Wednesday, 27 July 2016

WHAT IS THE SPECIFIC HEAT OF A SUBSTANCE?

WHAT IS THE SPECIFIC HEAT OF A SUBSTANCE?

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. We have also seen the concept of enthalpy in our previous post.

Today we will focus here over a very important topic of thermodynamics and that is specific heat. So let us see what is specific heat?

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.
Q = m c ∆T
c = Q / (m ∆T)
Where,
Required heat (Q) will be measured in joules
Mass of substance (m) will be measured in Kg
Increase in temperature (∆T) will be measured in K

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.
We can also express specific heat at constant volume of a system as the partial differential of specific internal energy with respect to temperature at constant volume and will be formulated as displayed in following figure.
Specific heat at constant volume of a substance

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.

We can also express specific heat of a system at constant pressure as the partial differential of specific enthalpy with respect to temperature at constant pressure and will be formulated as displayed in following figure.
Specific heat at constant pressure of a substance
We will come again with new topic i.e."Control volume system" in our next post in area of thermodynamics.
Do you have suggestions? Please write in comment box.

Reference:

Engineering thermodynamics by P. K. Nag
Basic thermodynamics by Prof. S.K. Som
Image courtesy: Google

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