 # Concept of Heat and Temperature|Zeroth Law of Thermodynamics

## Concept of Heat and Temperature

Familiar sensations of hotness and coldness are described with adjectives such as hot, warm, cold, cool etc. So, the quantity that indicates how warm or cold an object is called TEMPERATURE. However you must note here that these physical sensations of warmth and cold are not reliable because they depends on contrast as explained in this observation.
OBSERVATION :-
• Suppose if we place our right hand in hot water and left hand in cold water, if after a few minutes we place both the hands in water at room temperature then you will notice that to the right-hand water will appear to be cold and to left-hand, water will appear to be hot.
• From the above observation we can conclude that assessing the temperature of a body by mere sense of touch or comparing the degree of hotness of a body with respect to another body does not help in measuring the temperature quantitatively and accurately.
• Another observation about hot and cold bodies in contact is that, when they both are in contact, the temperature of cold body increases and that of the hot body decreases. This happens because energy is transferred from hot body to cold body when they are in contact and this is a non-mechanical process.

This energy which is transferred from one body to another without any Mechanical Work involved is known as HEAT. Heat is a form of energy and heat transfer from one body to another takes place by virtue of temperature difference only. Also heat transfer takes place from the body at higher temperature to a body at a lower temperature. So, we can also say that Heat is energy in transit. On microscopic level heat energy possessed by a body is due to the kinetic energy of the molecules that makes up the body. Heat energy can change from one form to another. S.I. unit of heat is Joule (J) and SI unit of temperature is Kelvin (K).

## 2. Zeroth Law of Thermodynamics

This fundamental law states that
If two bodies A and B are separately in thermal equilibrium with the third body C , then A and B are in thermal equilibrium with each other.
Now, Thermal equilibrium between two systems is the condition under which two substances in physical contact with each other exchange no heat energy.
OBSERVATION:-
1. Suppose there are two bodies A and B and a third body C ( assume it to be a thermometer).
2. Consider two bodies A and B , isolated from from each other.
3. Now we want to know whether A and B are in thermal equilibrium with each other.
4. For this we would put body A in contact with body C (thermometer) till they both are in thermal equilibrium .
5. At that stage reading of thermometer will become constant.
6. Thermometer is now put in thermal contact with body B.
7. We will wait till it is in thermal equilibrium with body B and note the temperature.
8. If two readings are same, then A and B are thermal equilibrium with each other.
The concept of thermal equilibrium or Zeroth law gives us the concept of temperature. All bodies which are in thermal equilibrium have a common property whose value is the same for all the bodies. We call this property as temperature. Thus , the temperature is that property of a body by which we know whether the body is in thermal equilibrium with other given body , or not.
Thus from above discussion, temperature of a system can be defined as the property that determines whether the body is in thermal equilibrium with the neighbouring systems. So, if a number of systems are in thermal equilibrium, then a common property of the system can be represented by a single numerical value called Temperature. So if two systems are not in thermal equilibrium then they are at different temperatures.