Heat: Introduction and Classification, About Heat, Example of Heat (For CBSE, ICSE, IAS, NET, NRA 2022)

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Heat: Introduction and Classification

  • It is energy that is transferred from one body to another as the result of a difference in temperature.
  • If two bodies at different temperatures are brought together, energy is transferred, that is heat flows-from the hotter body to the colder.
  • The effect of this transfer of energy is an increase in the temperature of the colder body and a decrease in the temperature of the hotter body.
  • A substance may absorb heat without an increase in temperature by changing from one physical state (or phase) as shown below,
    • As from a solid to a liquid (melting) ,
    • From a solid to a vapour (sublimation) ,
    • From a liquid to a vapour (boiling) ,
    • From one solid form to another (usually called a crystalline transition) .
  • The important distinction between heat and temperature was clarified during the 18th and 19th centuries.
  • The heat energy can be used for cooking, ironing, transportation, recreation etc. This form of energy also plays a vital role in nature.

About Heat

  • After an increase in temperature of a body, the vibrations of molecules or atoms increase.
  • These vibrations are then transferred from one part of the body to another.
  • Measuring of an energy with which the molecules vibrate in a system is termed as heat stored in that object.
  • It flows from a body at a higher temperature to a body at a lower temperature. The reason that the molecules vibrating faster transfer their energy to the molecules vibrating slower.
  • The vibration energy is also termed as its heat content. The heat content in the body makes it hot or cold. Greater the heat content, hotter will be the body.
  • It is a form of energy can be converted to other forms of energy.
  • For example, in motorized vehicles, heat is converted to mechanical energy. In electric bulbs, it is converted to light energy. In thermal power plants, it is finally converted to electrical energy.

Example of Heat

  • Let՚s take one example, let us take three beakers, first with cold water, the second with hot and the third with water at normal temperature. If we dip our finger in beaker 1 and then in beaker 3, we will observe that water in beaker 3 is hot as compared to that in beaker 1.
  • Whereas, if we dip our fingers in beaker 2 and then in beaker 3, we will observe that the water in beaker 3 is colder as compared to that in beaker 2.
  • This shows that we cannot rely on our sense of touch to judge the hotness or coldness of something and that՚s why the degree of hotness or coldness of any object is measured in terms of temperature.

Heat Transfer

Heat Transfer

Heat transfer occurs by three mechanisms:

  • Conduction is the transfer of heat between atoms and molecules in direct contact
  • Convection is the transfer of heat by movement of the heated substance itself, such as by currents in a fluid
  • Radiation is the transfer of heat by way of electromagnetic waves

Conduction

  • Conduction is heating a pot of water on an electric stove. The bottom of the pan is in contact with the hot stove top.
  • Heat flows from the burner to the bottom of the pan and even up the sides and possibly to the handle.
  • Conduction also occurs between the pan and the water, which are also in contact with each other.

Convection

  • Convection is a forced-air heating system.
  • Warm air is blown and mixed with cooler air to cause heating of the cooler air by the warmer air.

Radiation

  • heat transfer by radiation example is the sun or a hot fire.
  • The sun radiates electromagnetic waves that heat the earth as does a fire that heats your hands or body when you move nearby.
  • Combinations of these mechanisms are also used.

Classification of Heat

  • Hot
  • Cold

Hot

  • The Objects with high heat content are defined as hot or Hotness or coolness of an object is a relative term is always measured with respect to a reference object.
  • Hot objects Examples around us include the sun, fire, hot pans, air from a hairdryer, lava from volcanic eruptions, etc.

Cold

  • The Objects with lower heat content are defined as cold objects.
  • Cold objects Examples around us include ice, air from an air conditioner, cold drinks metal vessels kept in open in cold winter days etc.

Sources of Heat

Many sources of heat but following are the main sources of heat

  • Sun
  • Chemical
  • Electrical
  • Nuclear

Difference between Heat and Temperature

Difference between Heat and Temperature
ParameterHeatTemperature
DefinitionHeat is defined as the total energy of an object that has molecular motion inside itTemperature is defined as the measure of the thermal energy of an object
SI UnitJouleKelvin
SymbolQT

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