Physics HEAT

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HEAT

Heat : Energy in Transit

Heat is a form of energy which is transferred between a system and its surrounding as a result of temperature difference only. Once it is
transferred, it becomes the internal energy of the receiving body. When we say that a body is heated, it means its molecules begin to move with greater kinetic energy.

Heat is meaningful only as long as the energy is being transferred. Expression like "heat in a body " or "heat of a body" is meaningless.

`S.I` unit of heat energy is Joule `(J)`. Another common unit of heat energy is calorie (cal).

`1` calorie `= 4.18 J`

`1` calorie: the amount of heat needed to increase the temperature of `1` gm of water from `14.5^0` to `15.5^0C` at one atmospheric pressure is `1` calorie.

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`, then we can write,

`W=JH`

Where, `J` is called mechanical equivalent of heat. ` J ` is expressed in joule/calorie.