from liquid to the a solid statewithout any change in temperature. Theheat of solidification is equal to the heat offusion.

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Heat of vaporization (latent heat ofvaporization)- is the amount of heat

needed to change a unit mass of asubstance from liquid state to vapor statewithout any change in temperature.

Heat condensation (latent heat of

condensation)- is the amount of heat thatmust be removed from a unit mass of asubstance at boiling point to change tochange it from vapor state to liquid statewithout any change in temperature. Equalto heat of vaporization.

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Triple point&Critical point

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INTRODUCTION

Changes of state are affected by pressure. Thehigher the pressure, the higher the boilingpoint. This is the principle that underlies thepressure cooker. By heating the water in asealed container, the pressure can be raisedwell above 1 atm, and the temperature atwhich the water boils will be correspondinglyhigher than 100C.In this way food can becooked more rapidly than in an open pan.

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The triple point

Is the point that the three phases can existtogether in equilibrium or it is the pressureand temperature wherein the solid, liquid,

and vapor state of matter can occursimultaneously.

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The Critical Point

Is the temperature and pressure at whicha substance changes in its physical state.The critical point of a metal alloy is the

temperature during the cooling of thesubstance at which the molecularrearrangement takes place, giving rise to a

different form of the substance, usually theabsorption or evolution of heat.

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The temperature above which it is nolonger possible to distinguish liquid and

vapor is called critical temperature. Abovethis temperature, a gas cannot be liquefiedno matter how much pressure is applied.The pressure required to liquefy a gas atits critical temperature is called the criticalpressure. Some gases, such as helium,hydrogen, and nitrogen, have low critical

temperatures and require intensivecooling before they can be liquefied.

Ph di h

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Phase diagrams, showwhether a substance existsas a vapor, liquid, or solid

at a given temperature andpressure. The point wherethe three lines intersect in

a phase diagram showsthe pressure andtemperature where the

solid, liquid, and vapor allexist in equilibrium. Thispoint, which occurs forwater at 0.01C(32.02F),is known as the tri le oint.

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Heat transfer

Heat transfer is the process by whichenergy in the form of heat is exchangedbetween bodies or parts of the same body

at different temperatures. The discipline ofheat is concerned with only two things:temperature, and the flow of heat.

Heat is generally transferred byconvection, radiation, or conduction.

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Conduction- is a method of heat transfer in amaterial medium from point to point ormolecule to molecule. Conduction occurswhen to object at different temperaturesare in contact with each other. Heat flowsfrom the warmer to the cooler object until

they are both at the same temperature.conduction is the movement of

heat through a substance by the collisionof molecules. At the place where the two

objects touch, the faster molecules of thewarmer substance collide with the slowermoving molecules of the cooler object.

Conduction is the

transfer of heatalong a solidobject; it is this

process thatmakes the handleof a poker hot,

even if only the tipis in the fireplace.

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Temperature gradient- is the average dropin temperature per unit length of the

material.T2-T1

Temp. gradient = ----------

L

The conductor of heat from the hotterportion to the cooler portion is proportional

to the cross-sectional area of the bar, andthe time the heat is allowed to flow.

L

Q

T1T2

High temp.Low temp.

Q= ___ktTA__LWhere:

A, area of cross-sectiont, time when heat is allowed to flowK, thermal conductivity of a material

Q, heat conducted in caloriesL, thicknessT, change in temperature

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Convection is the transfer of heatby moving matter. In liquids and gasses,

convection is usually the most efficientway to transfer heat.

convection occurs when warmerareas of a liquid or gas rise to cooler areas

in the liquid or gas. The motion of the fluidmay be natural or forced. If a liquid or gasis heated, its mass per unit volumegenerally decreases as this happens,cooler liquid or gas takes the place of thewarmer areas which have risen higher.

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Radiation is very much faster process thanconduction and convection. Heat by

radiation travels with the speed of light. Itis believe that or radiant energy istransferred by means of electro-magneticwaves. These waves are believed to be

disturbances in the electric and themagnetic field intensities which change indirection and magnitude as the waveprogresses. The amount of radiated

energy is measured by the change intemperature of the body that absorbs it.