Explain the Process of Convection

You should know that thermal energy is transferred in solids by conduction.

When you heat a solid object, the heat will be transferred from the hot region to the cold region, due to the particles colliding in a knock-on effect. Solids are able to do this, as their particles are arranged in rows.

So how is thermal energy transferred in fluids (i.e. liquids and gases)? 

The process of thermal energy being transferred in liquids and gases is called convection. It is a process that occurs in all fluids. In this activity, you will learn what that process looks like, and why it occurs.

Imagine putting a saucepan of water on a stove and heating it. What would happen?

Well, the first energy transfer would be the conduction of heat through the bottom of the pan, as it is made of metal. But what happens next? Let's look at the water particles inside the pan. Here they are, sliding over each other randomly before any heat arrives. We can assume that all the particles are moving at the same speed.  

Once the metal in the pan conducts the heat to the water, the particles at the bottom of the pan will have thermal energy transferred to them. This means that they will gain kinetic energy, move faster, and spread out:

Here, the red particles represent the particles that have gained energy and have spread out. Obviously, they don't actually change colour! (However, if this process is being demonstrated in a laboratory in a transparent beaker, using food colouring allows this effect to be seen more clearly).

Now, having more space between particles will decrease the density of that region of water. If something has less density than the fluid surrounding it, then it will float: 

The water at the top of the pan was colder than at the bottom, but these faster-moving particles will rise to the top of the pan and push them to one side. In doing so, the thermal energy has been transferred from the bottom of the pan to the top. (It has transferred from hot to cold).

And that is how convection works in liquids and gases!

What is a convection current?

The process doesn't end there! If we continued to heat the pan and continued to look at the particles, we would realise that there are a couple more steps, resulting in a cycle.

In the last picture, with the red particles at the top of the pan, there was a massive gap at the bottom of the pan! In reality, the particles above this empty space would rush into it to fill it. Now, finding themselves at the bottom of the pan, and being heated from the bottom, it would then be their turn to gain kinetic energy, speed up, and spread out...

In this diagram, the red particles are the ones that have suddenly gained this kinetic energy, while the purple particles are the ones that had risen to the top at the beginning. They have lost their kinetic energy and are bunching together, making that region of the water more dense. What do you think will happen next?

Oh look! The particles that were at the top have now sunk back to the bottom! Meanwhile, the particles that had recently gained kinetic energy have risen to the top! This has occurred due to the less dense water floating and the more dense water sinking.

If the water continues to be heated, the particles at the bottom will keep gaining kinetic energy and rising to the top, while the ones at the top will be knocked back down to the bottom. This is called a convection current because we have created a situation where the particles are continually moving in a 'circuit' from the top to the bottom.

This process will continue until the supply of heat (e.g. the stove) is switched off. To make the process occur faster, you would just need a greater difference in temperature between hot and cold (so, turning up the heat on the stove), or having a lower mass of fluid (for example, it takes less time to boil a small pan of water than a large pan, as the water particles have fewer particles to push past, and rise and fall faster).

Now that you have seen the process, have a go at the questions!