Convection Vs Diffusion: Critical Facts


Convection is an essential mode of transport of heat. It occurs by diffusion and advection. In this post, you will read about Convection Vs Diffusion.

Diffusion is the movement of single molecules from a region of higher concentration to an area of lower concentration through gradients. In contrast, Convection is a process of transfer of a large number of fluid molecules. Both Convection and diffusion are in the same direction.

Now let us study Convection Vs Diffusion in detail.

Convection: A mode of transfer of heat

Convection is a process of movement of heat particles, which is experienced in daily routine.

The convection process, in simple words, is defined as the massive heat transfer of fluid matter that occurs through the movement of heat molecules. Where the hotter components rise upwards, and more excellent parts move downwards by convection currents. It takes place in liquid and gases and not in solids due to the nature of matter.

Convection Vs diffusion
Image Credit:  Convection cycle, Erik Ong, CC BY-SA 3.0, via Wikimedia Commons

Now let us know about the diffusion process.

Diffusion: Definition

The conduction process of heat also refers to diffusion, and it is the direct transport of momentum and energy molecules between source and sink.

Diffusion is a process that occurs spontaneously from a higher temperature substance to a lower temperature substance through a concentration gradient, and it follows the second law of thermodynamics. This process happens in the same solution of fluid. The movement of molecules is possible only till the concentration gradient is present.

Diffusion occurs as a part of Convection. The difference between Convection Vs diffusion is below.

Convection Vs Diffusion

Convection and diffusion can be defined as physical processes that include molecules’ movement that causes heat transfer.

             CONVECTION                 DIFFUSION
Convection is the process that includes the transfer of a large portion of liquid or gas molecules of the solution.Diffusion appears as the motion of molecules of a solution from a more concentrated region to less concentration.
It occurs due to the difference in pressure gradients of the two systems.It occurs through the region of a concentration gradient.
It transports heat between two systems.It transports both energy and momentum from one molecule to another within the liquid or gas.
Molecules flow in the same direction.Even in diffusion, molecules flow in the same direction.
Molecules of the fluid decide their motion concerning the pressure gradient.Molecules of the fluid decide their flow based on a concentration gradient.
It is a massive motion of molecules.It is a slight movement that includes single particles.
Example: Air conditioners work on the principle of Convection.Example: Removing the cap of soft drinks.

These are some basic facts on Convection Vs Diffusion.

You can read on: Convection Vs Advection.

Diffusion: How does it work

Diffusion is a process that is generally seen in all creatures; it may be living or non-living.

It is defined as the process of particles of substances from a more concentration region to a less concentration region through the concentration gradient. The process happens until there is the concentration gradient is present. In general, diffusion is seen only in fluids due to the atomic space and not in solids, which changes when molecules collide.

It is generally observed as an osmosis process. Now let us see about its types.

Types of Diffusion

There are two essential types of diffusion which are given as follows,

Simple diffusion

It is a diffusion process in which the molecules of substances move from one part of the component to another through a gradient in a chemical solution or semipermeable membrane in cells.

Example: Solubility of sugar grains in water

Facilitated diffusion

It is defined as the passive movement of molecules across the solution from higher concentration to lower concentration with a carrier substance.

Apart from these two, there are many other types of the diffusion process.

Convection: A Combination of diffusion and advection

The diffusion and advection processes are the essential parts of convection flow.

In general, the diffusion process is known as the random movement of a single particle of a fluid substance, also referred to as Brownian motion. Whereas, in advection, there is large transport of bulk matter of fluids. The combination of these two results in convection currents.

There is a certain difference between these processes.

Relation between Convection and diffusion: A convection-Diffusion equation

The convection-diffusion equation is also known as the drift-diffusion equation, which includes both Convection and diffusion. The general equation is given as follows,

Where,

  • c is the variable which symbolizes the concentration of temperature, mass etc.
  • D refers to diffusivity, or diffusion coefficient
  • v refers to the velocity of a fluid with time function.
  • R represents sources or sinks.
  • Refers to gradient and   refers to divergence component. But here, c refers to concentration-gradient.

It is even known as an advection-diffusion equation.

Conditions of R in Convection-diffusion equation

The convection-diffusion equation is a vital formula used in the calculation of heat transfer in many processes.

There is a presence of the term R in the equation, which refers to the substances indicated as sources or sinks.

  • For a chemical reaction, if R >0, it indicates that the reaction generates more chemical species, and it appears when heat transfer is due to friction.
  • If R < 0, it means that the reaction is losing the chemical species.

