Modes of Heat Transfer

Heat transfer occurs when one side of a metal bar is heated and the heat will be transferred from one face to another face by conduction when the air across the heat surface convection affects air and radiation takes place when the metal bar is placed in the vacuum. Convection, radiation and Conduction are three modes of heat transfer in physical mechanism and convection effects by the flow of fluids and radiation effects by metal surface.

“Thermal conductivity”

The “thermal conductivity” of a material is the amount of heat that passes normally in a unit of time through a plane of unit area of the material under a temperature gradient of unity flowing perpendicular to the plane of the material. Let, θ1 and θ2 be the temperatures of the near and far faces and x be the small separation with an area A and the amount of heat Q flowing from the hotter to cooler surface in a time t is proportional then, “(i) to the area Q ∝ A”, (ii) the temperature difference “Q ∝ (θ1– θ2)”, (iii) inversely separation “Q ∝ 1/x” and to the time “Q ∝ t”. Therefore, “Q ∝ {A (θ1– θ2) .t} / x” that is “Q= {KA (θ1– θ2).t}/x”, that is K is called Kappa and known as a “thermal conductivity” of the material. The “S.I unit of thermal conductivity” is “(W/ (m.k))” and the dimension the dimension is M¹L¹T-³θ-¹

Mode of Heat transfer

When transferred in the solid medium the conduction will occur, transferred via liquid or gasses convection will occur and heat transferred in electromagnetic waves then the radiation will start on the surface of the material. When the heat becomes energy that drives in molecular scale, if the molecule of the material vibrates at that position by the supply of heat, it transfers their energy to other molecules and allows them to vibrate. As a result, heat energy should be transferred from one face to another face when the temperature difference occurs in the material block and in the block two different temperatures are rising it makes a reason for the heat transfer.

 An example describes the whole theory of heat transfer and discusses the mode of heat transfer, which is when a block of ice can take in the water at room temperature; the water becomes a medium of heat transfer that helps to melt the ice.

Different types of mode of Heat 

There are three different types of modes of heat transfer that are convection, radiation and conduction.  Heat convention refers to the process of heat transfer of a fluid body where the fluid is moved and heated apart from the origin of heat transfer. “Q = A*I*ΔT. Here, Q is defined as the amount of heat flow passing through the unit time, A is defined as the area made by the fluid and ΔT is defined as a difference between the temperatures and I is known as energy transfer coefficient (convective heat transfer coefficient).

Heat radiation is the process of heat or energy transfer that starts by the movement of electrons in the material. Plank law, Kirchhoff law, Stefan-Boltzmann’s rule are three main laws related to the heat radiation. Radiation of heat transfer denotes mathematically, “Q=σ e A T4. Here, Q is defined as the energy or heat transfer by unit time, σ is Stefan boltzmann constant, “e” is emissivity of the body, and T is temperature of a body ,A heat transfer area .

If the temperature of any material body at any location x remains steady then there is no chance of heat emission at the unit time t, therefore the temperature difference between the two faces remains constant and gives the reason for the same amount of heat flow by each face at t. Consider, P1and P2be the amount of heat flow at t indifferent face of the material body, T1and T2are constant temperatures in both faces therefore, the total temperature gradient are constant in a whole material body that is “P1–  P2=P= KA(T1– T2)/L”. Where L= is the distance between two face of material and P= the amount of heat flow by the two surfaces of material at the time

Example of three modes of heat 

“Touching a hot iron” is one example of heat conduction. When ironing the clothes the heat is transferred from the iron to the cloth at the same time and the ironing is done. Uranium-238 transfers into Thorium-234 form of alpha particles that emit the radiation through the providing heat. Boiling water is an example of convection heat transfer as the heat transfer occurs from hot burner to water on the above side of the pot.

Difference and similarities between   three modes of heat transfer 

Difference: How the heat transfer from source to solid in direct contact is referred to in conduction. The heat transferred through fluid or gasses discusses in convection and radiation discusses the heat flow in a vacuum.

Similarities: In conduction heat transfer in physical contact, in convection heat transfer in the fluids and radiation there is no direct physical contact for the heat transferred.

What and how is the faster mode of heat transfer? 

Radiation is the fastest heat transfer in physics, there is no need for any media for the heat flow. In radiation heat can be affected by any source of heat such as speed of light and any state of the material cannot stop the radiation.

Conclusion

Conduction helps to transfer heat with the direct touch of the object and radiation does not require any type of direct contact of any objects.Chemical reaction or the effects of sunlight in plants are examples of radiation.