Heat transfer

By
Aman Rai
-
Specific heat

Heat and work are the forms of energy , in the previous post we had discussed about the work transfer.

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The form of energy which flows due to the temperature difference is heat and they phenomenon of its flow is heat transfer .

Heat transfer is :

  • Path function
  • Boundary phenomenon
  • Transient phenomenon
  • Not a property of a system
  • Inexact differential
  • Cyclic integral is non zero

Heat transfer is always written in form :

∆Q ( ✓)

dQ (×), bcoz heat transfer is Inexact differential.

Heat Capacity : ( C )

The amount of heat required to raise the temperature of system by unit degree is heat capacity and it is denoted by C .

C = ∆ Q / ( dT)

∆ Q= C × dT

OR

∆ Q = CdT

C is a coefficient called heat capacity it’s magnitude depends upon:

  • Size of the system
  • State of the system
  • Composition of the system

Unit of C = KJ/ kelvin.

Specific heat : (c)

It is amount of required to rise the temperature of unit mass of substance by one unit .

It is denoted by c ( small alphabet).

Unit of c = KJ/ (Kg – kelvin)

c= ∆ Q / ( M .dT)

∆ Q= M c × dT

∆ Q = McdT

Here ,

∆Q = amount of heat .

M = mass of substance

c = specific heat

C = heat capacity

dT = change in temperature

As we know that the amount of heat is path function , therefore specific heat is also a path function and therefore depend on the constant volume and constant pressure conditions , that helps in defining the path of the system.

Therefore ,

There are two types of specific heat :

(a) cp = specific heat at constant pressure

(b) cv = specific heat at constant volume

These two specific heats have a relationship which is called Meyer’s Relationship , and it valid only for :

(a) unit mass

(b) ideal gas

cp– cv = R

For Heat capacity Meyer’s Relationship :

CP – CV = R

Here

Cp = heat capacity at constant pressure

Cv = heat capacity at constant volume

And from the Meyer’s Relationship it is clear that the

Cp >Cv

Ratio of Cp / Cv :

The ratio of cp / cv is called specific heat ratio and is denoted by ¥ ( gamma)

¥ = specific heat ratio

Gas¥
Triatomic1.33
Diatomic1.4
Monoatomic1.67

Sensible heat –

The amount of heat required by any thermodynamic system to change the temperature of the substance is Sensible heat .

Latent heat –

The amount of heat required by the system or a body to change its phase without any interference in temperature is Latent heat .

Therefore , the specific heat of phase change is infinity.

Because , dT = 0 ;

Therefore , c ~ infinity.

For example :

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