In winters we use heaters to make the room warm, we use toasters to toast the bread, we put clothes in sun to dry them off, we use geysers to get hot water, we use microwaves to bake the cakes, we use press to iron the shirts, we use the stove to cook the food. What makes these tasks possible? It is the heat of the system that makes these things possible. Heat is the outcome of transfer of thermal energy from one object to another in the system and branch of physics which talks about thermal energy is called thermodynamics.
Energy is defined as the capacity of a body to do work. Thermal energy is a kind of energy that is formed by the movement of particles of the object. Thermal energy is a form of kinetic energy because it is formed by motion of particles.
Thermal energy or heat is responsible for the temperature of the body. This temperature can be measured in terms of Celsius or Fahrenheit. The device that is used to measure the temperature for household purposes is called thermometer. The temperature of the object or system is directly proportional to the speed of movement of particles of the object. It means, faster the speed, the higher the temperature and vice-versa.
CONVERSION TO OTHER FORMS:
Thermal energy can be converted to other forms of energy without any heat loss. This fact is exploited for the use of mankind. For example, in toasters, microwaves, stoves, thermal energy is converted to food energy. When a pot of water is placed on the stove, what happens? The thermal energy of stove makes the molecules of pot and subsequently the particles of water to move faster. As a result, internal energy of the heated element is passed to the pot and then to water. This transfer of thermal energy from on medium to another is called heat. Therefore, heat is transfer of thermal energy from medium to another. One should not confuse that heat and thermal energy are same. They are not same. If the internal energy continues to rise, the water reaches its boiling point and starts evaporating. This is because temperature of water rises. In big plants, thermal energy is converted to electrical energy for consumption in households, factories. Geothermal is the thermal energy of Earth. It is also being converted to electricity these days.