**Zeroth law of thermodynamics**

**Zeroth law of thermodynamics states that ****If objects A and B are separately in thermal equilibrium with a third object C **(the thermometer)**, then objects A and B are in thermal equilibrium with each other. **

**Zeroth law of thermodynamics introduces thermodynamic quantity called temperature**.

Two objects (or systems) are said to be in thermal equilibrium if their temperatures are the same.

In measuring the temperature of a body, it is important that the thermometer is in the thermal equilibrium with the body whose temperature is to be measured.

**Temperature**

- Temperature is a macroscopic physical quantity related to our sense of hot and cold.
- Temperature is basically a measure of the degree of hotness or coldness of a body.
- The natural flow of heat is from higher temperature to lower temperature, i.e. Temperature determines the thermal state of a body, whether it can give or receive heat.
- Two bodies are said to be in thermal equilibrium if and only if they are at same temperature. In this situation heat in the two bodies may or may not be equal.
- The temperature of a body is directly proportional to the kinetic energy of the random motion of the molecules or atoms of the substance.
- Temperature is one of seven fundamental quantities with dimensions (q). It is a scalar physical quantity with SI unit kelvin (K).
- The highest possible laboratory temperature is about $10^{8} \mathrm{~K}$ (in fusion test reactor) while the lowest $10^{-3} \mathrm{~K}$ (achieved in 1990 through nuclear spin colling).
- The theory has established that 0 K can never be achieved practically.

**Thermal equilibrium**

**Heat** is the transfer of energy from one object to another object as a result of a difference in temperature between them.

**Thermal equilibrium is a situation in which two objects in thermal contact cease to exchange energy by the process of heat.**

## Different types of temperature scales

(a) The Kelvin temperature scale is also known as thermodynamic scale.The Sl unit of temperature is the kelvin and is defined as $(1 / 273.16)$ of the temperature of the triple point of water. The triple point of water is that point on a P-T diagram where the three phases of water, the solid, the liquid and the gas, can coexist in equilibrium.

(b) In addition to Kelvin temperature scale, there are other temperature scales also like Celsius, Fahrenheit, Reaumer, Rankine, etc. Temperature on one scale can be converted into other scale by using the following identity :

$\frac{\text { Reading on any scale – lower fixed point (LFP) }}{\text { Upper fixed point (UFP) – lower fixed po int (LFP) }}$ = constant for all scales

Hence,

$\frac{t^{\circ} \mathrm{C}-0^{\circ}}{100^{\circ}-0^{\circ}}=\frac{t^{\circ} \mathrm{F}-32^{\circ}}{212^{\circ}-32^{\circ}}=\frac{\mathrm{tK}-273 \cdot 15}{373 \cdot 15-273.15}$

(c) Different temperature scales :

S.no Scale | Name of the Scale | Symbol for each degree | Lower Fixed Point (LFP) | Upper Fixed Point (UFP) | Number of Divisions on the Scale |

1 | Celsius | °C | 0°C | 100°C | 100 |

2 | Fahrenheit | °F | 32°F | 212°F | 180 |

3 | Kelvin | K | 273.15 K | 373.15 K | 100 |

For a better understanding of this chapter, please check the detailed notes of the Thermodynamics. If you want more Free Learning Videos and Study Material Then don’t forget to download the eSaral App.