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# What is temperature?

“The temperature of a body is the degree of hotness or coldness of the body.”or “Average kinetic energy of atoms or molecules.”When we touch a body,we feel it hot or cold.The temperature of a body tells us how hot or cold a body is.

When two systems are in thermal equilibrium,we say that they have the same temperature.Conversely,temperature is that property of a system which is equals that of another system when the two systems are in thermal equilibrium.For example ,suppose the systems are two gasses that initially have different temperatures,pressure and volumes.After we place them into contact and wait a sufficiently long time for them to reach thermal equilibrium,their pressures will in general not be equal,nor will their volumes;their temperatures,however,will always be equal in thermal equilibrium.It is only through this argument based on thermal equilibrium that the notion of temperature can be introduced into thermodynamics.

A candle flame is hot and is said to be at high temperature.Ice on other hand is cold and is said to be at low temperature.Our sense of touch is a simple way to know how much hot or cold a body is.However,this temperature sens is some what approximation and unreliable.Moreover it is not,always safe to touch a hot body.What we need is a reliable and practicable method to determined the relative hotness coldness of bodies.

To understand the concept of temperature,it is useful to understand the terms,thermal contact and thermal equilibrium.To store ice in summer,people wrap it with cloth or keep it in wooden box or in thermos flask.In this way ,they avoid the thermal contact of ice with its hot surrounding otherwise ice will soon melt away.Similarly,when you place a cup of hot tea or water in a room it cools down gradually.Does it continue cooling?It stops cooling as it reaches the room temperature.Thus,temperature determines the direction of flow of heat.Heat flows from a hot body to a cold body until thermal equilibrium is reached.

What happens when we touch a hot body?Take two bodies having different temperatures.Bring them in contact with each other.The temperature of the hot body falls.It losses energy.This energy enters the cold body at lower temperature.Cold body gains energy and its temperature rises.The transfer of energy continues till both the bodies have the same temperature.The form of energy that is transferred from hot body to a cold body is called heat.Thus “Heat is the energy that is transferred from one body to the other in thermal contact with each other as result of of the difference of temperature between them.”

Heat is therefore,called as the energy  in transit.Once heat enters a body, it becomes its internal energy and no longer exists at heat energy.

#### What is the internal energy of a body?

Internal energy is the sum of kinetic energy and potential energy associated with the atoms,molecules and particles of a body.

Internal energy of a body depends on many factors such as the mass of the body,kinetic,potential energies of molecules etc.Kinetic energy of an atom or molecule is due to its motion which depends upon the temperature.Potential energy of atoms or molecules is the stored energy due to inter molecular forces.

The device that is used to measure the temperature of a body is called Thermometer.

### Temperature scales:

Temperature is one of the seven base units,and we might therefore attempt to treat temperature as we treated other base units in the S.I system;establish a standard and relate all other scales to the standard.However,temperature has a nature different from that of other S.I base units,and so this scheme will not work in quite that simple a form.For instance,if we define one period of vibration of the light emitted by a Cesium atom as a standard of time,then two such vibrations lasts for twice the time, and any arbitrary time interval can in effect be measured in terms of the number of vibrations.But even if we define a standard of temperature, such as that of water boiling under certain conditions we have no procedure to determine a temperature twice as large. To pots of boiling water ,after all,have the same temperature as one pot.There is no apparent way using only this standard that we can relate the temperature of boiling water to that,for example,of boiling oil ;no amount of boiling water will ever be in thermal equilibrium with boiling oil.

To establish a measuring scale for temperatures we adopt the following procedure,which differs from the usual procedure for the S.I base units;we find a substance that has a property that varies temperature,and we measure that property.The substance we choose is called the thermometric substance,and the property that depends on temperature is called thermometric property.

A scale is marked on the thermometer.The temperature of the body in contact with the thermometer can be read on that scale.Three scales of temperature are in common use.These are:

1. Celsius scale or centigrade scale
2. Fahrenheit scale
3. Kelvin scale
1. Watch video to see temperature scales:

• ### Celsius scale or degree centigrade:

In nearly all countries of the world,the Celsius scale or degree centigrade is used for all popular and commercial and most scientific measurements. Anders Celsius (1701-1744) was a Swedish astronomer who,in addition to developing the temperature scale named for him,made measurements of the length of the arc of a median that verified Newton’s theory of the flattering of the earth at the poles.On Celsius scale,the interval between lower and upper fixed points is divided into 100 equal parts.The lower fixed point is marked as O °C and the upper fixed point is marked as 100 °C.It means on Celsius scale the freezing point of water is O °C and boiling point of water is 100 °C.

• ### The Fahrenheit scale:

The Fahrenheit scale ,used in the United States,employs a smaller degree than the Celsius scale,and its zero is set to a different temperature.Daniel Fahrenheit (1686-1744) ,a contemporary of Celsius ,was a German physicist who invented both the alcohol and mercury liquid thermometers and used them to study the boiling and freezing points of liquids.On this scale freezing point of water is 32 degree and boiling point of water is 212 degree. Fahrenheit scale is divided into 180 equal parts.

• ### Kelvin scale:

in SI units,the unit of temperature is kelvin(k) and its scale is called kelvin scale of temperature .the interval between the lower and upper fixed points is divided into 100 equal parts.Thus,a change in 1°c is equal to charge of 1 K. The lower fixed point on this scale corresponds to 273 Kn and the upper fixed point is referred o 373 K. The zero on this scale is called the absolute zero and is equal to _273°C. At the absolute 0 all thermometric properties are zero. There would be no kinetic energy.

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