Specific Heat

The Specific Heat is the amount of heat required to change a unit mass of a substance by one degree in temperature

The Specific Heat is the amount of heat required to change a unit mass of a substance by one degree in temperature. The heat supplied to a unit mass can be expressed as

dQ = m c dt         (1)

where

dQ = heat supplied (kJ, Btu)

m = unit mass (kg, lb)

c = specific heat (kJ/kg oC, kJ/kg oK, Btu/lb oF)

dt = temperature change (K, oC, oF)

Expressing Specific Heat using (1)

c = dQ / m dt         (1b)

Converting between Common Units

  • 1 Btu/lbmoF = 4186.8 J/kg K = 1 kcal/kgoC

Example - Heating Aluminum

2 kg of aluminum is heated from 20 oC to 100 oCSpecific heat of aluminum is 0.91 kJ/kg0C and the heat required can be calculated as

dQ = (2 kg) (0.91 kJ/kg0C) ((100 oC) - (20 oC)) 

     = 145.6 (kJ)

Example - Heating Water

One litre of water is heated from 0 oC to boiling 100 oCSpecific heat of water is 4.19 kJ/kg0C and the heat required can be calculated as

dQ = (1 litre) (1 kg/litre) (4.19 kJ/kg0C) ((100 oC) - (0 oC)) 

     = 419 (kJ)

Specific Heat Gases

There are two definitions of Specific Heat for vapors and gases:

cp = (δh / δT)p - Specific Heat at constant pressure (kJ/kgoC)

cv = ( δh / δT)v - Specific Heat at constant volume (kJ/kgoC)

Gas Constant

 The gas constant can be expressed as

R = cp - cv         (2)

where

R = Gas Constant

Ratio of Specific Heat

The Ratio of Specific Heat is expressed

k = cp / cv         (3)

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