# Explain how convection is a type of heat transfer

So, from my understanding convection is when a gas or liquid is unevenly heated and the warmer portion rises, as it is less dense, and the cooler portion sinks creating a cycle(something along those lines although not the best explanation). This sounds like a perfectly reasonable and logical physical occurrence, however, convection is referred to as a type of heat transfer. I just can't wrap my head around it. Where is the heat transfer bit? It sounds more like convection occurs because of heat transfer, not that is causes heat transfer. The specific example I am looking at is in my biology text book where a lizard is getting heat from its environment. A rock heating its body is referred to as conduction, as it is 2 bodies in direct contact with each other. The sun heating its body up is radiation. However the air heating it up is referred to as convection. Why is it not just conduction, as the air is a substance which is in direct contact with the lizards body?

• Hot (and cold) air/liquid is physically moving because of buoyancy. This transfers heat across the thermal gradient faster than it would conduct while still. Commented Apr 22 at 21:07
• That still sounds like convection is just a means through which conduction occurs? Commented Apr 22 at 21:50
• So if I take a hot brick out of a fireplace and throw it to you is that still 'conduction' - because that is not how it is understood in physics and engineering. Commented Apr 22 at 22:17
• Convection includes conduction. "Conductive heat transfer" refers to conduction alone. The distinction is made in part because $q=h(T-T_\infty)$ (flux $q$) is an enormously successful model for what's called convection; $q=k\nabla T$ is an enormously successful model for what's called conduction. Physics and physics pedagogy benefits by classifying concepts in terms of predictive models. I agree that divisions may seem semantically illogical at first if one is reading only definitions. Consider thinking of the terms as consensus shorthand for the type of mathematical model one plans to apply. Commented Apr 22 at 22:42
• @Chemomechanics your comment is essentially an answer, and a good one. Related physics.stackexchange.com/q/62423/226902 physics.stackexchange.com/q/23048/226902 Commented Apr 22 at 22:51

One distinction (loosely) between convection and conduction is the actual bulk movement of material. If you consider a scenario where a lizard breathes in warm air (forced convection), then the air is actually going inside the lizard, which, if you were to measure the internal temperature of the lizard then, you would find it to be warmer. If you imagine there is absolutely no conduction going on, then you can just think of this as taking some warm air, putting it inside the lizard, and then just measuring the temperature of that warm air (which is now inside the lizard). It's true that the warm air will then start to conduct heat to the lizard, but the point of convection is that the bulk material is actually moved from one location to the other, and in that other location the temperature became higher. Contrast this with conduction with zero convection where the air remains stationary outside the lizard and conducts heat to it. In reality, "thermal convection" is generally taken to mean a transfer of heat via a combination of conduction and advection (the flow of bulk fluid).

• That still sounds like convection is just a means through which conduction occurs? I realise I'm probably misunderstanding something but it still doesn't click for me why convection is referred to as a whole different type of heat transfer. It just sounds like it allows for conduction to happen Commented Apr 22 at 21:51
• Imagine firstly that no conduction occurs. Say I have a mass of warm air at point A and a mass of cold air at point B. Then I move the mass of warm air to point B, displacing the cold air previously there. Then I take the temperature at point B and find it to be higher. So, heat has transferred from point A to point B without the use of conduction. This is the advection part of convection. Commented Apr 22 at 22:05

Heat transfer by convection is governed by Newton's law of cooling which, for steady heat flow, is represented by the following equation:

$$\dot Q=hA(T_{w}-T_{\infty })$$

where

$$\dot Q$$ = heat rate

$$h$$= the convection heat transfer constant of the fluid [W/m$$^{2}$$.K]

$$A$$= the convection surface area (m$$^2$$)

$$T_w$$= the wall surface temperature (K)

$$T_{\infty}$$= the bulk fluid temperature far from the surface (K)

So, from my understanding convection is when a gas or liquid is unevenly heated and the warmer portion rises, as it is less dense, and the cooler portion sinks creating a cycle(something along those lines although not the best explanation).

What you are describing is convection where the convection heat transfer constant $$h$$ is that for natural convection. But there is also the possibility of forced convection, like moving air across a surface with a fan, in which case $$h$$ is higher, all else being equal.

Where is the heat transfer bit? It sounds more like convection occurs because of heat transfer, not that is causes heat transfer.

The heat transfer "bit" is Newton's law of cooling, discussed above.

The specific example I am looking at is in my biology text book where a lizard is getting heat from its environment. A rock heating its body is referred to as conduction, as it is 2 bodies in direct contact with each other. The sun heating its body up is radiation. However the air heating it up is referred to as convection. Why is it not just conduction, as the air is a substance which is in direct contact with the lizards body

There is heat transfer to the lizards body by conduction, but it is very inefficient. The movement of air warmer than the lizards skin across its surface increases the heat transfer rate to a greater level than conduction alone.

Hope this helps.