1. May 7, 2013

### rds-s

I have a question regarding heat transfer/ heat loss. When the sun goes down, lets say in December, is it possible that some objects lose heat (through radiation) at a quicker rate than other objects? If so, colder objects would lose heat, or emit radiation, to its surroundings (warming its surroundings). I doubt this is possible because I'm thinking it would violate the 2nd law of thermodynamics.

2. May 7, 2013

### SteamKing

Staff Emeritus
Objects colder than ambient cannot radiate heat; they absorb heat from hotter objects. This is a consequence of the Zeroth Law of Thermodynamics.

3. May 8, 2013

### Staff: Mentor

Some objects emit and absorb more radiation than others. For visible light, those objects appear darker (as they are not hot enough to emit significant quantities of visible light). If you heat them up, they can appear brighter than other materials. For infrared radiation, it is the same - some objects cool down quicker than others, if they are hotter than the environment. If you shine IR radiation on them, they will heat quicker as well.

4. May 8, 2013

### technician

I think you have this wrong !!!! What about an object at the same temperature as ambient?
What would happen when the ambient cooled to below the object temperature.... would the ambient cease to radiate and the object take over to radiate !!!! Think about the physics.
I think/hope that you are only confused about 'net' rate of heat transfer.

Last edited: May 8, 2013
5. May 8, 2013

### rds-s

So lets say the sun warmed a sidewalk abutting black pavement and the air surrounding it all. And then the sun goes down. Actually, for this scenario lets say the sun disappears. The pavement, sidewalk and air no longer have the warming energy of the sun and would then radiate (lose) thermal energy. I would imagine that the three objects radiate heat at different rates. Would that not mean one of the objects (sidewalk, pavement and air) would be cooler than the others? Or would all three objects have to cool down at the same rate?

6. May 8, 2013

### Integral

Staff Emeritus
Every material has a property called emissivity, this is determines how much energy it radiates. Yes it varies from material to material. Of course the temperature differential determines which way the energy goes.

A clear night sky has a very low temperature for radiation purposes. One of my profs claimed that if you took a good thermos jug put a little water in the bottom and pointed the opening at a dark (few stars) portion of sky, you could freeze the water, even on a warm summer night. I might take a while.

7. May 9, 2013

### christopher.s

Lets say we are walking on grass field shortly after the sun goes down. We walk onto a blacktop asphalt surface. Do we perceive more heat coming off of the blacktop? Probably yes.

The grass field might not be the best example of material but you get the idea.