## Physics XI Content

# Specific Heat – Method of Cooling

SPECIFIC HEAT | ||

“Amount of heat energy required to raise the temperature of unit mass of a substanceby one Kelvin or 1° C is known as specific heat of the substance.” | ||

MATHEMATICAL EXPRESSION | ||

If D Q is the amount of heat required to raise the temperature of “m” kg of the substance through ‘D T Kelvin , Then | ||

D Q µ m …….(i)D Q µ D T ……..(ii) or D Q = m c D T | ||

where ‘C’ is the constant called specific heat of substance. | ||

OR | ||

UNIT OF SPECIFIC HEAT | ||

Unit of specific heat is J/kg K in S.I. system | ||

Experiments show that specific heat of a particular material varies with temperature. Specific heat is not a precious concept for calculation because numbers of molecules per unit mass changes from material to material. | ||

DETERMINATION OF SPECIFIC HEAT | ||

Specific heat of a solid substance can be determined by the “Method of Mixture” using the concept of the “law of Heat Exchange” i.e. | ||

heat lost by hot body = heat gained by cold body | ||

The method of mixture based on the fact that when a hot substance is mixed with a cold substance, the hot body loses heat and the cold body absorbs heat until thermal equilibrium is attained. At equilibrium, final temperature of mixture is measured. The specific heat of the substance is calculated with the help of the law of heat exchange. | ||

Let | ||

Mass of substance = m _{s} kgMass of liquid = m _{l} kgMass of calorie meter = m _{c} kgInitial temperature of the substance = T _{s} KInitial temperature of the liquid = T _{l} KInitial temperature of the calorie meter = T _{c} KSpecific heat of substance = C _{s} = ?Specific heat of liquid = C _{l}Specific heat of the material of the calorie meter = C _{c}Final temperature of the mixture = T K | ||

According to the law of heat exchange | ||

Q_{LOST BY SUBSTANCE }= Q_{GAINED BY LIQUID} + Q_{GAINED BY CALORIE METER} | ||

M_{s }c_{s} (T_{s} – T) = m_{l }c_{l} (T – T_{l}) + m_{c} c_{c} (T – T_{c}) | ||

Which is the required value of specific heat of solid in J/ kg K. |