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### Heat and Temperature in Physics

Heat in Physics:

Heat is a form of energy. The molecules of a substance are in perpetual motion. The mean kinetic energy per molecule of the substance is proportional to its absolute temperature. In other words absolute temperature is a measure of the mean kinetic energy per molecule of the substance. When a gas is heated at constant pressure, some heat energy goes to increase the potential energy of the molecules by moving them apart against force of attraction, thereby increasing the volume of the gas; some heat energy goes to increase the kinetic energy of the molecules thereby changing the temperature of the gas and the rest of the heat is used by the gas in doing mechanical work in expanding the gas against external pressure. The sum of the potential and kinetic energies of the molecules of a substance is called internal energy of the substance. Therefore heat is another form of energy.

Temperature in Physics:

When we speak of the temperature of a body we associate this concept with the degree of hotness of the body. The sense of touch is the simplest way of distinguishing hot bodies from cold bodies. However our sense of touch is often unreliable and misleading. Also the range of our temperature sense is limited.

When two bodies at different temperatures are brought into contact, heat energy will flow from the hot body to the cold body until both attain a common temperature. Thus temperature is that property of a body which tells us the direction in which heat energy is transmitted when the body is placed in contact with another.

Temperature scale

Temperature of a body is expressed as a number. To obtain the number it is necessary to construct a scale of temperature and an instrument on which such a scale is employed. Such an instrument is known as a thermometer. The scale chosen must be precise, consistent and accurate.
All temperature scales make use of some physical property which changes with temperature. Thus we have mercury thermometer, constant volume gas thermometer, constant pressure gas thermometer, resistance thermometer, thermoelectric thermometer etc.  Then an easily reproducible state of some standard system is chosen. The temperature corresponding to these states are, called fixed points and arbitrary numerical values are given to these fixed points.

There are three important thermometric scales commonly used.

1. Celsius scale:

This is generally used in scientific work. It has two fixed points. (i) Lower fixed point and  (ii) upper fixed point. The temperature corresponding to the melting point of ice at standand atmospheric pressure (ice point) is taken as the lower fixed point and it is taken as 0°C. The boiling point of water at standard pressure (steam point) is taken as the upper fixed point and is given a value 100°C. The interval between these two temperatures is divided into 100 equal parts. Each of these divisions is called one degree celsius (1°C).

2. Fahrenheit scale

In this scale the ice point is taken as 32°F and the steam point 212°F. The interval is divided into 180 equal parts. Each division is called one degree Fahrenheit (1° F).

3. Kelvin scale

A temperature scale where the property of a substance is not involved is the thermodynamic scale of temperature (Kelvin scale). For this, a single fixed point is used as reference temperature and that is the triple point of water. It is the temperature at which water, water vapour and ice can co-exist in thermal equilibrium. In the Celsius scale, the triple point of water occurs at 0.01°C. The temperature of the triple point of water on the thermodynamic scale is arbitrarily taken as 273.16 Kelvin (273.16 K). The kelvin is defined as the fraction 1/273.16 of the temperature of the triple point of water. The thermodynamic scale of temperature is found to agree with the absolute scale of temperature. The magnitude of a degree in the Kelvin scale and Celsius scale are equal.

In Kelvin scale ice point is approximately 273 K and steam point is 373 K. Any temperature,

t°C = (273 + t) K.

Note: The melting point of ice is not a constant; it varies with pressure. But triple point of water is unique; it does not depend on the thermodynamic parameter like pressure, volume, etc. It occurs only at one particular set of values of pressure and temperature. Due to these reasons, in modern thermometry, the triple point of water is taken as a fixed point instead of ice point.