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does radiant heat dissipate with regard to distance?

does radiant heat dissipate with regard to distance?
Original post by sipoolw
does radiant heat dissipate with regard to distance?

Hello and welcome to TSR. The way this works is that other users guide you to the answer helping you to answer the problem for yourself.

To start: In order to answer this, you need to first understand the question.

Define radiant heat. Specifically, what is heat?
(edited 10 years ago)
Original post by sipoolw
does radiant heat dissipate with regard to distance?


What, exactly, do you mean by "dissipate" in this context?
Certainly, the further you are away from a source of radiant heat, the less you "feel" it.
The intensity is less and follows an inverse square law.
Reply 3
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sipoolw
OP
Thanks for your quick response.
Do you know where I can find this "inverse square law"?
I have tried looking online without success.
I had a question to answer which was to calculate the amount of heat energy radiated to a person within a room, and how much radiant heat energy the person would give back to the room when the walls were colder.
I wasn't given any dimensions of room size, so i have not factored in the distance the person is from a wall - which appears to be necessary.
In another study I am trying to find out whether heating (in a given circumstance) from a radiant panel uses less energy than convection heating.
For the question of amount heat radiated by an object you need the Stefan-Boltzmann law
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

For heat transfer generally look here
http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

Inverse square Law
I just typed Inverse Square Law into Google!
Here
http://hyperphysics.phy-astr.gsu.edu/hbase/forces/isq.html
http://en.wikipedia.org/wiki/Inverse-square_law
Reply 5
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sipoolw
OP
I have followed the Stefan Boltzmann law when I performed my calculation etc but I struggle to follow the logic if two object are very far apart, especially when the inverse square law is considered. I don't see how the same amount of heat energy can be radiated to an object close to a hot surface as to an object which is far away. Stefan Boltzmann law doesn't consider distance between two objects. Having paid 4500 pounds for a course, I find it obscene that I have to resort to google to find many of the answers. I did ask my tutor if distance had any bearing on radiant heat transfer to which the answer was "no". I would love to learn about the relationship between how hot a surface has to be and its distance to people to serve as an effective radiant panel. If anyone can point me in the direction of reports of manufacturers' reports I would appreciate it. (I have looked online for over 4 weeks, although I admit to not picking up the phone)
Original post by sipoolw
I have followed the Stefan Boltzmann law when I performed my calculation etc but I struggle to follow the logic if two object are very far apart, especially when the inverse square law is considered. I don't see how the same amount of heat energy can be radiated to an object close to a hot surface as to an object which is far away. Stefan Boltzmann law doesn't consider distance between two objects. Having paid 4500 pounds for a course, I find it obscene that I have to resort to google to find many of the answers. I did ask my tutor if distance had any bearing on radiant heat transfer to which the answer was "no". I would love to learn about the relationship between how hot a surface has to be and its distance to people to serve as an effective radiant panel. If anyone can point me in the direction of reports of manufacturers' reports I would appreciate it. (I have looked online for over 4 weeks, although I admit to not picking up the phone)


Energy conservation laws apply:

If you could focus the emitted em radiation into a perfectly parallel beam (rather like a perfect laser for instance) with no divergence or convergence and in a perfect vacuum (no intervening absorption) with the whole beam then incident on the target body, then yes, distance becomes irrelevant and all of the radiated energy will arrive at the target.

Distance becomes relevant if the propagating radiation from the source is divergent (conical beam say), in which case the received energy will be in the same ratio as that between the surface area of the target body to that of the surface area of the divergent energy front arriving at the same distance as the target.

i.e the transmitted energy is spread out over a larger surface area and that surface area increases with distance from the source whereas the target body surface area remains constant.

For radiation emitted uniformly in all directions (spherical outward propagation) this ratio increases as the square of the distance from the source.

Hence why radiant heaters use parabolic reflectors.
(edited 10 years ago)

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