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Heat

Heat is a form of energy which causes the sensation of hotness and coldness. When a body is heated, its energy increases and when it is cooled, its energy decrease.
SI unit – Joule (J) presently used standard unit
SI unit – Calorie (Cal) older unit of heat
1 kg = 1000 Joule, 1kcal = 1000 cal

Conservation

  • Heat Energy \(\rightarrow\) Mechanical Energy
    Steam engine pulls a train.
  • Heat Energy \(\rightarrow\) Sound and Light
    Cracker produce sound and light.
  • Heat Energy \(\rightarrow\) Light energy
    Charcoal heated emits light.

Effect of Heat

  • Kill harmful bacteria (milk).
  • Change state (ice).
  • Helps in cooking.
  • Expand on heating.
  • Accelerate chemical reactions.

Temperature
The degree of hotness or coldness of an object, on some chosen scale is called its temperature. Temperature is measured with a device called thermometer.
Three temperature scale

  • Celsius
    0°C – melting point
    100°C – boiling point
  • Fahrenheit
    32°F – melting point
    212°F – boiling point
  • Kelvin
    100°C = 180°F 

Most commonly used thermometer are mercury thermometers. Because of following:

  • Has uniform contraction and expansion.
  • Does hot stick to the wall of thermometer.
  • Only metal found in liquid state.
  • Silvery white liquid which can be seen very easily.
  • Sufficient law freezing point and high boiling point.

 

Laboratory

Chemical Thermometer

Size

Much longer

Shorter

Range

-10°C – 110°C

35°C – 42°C

Reading

Bulb remain in contact with the substance.

Read after removing from the body.

 

Transfer of Heat
Heat always flows from a hotter body to a colder body. There are three different processes by which heat transfer takes place.

  • Conduction
  • Convection
  • Radiation

Conduction
The process of transmission of heat energy inn solids without actual movements of particles from their position is called conduction.

  • Heat transfer will continue as long as these are a difference in temperature.
  • When the temperature becomes the same, the transfer of heat will stop.
  • Materials which allows heat allow heat to follow through them easily are called Good conductor of heat.
    Eg: All metals and their alloys.
  • Materials which do not allow heat to follow through them easily are called non-conductors or isolators.
    Eg: Wood, Rubber etc.

Convection
The movement of particle due to the temperature difference between different parts of the same substance is called convection.

Applications of Convection

  • Central Meeting of Building
  • Ventilation in Rooms
    Warm air rises up and leaves the room by ventilator. For this, fresh air enters through windows or doors. Convection set to keep room fresh.
  • Land and Sea Breeze
    During the day, sun shines equally on land and sea. Land faster hotter than sea. The hotter airoves land rises up then cooler air from the sea takes place. Convection current from the sea to the land to sea is called sea breeze.
    Similarly, convection current from the land to sea is called land breeze.

Radiation

  • It is made of heat transfer through vacuum.
  • The transfer of heat energy from a hot body to cold body directly, without heating the space in between the two bodies called radiation.
  • The heat energy transmitted by the process of radiation is called a radiant heat as thermal radiation.
  • Black bodies are better heat radiators than the white bodies.

Applications

  • White and light colored clothes all more suitable in summer.
  • The base of cooking utensils is painted black.
  • Dark colored clothes are more suitable in winter.

Thermos Flask
 A thermos flask is a very good example of how heat loss by all the three modes of heat transfer conduction, convection and radiation is minimized.


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