Introduction to Heat and its Effect - Heat - CBSE Class 7 Science

Definition of Heat:
Heat is a form of energy that is transferred from one object to another due to temperature difference. It is the energy that causes objects to become hot and is measured in units of joules (J) or calories (cal).

How is Heat Transferred?
Heat can be transferred through three different processes:

1. Conduction:
Conduction is the process of heat transfer through direct contact between particles of matter. In solids, the particles are closely packed, and when one particle gains heat energy, it vibrates and passes the energy to the neighboring particles. This continues until the heat is transferred throughout the object.

2. Convection:
Convection is the process of heat transfer through the movement of fluids such as liquids and gases. When a fluid is heated, it becomes less dense and rises, while the cooler fluid sinks. This creates a circular motion known as convection currents, which transfer heat from one place to another.

3. Radiation:
Radiation is the process of heat transfer through electromagnetic waves. It does not require any medium for transfer and can occur even in a vacuum. The Sun's heat reaching the Earth is an example of radiation.

Effects of Heat:
Heat has several effects on objects and substances. Some of the major effects include:

1. Expansion:
When an object is heated, its particles gain energy and move faster, causing the object to expand. This property is utilized in various applications, such as the functioning of thermometers and bimetallic strips.

2. Change in State:
Heat can cause substances to change their state. For example, when ice is heated, it melts and changes into water. Similarly, when water is heated, it boils and changes into steam. These changes in state occur at specific temperatures known as melting point and boiling point.

3. Chemical Reactions:
Heat can also initiate or speed up chemical reactions. Many chemical reactions require a certain amount of heat energy to occur. For instance, combustion is a chemical reaction that releases heat energy.

4. Transfer of Energy:
Heat is a form of energy, and it can be converted into other forms of energy. For example, when heat is supplied to a steam engine, it is converted into mechanical energy, which is used to do work.

In conclusion, heat is a form of energy that can be transferred through conduction, convection, and radiation. It has various effects on objects, including expansion, change in state, initiation of chemical reactions, and transfer of energy. Understanding the principles of heat is essential for comprehending a wide range of natural phenomena and technological applications.