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Basic Science & Engineering INDIA RRB ALP & Technicians Recruitment 2018 for Railway ALP CBT2 Basic Science Heat & Temperature
HEAT Heat is a form of energy which causes sensation of hotness or coldness. It is transferred between two or more systems or a system and its surroundings by virtue of temperature difference. The flow of heat is always from higher temperature to lower temperature. No heat flows from one body to other, when both the bodies are at the same temperature. The two bodies are said to be in thermal equilibrium. The SI unit of heat is joule. Its CGS unit is calorie, 1 cal -4.2 joule
Specific heat Capacity Specific heat capacity: It is the amount of heat energy needed to raise the temperature of unit mass of substance by 1'C (or IK). It is denoted by s or If the temperature of a substance of mass m changes from T to T dT when it exchanges an amount of heat dQ with its surroundings, then its specific heat capacity is given by c. 1 dQ m: dT
Unit of specific heat capacity: CGS unit of specific heat capacity is cal/g C SI unit of specific heat capacity is joule/kg K C water-1 cal/s*C-l cal/g K I. kcal/kg K-4200 joule/kg K temperature of one gram of water from 14.5 C to 15.5 c to raise the temperature of the 1 gram substance by 1 C is known For example, the specific heat capacity of water is: Definition of one calorie : Amount of heat required to raise lS known as one calorie. Gram specific heat capacity"c ": The amount of heat required as "gram specific heat".
Molar specific heat capacity"C": The molar heat capacity C is the heat required to raise the temperature of 1 mole of a gas by 1 C (or 1 K, " Heat required to raise the temperature of gram mole of a substance by A0 C AQ nAT Its SI unit is Jmol-1 K-l and CGS unit cal mo-loC-1 Thermal or heat capacity : It is the quantity of heat required to raise the temperature of the whole of that substance througlh 1 C. The thermal capacity ofmass m of the whole of substance of specific heat capacity s is equal to (ms) kilocalorie i.e., Thermal capacity -mass X specific heat capacitv
Water equivalent of a body : The thermal capacity of a body (ms) represents also its water equivalent, since the specific heat of water is unitv The wuater equivalent of a body is the amount of water that absorbs or gives out the same amount of heat as is done by the body when heated or cooled through 1 C Mass of water having the same thermal capacity as the body is called the water equivalent of the body W= mass x specific heat capacity Its SI unit is Kg and CGS unit g.
Calorimetry The branch ofphysics that deals with the measurement of heat is called calorimetry. A device in which heat measurement can be done is called calorimeter. Principle of Calorimetry or Law of Mixture According to the principle of calorimetry, Heat lost = heat gained Heat lost or gained by a body is given by, o-mxCx(92-91) Principle of calorimetry represents the law of conservation of heat energy. Temperature of mixture (T) is always lower temperature (T1) (TH and S higher temperature TiSTSTH
Latent Heat or Hidden Heat When state of a substance changes, change of state takes place at constant temperature (m.pt. or b.pt.) heat is released or absorbed and is given by, -mL where L is latent heat. Heat is absorbed if solid converts into liquid (at m.pt.) or liquid converts into vapour (at b.pt.) and is released if liquid converts into solid or vapours converts into liquid.
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