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Sunday, 25 December 2016

DIFFERENCE BETWEEN SPECIFIC HEAT AND LATENT HEAT

We were discussing various forms of work in thermodynamics such as displacement work or PdV work, paddle wheel work, flow work and shaft work in thermodynamics in our previous posts.
Today we will see here the basic difference between specific heat and latent heat. First let us see here the detailed explanation of specific heat of a substance and after that we will see here latent heat with the help of this post.

Specific heat

Specific heat of a substance is basically defined as the quantity of heat which is required to raise the temperature of the unit mass of substance by unit degree. Specific heat of a substance will be indicated by c and we must note it here that word c will be written in small letter for indicating the specific heat.

Let mass of substance (m) is 1 kg i.e. unit mass of substance and increase in temperature (∆T) is one degree Kelvin. Therefore c = Q or we may say that heat required to increase the temperature of one kg of substance by one degree Kelvin will be termed as specific heat of the substance.

Product of mass of substance and specific heat of substance will be termed as heat capacity and heat capacity of substance will be indicated by capital letter C.

Heat capacity of substance C = m. c
Heat capacity of substance C = Q / ∆T

Let us see here the unit of specific heat

Unit of specific heat will be J/Kg. K and it could be easily determined by the above formula, while heat capacity will be measured in J/ K.

Types of specific heat

There are basically two types of specific heat i.e. specific heat at constant volume and specific heat at constant pressure

Let us see here first specific heat at constant volume (cv)

Specific heat at constant volume of a substance is basically defined as the quantity of heat which is required at constant volume to raise the temperature of the unit mass of substance by unit degree. Specific heat at constant volume of a substance will be indicated by c v.

Specific heat at constant pressure (cp)

Specific heat at constant pressure of a substance is basically defined as the quantity of heat which is required at constant pressure to raise the temperature of the unit mass of substance by unit degree. Specific heat at constant pressure of a substance will be indicated by c p.

Latent heat (L)

Latent heat is basically defined as the amount of heat energy required for changing the phase of a unit mass of a substance at a given pressure and temperature. Latent heat is shown by L and it unit is J/Kg in S.I unit system.

Types of Latent heat

As we know that there are basically three phases of substance or matter and substance or matter can exist in these three phases i.e. solid, liquid and gas or vapour. Hence, there will be basically three types of latent heat as mentioned here.

Latent heat of fusion (Lf)

Latent heat of fusion is basically defined as the amount of heat energy which is required to melt the unit mass of solid in to liquid or to freeze the unit mass of liquid to solid at given temperature and pressure.

Latent heat of vaporization (LV)

Latent heat of vaporization is basically defined as the amount of heat energy which is required to vaporize the unit mass of liquid in to vapour or to condense the unit mass of vapour to liquid at given temperature and pressure.

Latent heat of sublimation (Ls)

Latent heat of sublimation is basically defined as the amount of heat energy which is required to convert the unit mass of solid in to vapour or amount of heat energy which is required to convert the unit mass of vapour to solid at given temperature and pressure.

We must note it here that latent heat of fusion will not be much affected due to pressure but latent heat of vaporization will be quite sensitive to pressure.

Reference:

Engineering thermodynamics, By P. K. Nag
Engineering thermodynamics, By S. K. Som
We will see another important topic i.e. Stirling cycle and Ericsson cycle in the category of thermal engineering

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