These are some conditions for sources and sinks.

Convective and Diffusive transport

Convective and diffusive transport are the two essential processes of heat transfer.

  • Convection is the horizontal direction of heat transfer in which a massive portion of fluid molecules is transferred between two systems. It is a transfer of mass, energy, or heat molecules. It includes convection currents which make the convective flow easily.
  • Diffusive transport generally depends on the size of the molecules, and it can carry only single molecules in random order. It comes after conduction and can help in the conduction of solids.
  • The difference between convective and diffusive transport is that Convection occurs due to pressure gradient and diffusion is due to concentration gradient.
Image Credit: “File:NF solute transport.jpg” by Yagnaseni Roy, David M. Warsinger is licensed under CC BY 4.0

Therefore, transport of NF solute involves both processes.

Examples of Convection and Diffusion

There are many in which Convection and diffusion are seen. Here are some examples of Convection and diffusion listed below.

  • When immersed in hot water, a hibiscus tea bag will diffuse and change its color to pink, and the same hot tea will become cooler after some time. It is Convection.
  • When you light a scented candle, it will get diffused into the whole room, giving warmth of freshness. Whereas in the candle, a process of Convection occurs.
  • When you add sugar to any liquid, they get dissolved easily and provides taste to the liquid. When you stir the sugar in a glass, the phenomenon of rotational velocity appears, which is a convection process.
Image credit: “Sugar & Water” by austinboardman is licensed under CC BY 2.0
  • When making instant cup noodles, you have to just add hot water, the water diffuses, and the noodles are ready to eat. But after a while, it becomes cold due to Convection.
Image credit: “Samyang Instant Noodles” by ffuentes is licensed under CC BY 2.0
  • When you drop a liquid ink in a liquid, say water, it diffuses the drop it to spread without any movement if you heat the solution, after some time, the movement causes low temperature.
Convection Vs diffusion
Image credit: “Smoke on the water” by Marcin Chylinski is licensed under CC BY 2.0

These are some real-life examples of both processes.

Frequently asked questions on Convection and diffusion | FAQs

What is the effect of Convection on diffusion?

Diffusion is a part of convection heat transfer.

Both diffusion and Convection flow in the same direction, but diffusion refers to the transfer energies of single particles. In contrast, Convection is a bulk motion in which each particle may affect the other in terms of movement and energy.

What is diffusivity?

Diffusivity is a term used to refer to the transport of substances.

It is defined as several particles of the substance that diffuses from one region to another through a unit area of cross-section per unit time. The condition is that the volume-concentration gradient must be unity.

Why is Convection not seen in solids?

Convection is an essential process of heat transfer that is not found in solids.

Solids are the states of matter in which atoms/ molecules are arranged in a rigid form. Due to this factor, there is no sufficient space to transfer massive substances, and even there is no opt for diffusion. This is the reason why Convection is not seen in solids.

Is the rate of convective transport slower than diffusion?

Diffusion transport in fluids is always slower compared to convective transport.

Convection is the transfer of massive fluid from one part to another, accompanied by both diffusion and advection. It dominates the diffusion rate because it occurs very slowly in liquids when measured in human length scales.

Is there any difference between convection flux and diffusive flux?

There is a slight difference between convective flux and diffusive flux, which is given as follows,

Convective flux is measured concerning average velocity, whereas diffusive flux is the difference between total flux, i.e., advection plus diffusion minus convective flux.

Mention the factors which increase Convection?

The factors which help to increase Convection are as follows,

  • The surface of the interaction of particles.
  • The velocity of air/wind.
  • The density of hot and cold air.
  • Nature of the process of Convection.
  • Nature of fluid.

What factors affect diffusion?

The factors which affect the diffusion rate are as follows,

  • Size and range of the particle.
  • The sharpness of the concentration gradient.
  • Rate of temperature.
  • Size of the area taken for the transport of substances.

Raghavi Acharya

I am Raghavi Acharya, I have completed my post-graduation in physics with a specialization in the field of condensed matter physics. Having a very good understanding in Latex, gnu-plot and octave. I have always considered Physics to be a captivating area of study and I enjoy exploring the various fields of this subject. In my free time, I engage myself in digital art. My articles are aimed towards delivering the concepts of physics in a very simplified manner to the readers. Let’s connect through - LinkedIn: https://www.linkedin.com/in/raghavi-c-s-260a801b1 EMAIL ID: raghavics6@gmail.com

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