One of the most common forms of energy is thermal energy. It is the energy due to which we feel hot or cold. If the energy flows into our body we feel hot and if it flows out of our body we feel cold. To prevent heat from flowing out of our body we wear woolen clothes during winter.
Thermal energy is also called heat. We receive heat directly from the sun along with light. The heat from the sun dries clothes, ripens crops and evaporates water from water bodies to cause rain.
We need heat to cook food, to light the fire, to run a thermal power station. Generally, we produce heat for all such purposes by burning a fuel or by passing electric current through a conductor.
Thermal energy is provided to water in a kettle when it is placed on fire. If we touch water in the kettle before we start heating it and then after some time of heating we find that the water becomes warmer. This degree of hotness or coldness of a body due to which we call it warmer is called Temperature. Heat and temperature are intimately related. Normally, more the heat given to a body higher will become its temperature.
When water is boiled in a kettle the steam built up in the kettle raises its lid up and when the steam escapes out the lid falls down. Thus, heat can do work, so, it is a form of energy. This property of steam was used to build steam engines - the devices which convert heat of steam into mechanical work.
The equivalence of work and heat was noticed and experimentally established by J. P. Joule. While boring the barrel of a gun with a blunt borer Joule found that so huge amount of heat was produced in the process that even water in which the process of boring was being carried out started boiling. Through further experiments he found that one Calorie (Unit of heat prevalent at that time) of heat is equivalent to 4.2 Joule of work.
Temperature is a quantity which tells us how hot a body is. If a hot body is kept in contact with a colder body for some time, the hotter body does not remain that hot and the colder body becomes some what hotter. Thus, heat is transferred from a hotter body (a body at higher temperature) to a colder body (i.e. a body at lower temperature).
Hence, temperature is the degree of hotness of a body which determines the direction of flow of heat. Heat always flows from a body at higher temperature to a body at lower temperature.