HEAT AS A FORM OF ENERGY IN TRANSIT

Many scientist have tried to explain the nature of heat.Up to the beginning of the nineteenth century, it was considered a weightless fluid called caloric which existed in every material body.Hot bodies were said to contain more caloric than the cold bodies.The caloric theory could explain satisfactory many processes such as heat conduction and mixing of substance in calorimeter.

This concept of heat fluid was challenged by Count Rumford.He observed while supervising the boring of cannon that as cannon barrels were being bored.A tremendous amount of heat was given off.According to the caloric theory since the metal chips cut off by the drill have lost caloric i.e the heat given off in the drilling process, the chips should not be the same as the original metal, which had not lost caloric.But Rumford was unable to find any difference between the chips and the original metal in respect to their ability to hold or give off heat.In order to further investigate it. Rumford made use of a very dull drill, which was unable to cut the metal.Heat was evolved in apparently unlimited quantity as long as the borer was rotated and the supply of heat was inexhaustible.He concluded that heat was due to the rotation of the borer and not from the metal itself.

As a result of this experiment and experiment performed by Joule, the scientists came to interpret heat not as the flow of substance but as transfer of energy when heat flows from a hot body to a cold one.It is energy that is being transferred from the hot to the cold object.Thus heat refers to energy that is transferred from one body to another because of difference in temperature.Heat is not the energy that a body contains in it.It refers to the amount of energy transferred from hot to a cold body.It is converted to the internal energy of the body.The internal energy is the sum of all the microscopic kinetic energy and potential energies of the molecules in the body.The S.I. unit of heat as for any form of energy is joule. 

TEMPERATURE

If we take two bodies,we can say by the sense of touch whether one is hotter than the other.It is not possible to determine,how much hotter ? it is at a particular time than the other .We find that it is not possible to determine the degree of hotness by sense of touch.The quantitative determination of the degree of hotness may be termed as temperature.Before describing the measurement of temperature it is appropriate here to define thermal equilibrium, when two bodies at different temperature are brought in thermal contact with each other .The heat starts flowing from the hot body to cold body till the temperature of the bodies become same.then they are said to be in equilibrium.

SCALES OF TEMPERATURE

It is not possible to determine the temperature of the body accurately by simple sense of touch or by comparing the degree of hotness so a temperature scale is need to measure the temperature quantitatively.For this we must have two reference points that are fixed and easily reproduce-able.That is , the value of the fixed points must always be the same under similar conditions.The scale depends upon these fixed points.

The melting point of ice and boiling point of water at standard pressure (76 cm of Hg) are taken to be the two fixed difference between these two points is divided in different ways called scales of graduation.

On the Celsius (centigrade) scale this interval between these fixed points is divided into hundred equal parts.The lower fixed point is marked 0 and upper fixed points is 100.Each part thus represent one degree Celsius (1 C). This scale was suggested by Celsius in 1742.  

On the Fahrenheit scale the lower fixed point is marked 32 and the upper fixed point is 212, and the interval between them is equally divided into 180 equal parts.Each part represent one degree in Fahrenheit.

There is another scale called the Kelvin scale.The lowest temperature on this scale is 273C.Thus the zero on the kelvin scale well be 273 on kelvin scale written as 273K and 100 on Celsius scale will 373K.The size of the degree on the kelvin scale is the same as that of Celsius scale.

In order to drive relationship between centigrade and Fahrenheit scales let the two thermometers be placed in a bath and the mercury in each thermometer rises to the same level.

THERMOMETRIC PROPERTIES

Property of the substance which changes uniformly with the change of temperature is named thermometric property.For example the volume of a liquid in a vessel, the volume of a fixed mass of a gas maintained at constant volume, electrical resistance of a metal are some of the many measurable physical properties which changes with change of temperature.

THERMAL EXPANSION

It is matter of daily observations that in general bodies undergo change in size on heating is termed Thermal expansion.Solids, when heated show increase in length,area or volume.Whereas liquids and gases expand in volume.The property of thermal expansion varies from substance to substance.When a solid is heated,its molecules vibrate more energetically against the action of inter molecular forces producing greater displacement.Since the average distances among the molecules increase, the size of solids of the solids increases.

LINEAR EXPANSION

Expansion in length of solids on heating is called linear expansion.The observed expansion in length depends upon the original length and the change in temperature.

It has been found experimentally that the change in length is directly proportional to the original length and the change in temperature of the solid.

Suppose L is the length of a uniform thin metallic rod at some initial temperature.When it is heated through a small temperature  ΔT,an increase of length ΔL takes place.

BIMETALLIC THERMOSTAT 

One of the examples of thermal expansion is of thermostat devices.Thermostat are well known devices commonly used for maintaining required temperatures.We describe below a common type of thermostat called bimetallic strip.The bimetallic strip works as an electric contact breaker in an electrical heating circuit.The circuit is broken when the desired temperature of the bath is reached.Due to the difference in the coefficients of linear expansion of the two metals,the metallic strip bends in the form of a curve and the circuit broken.In figure 11.2 (a) the metallic strip is in contact downwards as it becomes hot and contact at P is broken Thus the current stops flowing through the heating coil.When the temperature falls, the strip contracts and the contact at P is restored.The two metal strips are very well joined.

BIMETALLIC THERMOMETER 

Similarly a bimetallic strip can be used to make a thermometer.In this case the bimetallic strip often is in the form of a coil.Its one end is fixed and the other end is attached to a pointer.This kind of thermometer is usually ordinary air thermometer, over thermometer, and in automobiles for automatic choke.

GAS LAWS

It is known fact that gases have no fixed volumes or shape and their volume can altered by changing the pressure as well as the temperature.Any gas can be described with the help of four variables i.e. pressure,volume,mass and temperature.The relation between two any two variables is found experimentally while keeping the other constant.

BOYLE'S LAW

Let us consider the relation between the pressure and volume of a given mass at constant mass and temperature.It was found experimentally by Robert Boyle in 1660 that for a fixed mass of a gas at constant temperature, the product of pressure (P) Volume (V) is constant.This is known as Boyle's Law.

CHARLES'S LAW

When a given mass of a gas is heated at constant pressure experiments show that the volume V of a given mass of a gas is directly proportional to its absolute temperature T.

This law was investigated by Charles and is known as Charles law.On plotting a graph of volume of the gas against its temperature, a straight line is obtained.Its shows that equal changes its temperature lead to equal changes in volume at constant pressure.From the graph it is found that 0C the gas still processes a volume V o.When the straight line of the graph is extrapolated to lower and lower temperature axis.

THE PROPERTIES OF GASES: KINETIC INTERPRETATION 

The properties of matter in bulk can however be predicted on molecular basis by a theory known as kinetic theory.The first step in the construction of a theory is to set up some sort of model which is simple enough to be treated mathematically.The characteristics of the model are described by a set of fundamental assumptions which for the kinetic theory of gases are:

1. A gas consists of particles called molecules.Depending on the gas each molecule will consist of an atom or a group of atoms.All the molecules of a gas in a stable state are considered identical.
2. The molecules are separated by distance large as compared to their own dimensions.The diameter of a molecule, considered as a sphere, is about 3e-10.
3. Any finite volume of a gas consist of a very large number of these molecules.This assumptions justified by experiments.At standard conditions there are 3e+25
4. The molecules move in all directions and with various speed making elastic collision with one another and with the walls of the container.The walls of a container can be considered perfectly smooth.
5. Molecules exert no forces on one another except during collisions.Therefore in between collisions with other molecules or with the walls of the container, and in the absence of the external forces, they move freely in straight lines.
6. Newtonian mechanics is applicable to the motion of molecules.