Page 306 - General Knowledge

GENERAL KNOWLEDGE 2019

 When there are more than two factors which determine a value to compare only two of them,
we must make the third factor a common standard value.
 To compare how different materials react to the same amount of heat, i.e how two of the
above factors depend on each other we must make the third factor common, i.e. all materials
must have equal mass and we take unit mass or 1 kg as the standard value.

Heat Flow
 When heat flows from one material to another, the temperature of the contact layer of the
cooler material increases.
 From the contact layer thermal energy has spread throughout the cooler material by
conduction or convection.
 The temperature in the contact layer increases depends on the specific heat capacity of the
material.
 The specific heat capacity c is the amount of energy it takes to raise the temperature of one
kg of material by 1 degree Kelvin or Celsius.

o
 Specific heat capacities: (kcal/(kg C))
 The specific heat capacity of water is approximately 4 times higher than that of air. The
exact specific heat capacity of a substance depends on the condition under which it is
measured.
 For gases, the specific heat capacity measured at constant volume is different from the
specific heat capacity measured at constant pressure.

Specific Heat
 The amount of heat required to raise the temperature of unit mass the substance through
1°C is called its specific heat.
 It is denoted by c or s. Its SI unit is joule/kilogram-°C'(J/kg-°C).
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-1
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 Its dimensions are [L2T -2 θ -1]. The specific heat of water is 4200 J kg °C or 1 cal g C -
1
, which high compared with most other substances.
 Gases have two types of specific heat
 The specific heat capacity at constant volume (C v)
 The specific heat capacity at constant pressure (C r)

 Specific heat at constant pressure (C p) is greater than specific heat constant volume (C V),
i.e., C p > C v .
 For molar specific heats Cp – CV = R where R = gas constant and this relation is called
Mayer‘s formula.
 The ratio of two principal specific heats of a gas is represented by

 The value of y depends on atomicity of the gas.
 Amount of heat energy required to change the temperature of any substance is given by,

where, m = mass of the substance

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