Difference between Conduction and Convection

Conduction and convection are two different forms of heat transfer. The major difference between the two processes is the mode of transfer of heat. There is also a third mode of transfer of heat which is radiation.

In conduction, the transfer of heat takes place through the direct contact between the source of heat and the object to which heat is transferred while in convection there is no direct contact.

Before diving deep into the difference between conduction and convection, let’s find out what heat is and how the transfer of heat takes place.

Heat is basically a form of energy that flows from a body at higher temperature to a body at lower temperature, or we can say that it flows from a body having higher kinetic energy to a body having lower Kinetic Energy.

Conduction vs Convection

Now, let’s briefly differentiate between the two through a table.

Basis of Comparison Conduction Convection
Mode of Heat transfer. For conduction to occur there should be direct contact between the bodies. Direct contact between the bodies is not required.
Observed in This form of heat transfer is generally observed in solids. This form of heat transfer is observed in fluids and gases.
Cause of occurrence Transfer of heat takes place due to differences in temperature. Transfer of heat takes place due to the difference in densities between the two bodies.
Rate of occurrence of the process This process is quite slow. Heat transfer through this process takes place at a faster rate.
Arises from Molecules at rest or free electrons. Molecules in motion.
How it takes place It takes place due to molecular collision. It takes place due to the diffusion of heated particles.

What is Conduction?

Conduction is that mode of transfer of heat energy that requires direct contact between the two bodies to transfer heat. If we observe at the microscopic level, there occurs a collision between the molecules to transfer the Kinetic energy. It is the most common form of transfer of heat.

The molecules with higher kinetic energy collide with the molecules of lower kinetic energy and transfer of energy takes place, in this way the particles with lower kinetic energy acquire more energy. This process continues and energy gets transmitted till the temperature of the two bodies becomes the same.

The process of conduction depends on the following factors: cross-section of the material, physical material properties, temperature gradient, and length of the travel path. The larger the exposed surface area, the more heat is lost. If the length or size of the object to be heated is more, more amount of heat is required to heat it. The Heat will flow from a body at a higher temperature to a body at a lower temperature.

Physical properties also affect the rate of heat flow through conduction, the material with a higher coefficient of thermal conductivity will have a higher rate of conduction and vice versa.

The rate of heat transfer in conduction is given by the equation:

    \[Q=k\: A \: \frac{T_{hot}-T_{cold}}{d}\]

Where Q = amount of heat transferred per second,

A = area in contact, k is the thermal conductivity of the material, d is the thickness of the material, Thot and Tcold are the temperatures of the hotter region and colder region respectively.

What is Convection?

It is another eminent mode of transfer of heat energy. In this mode of heat transfer, there is the displacement of fluid molecules from one region to the other. The most common example of convection which can often be seen in our homes is when we heat water, small bubbles of water can be seen rising from the bottom of the container. The hotter water expands and its density decreases so it rises to the top and provides energy to other water molecules while the colder water sinks to the bottom, this process keeps repeating.

The equation for the rate of heat flow by the process of convection is as follows:

Q = hc · A · (Ts – Tf)

Where Q= heat transferred per unit time, A = area of heat transfer, hc is the convective heat transfer coefficient, Tf is the temperature of the fluid and Ts is the temperature of the surface.

Gravity does not play any role in conduction and radiation because there is no actual movement of particles in these two processes but there is the movement of particles of medium in convection so here gravity plays an important role. The rate of heat flow in convection is faster than conduction but slower than radiation.

Conclusion

Conduction can take place in solids, liquids, and gases but it is maximum in solids because the molecules are closely packed in case of solids. Convection is observed in fluids and gases. Conduction takes place due to differences in temperature while convection occurs due to differences in the densities of the objects. We have explained the differences between conduction and convection in great detail.

Author
Deepak Yadav
Aligarh Muslim University, Aligarh

References

1. https://www.machinedesign.com/learning-resources/whats-the-difference-between/document/21834474/whats-the-difference-between-conduction-convection-and-radiation
2. https://circuitglobe.com/difference-between-conduction-and-convection.html

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