Heat is a very important source of energy. We use heat for different purposes in our daily life. Heat is the total kinetic energy of molecules that provides us with the sensation of warmth. Heat can flow from hotter to colder bodies. 

Heat as energy

Heat energy is the result of the movement of tiny particles called atoms, molecules or ions in solids, liquids and gases. Heat energy can be transferred from one energy to another. The transfer or flow due to the difference in temperature in two objects is called heat.

For example, an ice cube has heat in it and so does a glass of lemonade. If you put the ice in the lemonade, the lemonade (which is warmer) will transfer some heat to the ice. In other words, it will heat up the ice. Eventually, the ice will melt and the lemonade and the water from the ice will have the same temperature. This is known as reaching the state of thermal equilibrium.

The total amount of heat possessed by a body i.e. internal energy depends on:

1. The number of molecules of the body i.e. mass of the body. It means the larger the number of molecules, the higher is the value of internal energy of the body.


2. The average kinetic energy of the molecules( temperature of the body).                                     ∴ Amount of heat ∝ No. of molecules of the body * Average kinetic energy of molecules     i.e. Amount of heat ∝ Mass of the body* temperature

The nature of molecules is different for different bodies having the same mass are heated equally, their average kinetic energy will be different. That is, a molecule of a body vibrates about its mean position at a different speed from the molecule of another body at the same temperature.

Units of heat energy

Energy is measured in joule in the SI system. As heat is a form of energy, it is also measured in joule. In the CGS system, it is measured in calories. The amount of heat energy required to raise the temperature of 1g pure water by 1°C or 1K is one calorie heat. One calorie heat is equal to 4.2 joules of heat. Caloriemeter is used to measure heat.

Converting 4200 J into calrie i.e. SI system to CGS system.

∴ 4200 J= (4200/4.2) calories          (since, 4.2 J= 1 calorie)

or, 4200 J= 1000 calories

Thus, 4200 J and 1000 calories are equivalent.

1000 calorie is also called one kilocalorie.

One kilocalorie(kcal) heat is defined as that heat which is required to raise the temperature of 1kg pure water by 1°C or 1K.

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