I have tried to explore the information but still not very clear on the exact difference between diffusion, convection and advection. Can anyone help me out to clear my concept?
Fluid Dynamics – What’s the Exact Difference Between Diffusion, Convection, and Advection?
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To pretty much everything you stated in your question, "no".
That convection requires a medium is not the main difference, it is simply the most obvious aspect of what is a fundamentally different mechanism for transferring energy. Convection is the transfer of energy by movement of a medium, whereas radiation is the transfer of energy by, well, thermal radiation. Conduction also requires a medium, but, again, it is a fundamentally different mechanism than either convection or radiation; in this case it is the transfer of energy through a medium.
Unfortunately, analogies are hard but if you can visualize the particles involved, it would help. Picture the red hot iron you mentioned. On a molecular level, the material is emitting lots and lots of photons (hence why it is glowing red). The creation of these photons takes energy; energy from the heat of the iron. These photons leave the iron, pass through the environment, and eventually collide with some other object where they are absorbed and deposit their energy. This is radiative heat transfer. If that energy is deposited on your retina or a CCD (like in a digital camera), an image forms over time. This is how infrared goggles work and they would work equally well in high vacuum as here on earth.
In conduction, the next simplest example, there is no generation of photons (physics nerds forgive me for the sake of simplicity). The individual atoms in the object are vibrating with heat energy. As each atom gains energy from its more energetic neighbors, so it gives up energy to its less energetic ones. Over time, the heat "travels" through the object.
In convection, the molecules of gas near the object gain energy, like in the conduction case, but those same molecules that gained energy then travel through the environment to some other location where they then give off their heat energy.
In summary:
- radiation = generated and absorbed photons
- conduction = molecules exciting their neighbors successively
- convection = molecules heated like in conduction, but then move to another location
Convection is the movement of a fluid, typically in response to heat.
Advection is the movement of some material dissolved or suspended in the fluid.
So if you have pure water and you heat it you will get convection of the water. You can't have advection because there is nothing dissolved or suspended in the fluid to advect.
If you have silt suspended in the water and heat it then you will get convection of the water and advection of the silt.
If you have silt suspended in the water and the water is just flowing in a river you will get advection of the silt, but you would not normally describe the water movement as convection.
Best Answer
Convection is the collective motion of particles in a fluid and actually encompasses both diffusion and advection.
We typically describe the above two using the partial differential equations: \begin{align} \frac{\partial\psi}{\partial t}+\nabla\cdot\left(\mathbf u \psi\right)&=0\tag{advection}\\ \frac{\partial\psi}{\partial t}&=\nabla\cdot\left(D\nabla\psi\right)\tag{diffusion} \end{align} where $\psi$ is the quantity in consideration, $\mathbf u$ is the fluid velocity and $D$ the diffusion coefficient (sometimes called the diffusivity).
There are some nuances to the combined effect for convection (e.g., forced, natural, gravitational mechanisms), but the general definition for it is the total motion.