Basic concepts of mechanical engineering Part 3 ( temperature, units of heat, specific heat capacity)

In the last post, we have seen about work, power, energy, and heat click here to read that.
In this part 3, we will see these things down below

8. Temperature
9. Units of Heat
10. Specific Heat capacity

8. Temperature

   One is well familiar with the qualitative statement of the state of a system such as cold, hot, too cold, too hot etc. based on the day to day experience. The degree of hotness or coldness is relative to the state of the observer. The temperature of a body is proportional to the stored molecular energy i.e. the average molecular kinetic energy of the molecules in a system.

Definition:

qualitative indication f the relative hotness can be exactly defined by using thermodynamic property known as temperature.

Unit of temperature:

In the International system(SI) of a unit, the unit of thermodynamic temperature is Kelvin. It is denoted by the symbol K.  However, for practical purposes, the Celsius scale is used for measuring temperature. It is denoted by degree Celsius(°C)

Absolute zero temperature:

It has been found that a gas will not occupy any volume at a certain temperature, This temperature is known as absolute zero temperature. This is the lowest temperature that can be measured by a gas thermometer.

Temperature scale:

A look at the history shows that for quantitative estimation of temperature a German instrument maker Mr. Gabriel Danniel Fahrenheit (1686-1736) came up with an idea of the instrument like a thermometer and developed mercury in glass thermometer. In the year 1742, a Swedish astronomer Mr. Anders Celsiys described a scale for temperature measurement. This scale, later on, became very popular and is known as Centigrade Scale or Celsius scale. Some standard reference points used for international practical Temperature scale is given in the table.

Sr no.	State	°C	K
1	Ice point	0	273.15
2	Steam point	100	373.15
3	Triple point of water	0.010	273.16
4	Absolute zero	-273.15	0

9. Units of Heat

Heat is a form of energy.In SI system, unit of heat is taken as Joule. Kilojoule(kJ) and Megajoule (MJ) are recommended larger units of heat.

Calorie(cal.) is also unit of heat. Generally, Kilocalorie(kcal) is a quantity of heat required to raise a temperature of unit mass of water by one degree Celsius or Kelvin. 

1 kcal = 4186.8 joules = 4.1868 kilojoules

10. Specific heat capacity

specific heat capacity is also known as specific heat. The specific heat capacity of a substance may be defined as the quantity of heat required to raise the temperature of unit mass of the substance by one degree.

The unit of specific heat is J/kg°C. Ths unit is small, so kJ/kg-K or kJ/kg°C are recommended larger units.

Mathematically, the heat transfer rate Q is written as 

              Q = m×C×∆T

           ∴  C = Q/(m×∆T)

where, C = specific heat in kJ/kg⋅K, m = mass of substance in kg, ∆T = temperature difference in K.

• The product of mass and specific heat (m.C.P s called the Heat capacity of the substance.
• specific heat is a function of temperature, hence it is not constant but varies with temperature. Generally, it is assumed that it is constant.  

Specific heats in thermodynamics:

The solids and liquids have only one value of specific heat but a gas is considered to have two distinct values of specific heat capacity.

i) a value when the gas is heated at constant volume, C_v

ii) a value when the gas is heated at constant pressure C_p

the specific heat at constant volume C_v may be defined as the heat required to increase the temperature of the unit mass of a substance by one degree as the volume is maintained constant.

the same way one can define the specific heat at constant pressure(C_p), here pressure is maintained constant.