Can hot water cool down through radiation?

[gaurav_samanta]

I have two more questions:-
(1) Can we calculate the time it takes specifically for water to radiate all its heat?
(2) Heat is illustrated as kinetic energy of molecules, so why collision between particles should result in the conversion of kinetic energy into radiation. Why conservation of momentum simply does not apply?

 

[anuttarasammyak]

(1) We have Newton's law of cooling.
(2) Collisions of molecules do not play main role in generating radiation. Radiation comes mostly from vibration of molecule, not from translation motion of molecules.

 

[Drakkith]

Summary:: What everyone tells me is that hot water transfers its energy to air. But I have also heard of water freezing down in the vacuum of space. How does that happen?

Most of it occurs by the boiling of part of the water. The lack of surrounding air pressure drops the boiling temperature of water to a very, very low temperature, and as some of the boil off they take energy with them, cooling off the remaining water.

(1) Can we calculate the time it takes specifically for water to radiate all its heat?

The water will freeze first and then the ice will gradually sublimate and escape into space, eventually leaving nothing behind but a dispersed cloud of water vapor. Once this occurs the gas will either cool off or heat up depending on the environment that its in.

(2) Heat is illustrated as kinetic energy of molecules, so why collision between particles should result in the conversion of kinetic energy into radiation. Why conservation of momentum simply does not apply?

Molecules and atoms are composed of charged particles. Collisions of charged particles result in the acceleration (which includes deceleration) of said particles, and the acceleration of charged particles results in the creation of EM radiation. Kinetic energy from the particles is gradually turned into electromagnetic energy in the form of EM waves and the particles slow down overall as a result. Momentum is also transferred, as EM waves have momentum, so conservation of momentum holds up just fine.

 

[gaurav_samanta]

(1) We have Newton's law of cooling.
(2) Collisions of molecules do not play main role in generating radiation. Radiation comes mostly from vibration of molecule, not from translation motion of molecules.

I don't think Newton's law of cooling applies on radiation. Correct me if I am wrong.

 

[gaurav_samanta]

Collisions of charged particles result in the acceleration (which includes deceleration) of said particles, and the acceleration of charged particles results in the creation of EM radiation.

Does that also happen in our macroscopic world? Do some materials tend to decelerate more than others? What's is this property of materials called?

 

[Drakkith]

Does that also happen in our macroscopic world?

Sure, all you need is a large, charged object. Unfortunately the intensity and frequency is proportional to the magnitude of the acceleration, and macroscopic objects can't be accelerated as hard as microscopic particles, so the emitted radiation is very hard to detect and of a very low frequency.

Do some materials tend to decelerate more than others?

Sorry, I'm not sure what you're asking.

 

[gaurav_samanta]

Do some materials tend to decelerate more than others?

My question is does the intensity and frequency of radiation depends only on the mass of object and magnitude of the acceleration, or the chemistry of the object also has a role?

 

[gaurav_samanta]

Also please tell me can we calculate the time it takes specifically for water to radiate all its heat?
Is 'Newton's law of cooling' the answer?
Is the math involved pretty complicated?

 

[Cutter Ketch]

Does that also happen in our macroscopic world? Do some materials tend to decelerate more than others? What's is this property of materials called?

The efficiency with which a surface emits thermal radiation is called emissivity. The number is in relation to a theoretical maximum: a “black body”. The emissivity is a number between zero and one. Thermal radiation is a very strong function ($\Delta T^4$) of the temperature difference between the body in question and the environment into which it radiates. At large temperature differences (say, hot enough to glow) it can be the dominant way heat is transferred (standing near a bonfire, or pit of lava, or toasting bread). However at the temperature differences usually encountered in daily life thermal radiation transfers heat at a much slower rate than convection or conduction and often is ignored by comparison. However it isn’t completely negligible. If you do your best to suppress the others your coffee will still cool down faster than you want. A good example of this is a thermos bottle where they use a vacuum to suppress convection and conduction. ( still conductive losses at the neck.). If you get a chance to take one apart you’ll notice they coat the surfaces in shiny metal. This is to suppress the emissivity. It wouldn’t work nearly as well if they skipped that because thermal radiation still matters even at boiling water temperature differences. You can also see thermal radiation with a thermal camera. The whole world is glowing. At ~ 300K the glow is brightest at about 9 um. Animals which are only a few degrees warmer than the environment glow brightly.

 

[anuttarasammyak]

Also please tell me can we calculate the time it takes specifically for water to radiate all its heat?
Is 'Newton's law of cooling' the answer?
Is the math involved pretty complicated?

We can consider it from Stephen Botlzman law which says radiated energy per surface area per time is

$$E=\epsilon\sigma T^4$$

So the differencial equation

$$S\epsilon\sigma T^4(t) dt + CdT(t)=0$$

where S is area and C is specific heat capacity of the body, would hold meaning energy conservation.

Solving the equation
$$T(t)=[T_0^{-3}+\frac{3S\epsilon\sigma}{C} t]^{-\frac{1}{3}}$$
where $T_0=T(0)$ initial temperature of the body and perfect vacuum, even 3K cosmic background radiation does not exist, surrounds the body. I derive it myself so please check it carefully.

 

[Drakkith]

My question is does the intensity and frequency of radiation depends only on the mass of object and magnitude of the acceleration, or the chemistry of the object also has a role?

It depends just on the magnitude of the charge and the magnitude of the acceleration. Mass and chemistry have nothing to do with the frequency or how much EM radiation is emitted from an accelerating charge (though mass will of course affect the rate of acceleration of a charged particle or object).

 

[russ_watters]

Also please tell me can we calculate the time it takes specifically for water to radiate all its heat?
Is 'Newton's law of cooling' the answer?
Is the math involved pretty complicated?

Newton's law of cooling does not work well for radiation.

The math for heat transfer can be simple or complex depending on the specifics of the scenario, the approach to solving it and desired accuracy of the model. You should describe for us a specific problem and we'll help you solve it.