# Heat And Temperature

Ice is cold while burning charcoal is cold. These sensations are produced in us because of heat.
Heat is a form of energy that produces the sensation of warmth in us.
And,
The degree of hotness and coldness of a body is called its temperature.
When different bodies are heated, different physical changes like expansion, contraction, change of state, etc. are observed in them. Heat causes temperature and temperature is the effect of heat.

## Molecular Theory of Heat

​All the substances are composed of large number of molecules. Above absolute zero temperature i.e. $0°K$, these molecules are in random motion. So, they possess kinetic energy. According to the molecular theory, The sum total of kinetic energy of all the molecules of a body is equal to the amount of heat contained by the body. and, The average kinetic energy of all the molecules is equal to the temperature of the body.

Heat is a form of energy. So, its SI unit is Joule and its CGS unit is calorie. A calorimeter is used to measure heat. The SI unit of temperature is Kelvin $(°K)$ and its CGS unit is Celsius $(°C)$. A thermometer is used to measure temperature.

#### Q. What is the temperature of vacuum?

According to the molecular theory, the temperature of a body is the average kinetic energy of all the molecules of the body. Since, vacuum does not contain any molecule, so the average kinetic energy is zero. So, the temperature of vacuum is zero.
​But, According to the theory of radiation, No space in universe is completely free from radiation because it does not need medium to transfer. So, vacuum must have certain temperature which depends upon the energy density of the radiation. So, the temperature of vacuum is indeterminate.

## Flow of Heat

Heat fows from a body at a higher temperature to another body at lower temperature. When a body is heated, the amplitude and the frequency of the vibration of the molecules increases i.e. kinetic energy increases. The hot body is more energetic than the cold body. When the hot body is in contact with the cold body, the energetic molecules of the hot body give up some of kinetic energy to the less energetic molecules of the cold body. So, heat flows from the hot body to cold body until the thermal equilibrium is attained.

Under thermal equilibrium,

Heat loss by the hot body $=$ Heat gained by the cold body

This is the principle of calorimetry.

## Scales of Temperature

​This scale of temperature is also known as Celsius scale. It was introduced by Swedish astronomer Anders Celsius. This scale is used worldwide. Anders Celsius divided the temperature interval between the melting point of ice and the boiling point of water into $100$ equal parts. Each part is called degree Centigrade or degree Celsius $(°C)$. The melting point of ice is denoted as $0°C$ and the boiling point of water is denoted as $100°C$.

### Fahrenheit Scale

​This scale of temperature was introduced by German physicist Daniel Gabriel Fahrenheit. On this scale, the temperature interval between the melting point of ice and the boiling point of water is divided into $180$ equal parts. Each part is called degree Fahrenheit $(°F)$. The melting point of ice is denoted as $32°F$ and the boiling point of water as $212°F$. The lower fixed point, $0°F$, was established as the freezing temperature of brine (solution of equal parts of ice, water and NaCl).

### Réaumur Scale

​The Réaumur scale, also known as Octogesimal division, is named after René Antoine Ferchault de Réaumur. On this scale, the temperature interval between the melting point of ice and the boiling point of water is divided into $80$ equal parts. Each part is called degree Réaumur $(°R)$. The melting point of ice is denoted as $0°R$ and the boiling point of water as $80°R$.

### Absolute Scale

​This scale of temperature was introduced by Lord Kelvin and is also known as Kelvin’s scale of temperature or thermodynamic temperature. Its unit is degree Kelvin $(°K)$. The size of degree of this scale is same as the Centigrade scale. But the zero of this scale is $-273.15°C$. $-273.15°C$ is the temperature at which the velocity and the kinetic energy of the molecules of a system is negligible or zero. So, it is the lowest possible temperature. This temperature is known as absolute zero and is denoted as $0°K$. $K=C+273.15$ where, $K$ is temperature in Kelvin and $C$ is temperature in Celsius. In absolute scale, melting point of ice is $273.15°K$ and boiling point of water is $373.15°K$.

### Relation Between Temperature Scales

​All the temperature scales have two fixed points and different temperature intervals. If $T$ be the temperature of a body, $LFP$ be the lower fixed point, $UFP$ be the upper fixed point in any scale, then, $\frac{T-LFP}{UFP-LFP}=\text{constant}$ If $C$, $F$, $R$ and $K$ be the temperature of the body in Celsius, Fahrenheit, Réaumur and Kelvin scale respectively, then, $\frac{C-0}{100-0}=\frac{F-32}{212-32}=\frac{R-0}{80-0}=\frac{K-273.15}{373.15-273.15}$ $\frac{C}{100}=\frac{F-32}{180}=\frac{R}{80}=\frac{K-273.15}{100}$ $\frac{C}{5}=\frac{F-32}{9}=\frac{R}{4}=\frac{K-273.15}{5}$