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Chapter 1: Heat and Temperature-4 (in Hindi)
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In this discuss the changes of state.

Vishal Garg
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  4. Some common changes of states: (i) .Melting: When heat is supplied, solid substance changes into liquid, this change ofstate ofsubstance is called melting Heat Solid Liqui The temperature at which the solid and the liquid states ofa substance coexist in thermal equilibrium with each other is called its nelting point. Freezing: When heat is released, liquid changes into solid, this change of state of substance is called freezing (ii) Liquid _CooSolid (iii) Condensation When vapour is cooled, it changes into liquid, this change of state is called condensation Coo1 v apour liquid Evaporation Conversion of liquid into gaseous state at all the temperatures is called evaporation or boiling. Heat Liquid Vapour The temperature at which the liquid and vapour states of a substance coexsist in thermal equilibrium with each other is called its boiling point (V) Sublimation It is the conversion of a solid directly into vapours SolidHeat apour Sublimation takes place when boiling point is less than the melting point


  5. Heating Curve Ifto a given mass (m) ofa solid, heat is supplied at constant rate and a graph is plotted between temperature and time, the graph is as shown in fig. and is called heating curve. From this curve it is clear that: -b.pt. Boiling BAO Melting 2 Tim 3 4 egelation : The phenomenon in which ice-melts when pressure is increased and again removed is called regelation. freezes when pressure


  6. Heat Transfer Conduction Conduction is that mode of transmission ofheat in which heat is transferred from a region of higher temperature to a region of lower temperature by the aid of particles of the body without their actual migration. Conduction requires material medium Variable state and steady state: When one end ofa rod is heated, then initially the temperature of various points of the rod changes continuously and the rod is said to exist in a variable state After some time, a state is reached, temperature of each cross- section becomes steady. This state is kno state. If rod is in contact with atmosphere or surroundings and it is heated at one end then distance temperature curve is as shown in fig. alongside. when the wn as steady versuS Distance from hot end


  7. And if rod is insulated with surroundings then graph is straight line. Temperature gradient: The rate ofchange of temperature with distance between two isothermal surfaces is called temperature gradient. Ifthe temperature ofthe isothermal surfaces be and - and perpendicular distance between them is Ax then, Temperature gradient between them Change in temperature Distance between two surfaces Ax Ax The negative sign show that temperature decreases as the distance x increases in the direction of heat flow. Unit in SI System /metre