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UNIT 3 HEAT AND

TEMPERATURE

UNIT 3 HEAT AND

TEMPERATURE



The discovery of fire and the controlled use of fire are

among the most valuable human discoveries Nearly a

million years ago, humans already knew how to use

fire to create light and heat, to cook plants and

animals, and to keep predators away On basis,

whenever standing close to a heat source, we feel hot

However, if we touch a good thermal flask on its

surface, we do not feel hot at all Have you ever asked

yourself why there is a difference; in other words,

“what is heat” and “how is heat transferred from an

object to the other”?

To answer these questions, we should recall the basic

principle that heat can only be transferred via a specific

environment of matter According to the Kinetic Theory of Matter, matter is composed of a large number of atoms or

molecules These atoms or molecules carry (contain) both the kinetic energies resulted from their motion and

potential energy resulted from interaction or the changing

of position of the forces among them

Back3.1: A reconstruction of Homo erectus making fire

A body of matter can be viewed as a system of atoms, and each system has a typical amount of energy known the “internal energy” of that system, which is sum of the total kinetic and potential energies of all the atoms

or molecules in the system at rest as a whole When that internal energy is transferred between two bodies as a result of their different temperatures, this energy is

called heat.

Heat is thus the amount of internal energy flowing from a body at a higher temperature to a body at a lower one, raising the temperature of the latter and lowering that of the former substance, provided that the volumes of the bodies remain constant as shown

in Fig 3.3

Fig.3.3: Transfering of energy from one part

of a substance to another.

Heat is energy in transform: when the body cools

down, its internal energy decreases, when it is heated,

its internal energy increases Whenever two bodies

with different temperature are brought into contacts,

thermal energy always flows from the hotter body to

the cooler one until they are both at the same

temperature When this occurs, we say two bodies are

in thermal equilibrium Therefore, in principle, heat

does not flow from a lower to a higher temperature

environment unless another form of energy has

transferred in the opposite direction, or work is also

presented

[2]

Until the beginning of the 19th century, the effect of

heat on the temperature of a body was explained by

postulating the existence of an invisible substance or

form of matter termed caloric According to the caloric theory of heat, a body at a higher temperature contains more caloric than one at a lower temperature So, when two bodies at different temperatures are brought into

contacts, hotter body loses some caloric to the cooler

one until they are both at the same temperature.

[3]

Although the caloric theory successfully explained some phenomena of heat transfer, experimental evidence was presented by the American – born British physicist Benjamin Thompson (later known as Count von Rumford) in 1798 and by the British chemist Sir Humphry Davy in 1799 suggesting that heat, like work, is a form of energy in transform Between 1840 and 1849 the British physicist James Prescott Joule, in series of highly accurate experiments, conclusively confirmed that heat is a form of energy in transform and that it can cause the same changes in a body as work

[3]

The sensation of warmth or coldness of a substance on contact is determined by the property known as

temperature Although it is relatively easy to compare

the relative temperature of two substances by the sense

of touch, it is impossible to evaluate the absolute

magnitude of the temperatures by subjective reactions Temperature depends on the average kinetic energy of the molecules of the substance

[4]

Adding heat to the object not only raises its temperature,

which makes us feel warmer when touching it, but may

also produces alterations to several physical properties,

which can be measured with precision by other means

Therefore, temperature is a measure of the intensity of heat

or cold Two identical substances may have the same

temperature but may possess different quantities of heat

Heat energy will produce a measurable change in

temperature when enough energy is absorbed by matter to cause a significant increase in the average kinetic energy of its molecules

[4]

As its temperature varies, a substance expands or contracts accordingly, and its electrical resistivity changes In

contract, in the gaseous form as a special case, molecules

move incessantly and the more quickly the molecules

move, the higher temperature of the object is, it also exerts varying pressure Therefore, any state of gas can be

described by three state parameters such as pressure (P),

volume (V), and temperature (T).

[4]

Temperature is expressed in degrees and can be measured by

five different temperature scales: the Celsius or Centigrade

scale; the Fahrenheit scale; the Kelvin scale; the Rankine scale; and the international thermodynamic temperature scale 1) In

most countries, temperature is used in the Celsius or Centigrade scale This temperature scale, the boiling point of pure water is 1000C and freezing point is 00C, we noticed as TC(0C) 2)

Until the 1970s, Fahrenheit scale was commonly use in

English-speaking countries The conversion formula from

Celsius (C) to Fahrenheit (F) is: Hence, in this

latter scale, the boiling point of pure water is 2120F and the

freezing point is 32^0F ”

[5]

Kelvin scale is popular in scientific applications This scale is convenient for recording extremely low temperatures because

there is no negative temperature, i.e., the lowest temperature is

0K Like other temperature scales, the freezing and boiling

points of water are 273.16 K and 373.16 K, respectively 4)

