Radiation - mode of heat transfer

[Sahil Dev]

Hi All,

I'm trying to understand the radiation mode of heat transfer. I have been reading the book by Incropera - "Fundamentals of heat and mass transfer" and have a few very basic questions.

1) Firstly, thermal radiation is the energy emitted by matter that is at non-zero temperature. Radiation may be emitted from solid surface or liquids or gases. The energy of radiation is transported by electromagnetic waves (or alternatively, photons). While the transfer of energy by conduction or convection requires presence of material medium, radiation does not. In fact, radiation occurs most efficiently in a vacuum.

QUESTION 1: Does it mean that radiations are emitted by matter (solid or liquid or gases) but transported in a vacuum. I do not get quite correctly when we say that transport through radiation does not involve a material medium like in case of conduction and convection.

2) Definition of black body. Be definition, the upper limit of the energy released by the surface is given by Stefan Boltzmann law, which is;

E_b= σ * T⁴

It is also said (in the text that I have been reading) that such a surface is called an ideal radiator or black body.

My question is very basic / more of a stupid layman question now.

QUESTION 2: It is always said that during cold season, you wear a black shirt, you would not feel cold.

I'm trying to relate the above statement to the definition of black body (the ideal radiator in Stefen Boltzmann law), by the definition of ideal radiator (the black body), shouldn't the black body (my black shirt-say) emit MORE heat to surroundings than a white shirt?

 

[Bandersnatch]

Hi Sahil Dev, welcome to PF!

Re: 1)
Remember that electromagnetic radiation needs only the electromagnetic field to travel in. It doesn't need a medium, but neither does the presence of some medium necessarily prevent it from propagating.
Radiation can travel through any transparent material, but the more transparent the less of it gets stopped (think of photons 'hitting' atoms on their way). Obviously in a vacuum there's nothing to stop the radiation, so it's the best 'conductor'.

Re: 2)
The black body is called so because it is assumed that it doesn't reflect, nor transmit any radiation. Every bit of radiation that hits it is absorbed, so every bit of radiation that you detect leaving it must be produced internally.

That's also what your black shirt does (approximately) - it absorbs all visible light, which heats it up more when compared to a coloured (reflects a given colour) or white (reflects all visible light) shirt.

 

[Drakkith]

QUESTION 2: It is always said that during cold season, you wear a black shirt, you would not feel cold.

I'm trying to relate the above statement to the definition of black body (the ideal radiator in Stefen Boltzmann law), by the definition of ideal radiator (the black body), shouldn't the black body (my black shirt-say) emit MORE heat to surroundings than a white shirt?

A black shirt would emit more radiation than a white shirt if you were emitting a significant amount of EM radiation in the visible range. But you are not, so it ends up making you warmer because it absorbs more visible light than a white shirt does. A perfect black body is a perfect absorber/emitter at all wavelengths, but your shirt is not. It can be a poor absorber/emitter in different bands despite being very good in the visible range.

 

[Sahil Dev]

Thanks a lot Drakkith and Bandersnatch

Drakkith, Sir;

You said;

A black shirt would emit more radiation than a white shirt if you were emitting a significant amount of EM radiation in the visible range. But you are not, so it ends up making you warmer because it absorbs more visible light than a white shirt does.

1) I'm sorry, but I'm associating the 'word / terminology' radiation as heat energy emitted by matter (solid, liquid, gas) - the matter should be at a non-zero temperature to emit radiation (heat energy). Am I right?
2) Why do you use the term 'electromagnetic radiation' ?
3) And what do you mean by a visible range of radiation? What is visible and non visible range ?

Sorry for being too basic.

 

[Drakkith]

1) I'm sorry, but I'm associating the 'word / terminology' radiation as heat energy emitted by matter (solid, liquid, gas) - the matter should be at a non-zero temperature to emit radiation (heat energy). Am I right?

That's right. At any temperature above absolute zero an object will emit electromagnetic radiation. Note that 'heat energy' is technically an incorrect use of terminology. Heat is the transfer of energy that doesn't result in useful work and doesn't involve a transfer of matter. When you touch a hot object to a cold object, thermal energy flows from the hot object to the colder object. We tend to call this 'heat flow' or 'heat transfer' because no matter is transferred from one object to the other and no useful work is performed on either object.

See the following links for more information.
http://en.wikipedia.org/wiki/Heat
http://en.wikipedia.org/wiki/Thermal_energy

2) Why do you use the term 'electromagnetic radiation' ?

Electromagnetic radiation is the type of radiation emitted by the object. There are other types of radiation, such as alpha and beta radiation, but both of these are very different than EM radiation. EM radiation is made up of electromagnetic waves. Radio waves and visible light are both EM waves, they just have a different frequency/wavelength.

3) And what do you mean by a visible range of radiation? What is visible and non visible range ?

Visible light composes a small section of the electromagnetic spectrum, approximately falling between 700 nanometers (red) and 350 nanometers (violet). Non-visible EM radiation is everything else, including radio waves, microwaves, infrared radiation, UV radiation, X-Rays, and Gamma Rays.

See here for more: http://en.wikipedia.org/wiki/Electromagnetic_radiation

 

[Sahil Dev]

I understand, however still a fundamental doubt.

Electromagnetic radiation is the type of radiation emitted by the object

So, electromagnetic radiation is emitted by a solid, liquid or gas.

What I don't get quite right, is this sentence in the definition of transfer by radiation

While the transfer of energy by conduction or convection requires presence of material medium, radiation does not

Radiation is emitted by a solid or liquid or gas. Isn't this (a solid or liquid or gas) a medium ?

OR does it mean the transfer of heat by means of radiation is NOT to a medium ?

I'm confused but will be grateful if helped.

 

[Drakkith]

Radiation is emitted by a solid or liquid or gas. Isn't this (a solid or liquid or gas) a medium ?

A medium is something that an object or disturbance travels through. For example, a sound wave travels through the air, meaning the air is the medium that the sound wave travels through. Conduction and convection require a medium. A hot stove top heats the water in a pot, which rises until it reaches the top, allowing it to transfer the heat to the air in the process known as convection. The hot stove top directly heats the bottom of the pan and the heat moves through the metal of the pot (the medium) in the process of conduction.

In contrast, EM radiation requires no medium. The energy radiated out from the Sun reaches the Earth after traveling millions of miles through the vacuum of space.