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Physics Theory : Heat

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Heat is an invisible energy which causes in us the sensation of hotness (when it flows into our body) or coldness (when it flows out of our body).
Heat can bring about a change in the temperature, state and dimensions of a given body.


The SI unit of thermal energy (heat) is Joule, which is abbreviated as J.
This SI unit is named after British physicist James Joule (1818-89) who discovered the first law of thermodynamics (the conservation of energy).
Two more units used for heat are erg and calorie.
1 Joule = 1 kg m2 s−2 = 107 g cm2 s−2 = 107 ergs.
1 calorie = 4.184 Joules.
1 calorie of heat is required to raise the temperature of 1 g of water by 1oC.

1 calorie of heat is lost when 1 g of water cools by 1oC.


Heat energy always flows from a (hot) body at a higher temperature to a (cold) body at a lower temperature, till both the bodies attain the same temperature.
The method of mixtures assumes that the heat lost by the hot body equals the heat gained by the cold body.
Heat transfer occurs by three possible modes : conduction, convection, and radiation.


Conduction is a mode where heat energy is transferred by molecules to neighboring molecules without the molecules actually moving from their positions.
Heat travels through solids by the process of conduction only.


Convection is a mode of heat transfer where molecules actually come to the heat source and then after absorbing heat energy move away from the source.
Convection occurs only in fluids (liquids and gases), where the particles can move freely.
An exception is a liquid metal like mercury which gets heated by conduction because it is a very good conductor of heat.


Radiation is a mode for the transfer of heat energy directly from a hot body to a cold body without heating the space between the two bodies.


Thermal expansion is the expansion produced in matter due to the absorption of heat energy.
All states of matter show thermal expansion.
Solids expand the least, liquids more and gases the most for the same rise in temperature.
Linear expansion is the expansion in the length of a solid.
Superficial expansion is the expansion in the area of a solid.
Cubical expansion is the expansion in the volume of a body.
Liquids and gases show only cubical expansion, but no linear and superficial expansion because they do not have a definite shape.
Different gases have the same rate of expansion for the same rise in temperature, as compared to solids and liquids which show different rates of expansion for the same rise in temperature.


Calorific value of a fuel is the amount of heat produced when a unit mass of it is burnt.
It is usually specified in kJ/kg (kilojoules/kilogram) or MJ/kg (megajoules/kilogram).


Physics Quiz on Heat.
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