Temperature and Heat: Class 11 Physics Notes, Definition, Source & Comparision

Physics Thermal Properties of Matter 2025

Vikash Kumar Vishwakarma
Updated on May 16, 2025 10:56 IST

By Vikash Kumar Vishwakarma

Temperature and Heat are the most used terms in daily life. They are the fundamental concepts of thermodynamics and physical science. Students can understand how energy is transferred between two objects through this topic. Temperature and heat are fundamental for studying thermal expansion, change of state, heat transfer, Newton's law of cooling, calorimeter, and thermal expansion.

Also Read: Class 11 Physics NCERT Solutions | NCERT Solution for Class 11 Physics Chapter 10 Thermal Properties of Matter

Table of content
  • Temperature and Heat Definition
  • Understanding Temperature and Heat
  • Mechanical Equivalent of Heat
  • How does heat transfer?
  • Source of Heat
  • Comparision Between Heat and Temperature
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Temperature and Heat Definition

The NCERT definition of Temperature and Heat is as follows: 

  • Temperature: Temperature is a relative measure, or indication of hotness or coldness.  
  • Heat: The energy transferred between two (or more) systems or a system and its surroundings by virtue of a temperature difference

Read More: NCERT Solution

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Understanding Temperature and Heat

Temperature indicates how hot or cold a body is, based on the average kinetic energy of particles of that material. When two objects at different temperatures come in contact, thermal energy flows from the warmer object towards the cooler object. This energy transfer continues until both objects attain the same temperature, the so-called thermal equilibrium.

Resources: Class 12 Physics Solutions

The energy that is transmitted due to the temperature difference is called heat. In short, heat is energy in transit because of the temperature difference. When this energy becomes absorbed by a body, it transforms into that body’s internal energy. 

With regard to the term heat, it is confirmed that it applies only when energy is being transferred. It is, hence, wrong to say that a certain amount of heat is contained by a certain body, as once the transfer has taken place, the energy is no longer considered as heat; it is recognised as internal energy.

When an object is heated, it undergoes many changes such as a rise in temperature, expand or change the shape. 

                                                                                                         
 

When we say that a body is heated it means that its molecules begin to move with greater kinetic energy. S.I. unit of heat energy is joule (J). Another common unit of heat energy is calorie (cal).

 

1 calorie = 4.18 joules.

1 calorie :  The amount of heat needed to increase the temperature of 1 gm of water from 14.5 to 15.5 ºC at one atmospheric pressure is 1 calorie.

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Mechanical Equivalent of Heat

In early days heat was not recognized as a form of energy. Heat was supposed to be something needed to raise the temperature of a body or to change its phase. Calorie was defined as the unit of heat. A number of experiments were performed to show that the temperature may also be increased by doing mechanical work on the system.

These experiments established that heat is equivalent to mechanical energy and measured how much mechanical energy is equivalent to a calorie.

If mechanical work W produces the same temperature change as heat H, we write,   

W = JH

where J is called the mechanical equivalent of heat. J is expressed in joule/calorie. The value of J gives how many of joules of mechanical work needed to raise the temperature of 1 g of water by 1°C.

Example 1.       What is the change in potential energy (in calories) of a 10 kg mass after 10 m fall?

Solution :         Change in potential energy

                     

 

 

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How does heat transfer?

The transfer of heat can take place mainly in three ways: conduction, convection, and radiation 

Conduction:  

  • Heat transfer from one molecule to another substance 
  • Example: touching hot metal 

Convection:

  • Heat transfer through the movement of fluid 
  • Example: water boiling on gas 

Radiation

  • Heat transfer through electromagnetic waves 
  • Example: Fire, Sun
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Source of Heat

There are various sources of heat, however, the most efficient sources are 

  • Chemical: burning coal, wood, gasoline and natural gas 
  • Nuclear: Nuclear reactor 
  • Electrical: Electric heater, ovens, water heater, electric stove, joule heating 
  • Sun
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Comparision Between Heat and Temperature

Below is a comparison of heat and temperature on various aspects.

Heat 

Temperature 

Heat is an energy that is transferred from within two object  

Temperature is expressed as the degree of hot or cold 

SI unit is the Joule  

SI unit is Kelvin, Degree Celsius 

Denoted by the 'Q' symbol 

Denoted by the 'T' symbol 

qna

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