Another absolute temperature scale, the Rankine temperature

scale, is used in some engineering applications Absolute zero,

or 0°R, is the temperature at which molecular energy is a

minimum, and it corresponds to a temperature of - 459.67°F

The freezing point of water and the boiling point of water

correspond to 491.67°R and 671.67°R (see Fig 3.4)

[6]

[6]

Thermal systems change as the thermodynamic variables

change (P,V,T) It’s possible to have processes in which

only two parameters change, another parameter keeps

being unchanged, these processes are called isoprocess

The process of state transformation in which the

temperature is kept unchanged is called isothermal

process It is Boyle- Mariotte’s law “a certain amount of

gas, the pressure inverses proportionally to the volume or

PV = const while T=const.” In addition, the process of

state transformation when the volume is unchanged is the

isochoric process

[5]

This can call as Charles’s law “a certain amount of gas,

the pressure is directly proportional to the absolute

temperature, while V=const.” Moreover, the

process transforming the state without changing the air

pressure is called isobaric process It is Guy – Lussac’s law

“a certain amount of gas, the volume is directly

proportional to the absolute temperature

l ; P=const.”

[5]

In cryogenics, or low- temperature research, temperatures as

low as 0,003 K have been produced by the demagnetization of

paramagnetic materials Momentary high temperatures

estimated to be greater than 100,000,000 K have been achieved

by nuclear explosions

[6]

[6]

As we have known that heat energy always travels from

the hotter object (higher temperature) to the colder object

(lower temperature) Heat transfer between objects of

different temperatures until an equilibrium point is reached

as shown in Fig 3.6 However, one question is raising that how can the heat transfer? Heat travels from one object to

another object in three ways (1) by conduction, (2) by

convection, and (3) by radiation .

[7]

especially in solids matter such as metals when we heat up a piece of metal (see Fig 3.7.) The electrons

in a piece of metal can leave their atoms and move inside the metal body as free electrons The parts of the body’s atoms left behind are now charged metal ions The ions are packed closely together and they vibrate continually The hotter the metal, the more kinetic energy these vibrations have This kinetic energy is transferred from hot parts of the metal to cooler parts by the free electrons These move through the structure of the metal, colliding with ions as they go.

CONDUCTION:

In this process, heat will continue to transfer until every part

of the object has the same temperature While metals are good

conductors for heat, non-metals and gases are usually poor heat conductors Poor heat conductors are called insulators Heat

energy is conducted from the hot end of an object to its cold

end

CONDUCTION:

Convection is another mean of heat transfer It occurs only in fluids (liquid and gas) When the liquid is heated, the particles

of the liquid receive energy, the liquid also expands and

becomes less dense The hot particles rises up against gravity

then transfer some energy to the cooler particles around, and

cool down After that, the hot fluid becomes more dense and

falls (sinks) down to the bottom While this process occurs, the cooler particles at the top moves down to replace the hotter

ones and starts a new cycle An example of convection is the

Earth atmosphere During the day, the ground heats up more

quickly than water in the sea, because water has a greater

specific heat The air in contact with the warm ground is heated

CONVECTION

It expands and become lighter As a result, the warm air rises

up, resulting in air currents and cool air moves down to fill the

space This creates a air cycle or sea breeze

In this way, thermal convection cycle is set up, which transfer

heat away from land (see Fig 3.8)

CONVECTION

The next example is the convection current occurs in the

aesthenophere The inner core of the Earth generates huge

amount of heat energy and travels out through the outer core

and to the aesthenophere (mantle) where it is much cooler It

heats the viscous material at the bottom of the mantle and make

up the mantle moves The hotter material at the bottom of matle moves towards the top and begin cool down as heat lost to the

crust The cooler material at the top of the mantle starts sink to

the bottom, it warms up and then moves towards to the top

again This cyling of hot and cool material is called the

covection current As a results, the trench and the ridge is

created by the mantle convection (see firgure 3.8)

CONVECTION

CONVECTION

The transfer of heat by convection and conduction

requires a material medium for the process to take place However, the Sun can transfer its heat to the Earth

through the vacuum Because heat from the Sun reaches the Earth via electromagnetic waves The process of

transferring heat energy through space by means of

electromagnetic wave is known as radiation (see Fig

3.9) Electromagnetic waves carry energy and can travel through a vacuum The heat we get from the Sun is

transmitted through the vacuum of space by radiation

RADIATION

Another example of heat transfer by radiation is heat from

a campfire or wood stove We can readily feel warmth of

the fire on exposed skin Some of the radiant energy that

falls upon matter may be absorbed and converted to heat

Dark-colored objects are good absorbers and poor

reflectors of heat energy Black rough surfaces absorb heat and transfer heat well White or light-colored objects and

highly polished, smooth surfaces such as aluminum reflect most of the heat rather than absorb it.

RADIATION

RADIATION