Difference between convection and radiation

[TSN79]

With heat transfer I'm having trouble seeing the difference between convection and radiation. My book says convection is the heat transfer between a solid and a liquid or gas, like an oven in a room. Sunlight is supposed to be radiation, but if one looks at the sun as being the oven, and the air around us as being the gas, how is that different from convection?

 

[Integral]

Convection REQUIRES a medium to carry thermal energy from point A to point B. There is no medium necessary for radiation. The sun's rays heat the Earth and the Earth's atmosphere even thought they are separated by the vacuum of space. Once the sun heats the atmosphere, convection currents develop which further distribute the heat. On the surface of the Earth we receive some energy directly radiated by the sun and some due to convection mixing. Consider the difference between a clear day or a cloudy day. On a cloudy day you only get thermal energy from convection, the heat of a clear bright summer day is due to solar radiation more then convection.

In the winter the suns radiation is not as intense as in the summer because of the angle of the Earth's surface with respect to the suns rays. To see this take a square of cardboard, pierce its center with a pencil. Then on a summer day point the pencil at the sun in such a manner that it does not cast a shadow on the cardboard. Observe the size and shape of the shadow on the ground. Do the same thing on a winter day. You will see that the same piece of cardboard will cast a bigger shadow in the winter. The cardboard is intercepting the same amount of the suns rays so this shows that in the winter a square meter of the suns rays must heat a larger area of the Earth's surface then it does in the summer. Therefore in the summer the ground is warmer then in the winter. This is all radiant energy, if it is cloudy or at night you do not have the same effects.

 

[TSN79]

Still confused, sorry. So when an oven heats up a room, what is the medium then, air? But won't there always be a medium present to carry the heat, so so we'll practically always deal with convection? Is a vacuum not able to transfer heat?

 

[Integral]

In our atmosphere and in our homes convection is a key heat transfer mechanism.

But, if you have ever broiled anything in your oven, radiation is the main mechanism. If you have ever stood by a campfire and felt the heat of the flames on your hands, that is radiation, the sun, as I mentioned in my first post is radiation.

No, vacuum cannot transfer heat via conduction or convection the only heat transfer mechanism which works through a vacuum is radiation. This is why we have vacuum thermos bottles, this is why you have double pane windows. Double pane windows are to close together to allow convection currents to develop so the only mechanism allowed is radiation, for small temperature differentials radiation is not able to transfer very much heat.

Convection heating implies currents in some fluid, be it air or water, radiation does not require a medium, just like light does not need a medium. But then light is a prime example of radiative energy transfer. The mechanism for radiative heat transfer is Electro Magnetism, and that may be best for you way to differentiate between convection and radiation. Convection is heat transfer via currents in a fluid, radiation is heat transfer through electromagnetic interactions ONLY.

 

[dextercioby]

To me, the more interesting subject is :"difference between convection & conduction" when it comes to heat transfer.

Daniel.

 

[Claude Bile]

Convection is just the movement of hot matter from one point to another. When hot matter moves, it carries whatever thermal energy it posseses with it, thus it is a heat transfer mechanism.

I do not blame you for your confusion, the definition quoted by your textbook is an awful one, I prefer the definition in my elementary physics textbook "Convection is the transfer of heat by mass motion of a fluid from one region of space to another".

I hope that alleviates some confusion.

Claude.