30
votes
Why is rock or metal often cold to the touch but wood or plastic is not?
What we perceive as an object being "hot/ cold to the touch" is related to the rate at which heat is transferred from the object to your hand. In the case of touching an object with your ...
- 500
23
votes
Accepted
Why is rock or metal often cold to the touch but wood or plastic is not?
In a room at normal room temperature, certain materials, such as
metal, glass, ceramic, or rock, will feel cold to the touch, but
others, such as wood or plastic, do not so much.
When you touch ...
- 69k
21
votes
Accepted
Why haven't we found thermal superconductors?
In an electrically insulating crystaline solid, heat is mostly transferred by phonons rather than by photons. What provides thermal resistance is phonon scattering via impurities or umklapp ...
- 51.2k
14
votes
Accepted
Can a tea light really heat a greenhouse?
"a greenhouse in the winter does not retain heat well"
This is the crucial point: the rate of heat loss will be roughly proportional to the temperature difference between the inside of the ...
- 34.9k
6
votes
Cup of warm water suspended in a pot of water held at a steady boil
The illustration here is enlightening. The vapor bubbles form at the bottom of the pot, where the temperature of the heat source exceeds 100°C, as is required in practice for boiling*. As they float ...
- 23.8k
4
votes
Cup of warm water suspended in a pot of water held at a steady boil
Just reaching the boiling temperature is not enough for boiling. Boiling happens when the the vapour pressure becomes equal to atmospheric pressure. When the temperature of $100ºC$ is reached then too ...
- 218
4
votes
Cup of warm water suspended in a pot of water held at a steady boil
I don't have the rep to add to the above answer. But FYI I did attempt this experiment IRL and can say that the temp in the cup never got much above 90 deg C. I did leave it at a rolling boil for 10 ...
- 41
4
votes
Why is the specific heat of conductors and insulators not same at high $T$?
The Dulong-Petit law says that at high temperatures, the molar heat capacity of a solid substance approaches $3R$, regardless of whether it is an electrical conductor or insulator. It is a ...
- 67.5k
4
votes
On the non-quasistatic transfer of heat
Do we arrive at the same final temperature as if the same spheres had equilibrated via a quasistatic process? If so then, rigorously, why?
Of course; by conservation of energy, since nothing else ...
- 23.8k
3
votes
Can a tea light really heat a greenhouse?
This is a heat transfer rate problem.
The rate of heat generated by the combustion minus the rate of heat transferred through the walls to the outside environment. The bigger this number, the warmer ...
- 31
3
votes
Why is rock or metal often cold to the touch but wood or plastic is not?
I can confirm that the granite slab feels cooler than paper or wood, despite having a greater temperature.
I experimented with the following materials: A steel handle, a granite slab, a wooden chair, ...
- 9,658
3
votes
On the non-quasistatic transfer of heat
I submit to you that the way you have phrased the question is actually ill-defined. You have tacitly assumed two pieces of matter whose interaction is strictly thermal energy exchange without any ...
- 18.1k
3
votes
Heat Transfer between 2 bodies
Since you don't mention any type of thermal contact resistance, I'm going to ignore that aspect and assume that the contact is thermally perfect.
As @Gert notes, the bodies must be initially separated ...
- 23.8k
2
votes
On the non-quasistatic transfer of heat
If you equilibrate the two spheres using a quasi-static path or an irreversible path, in both cases you end up with the same temperature, $(T_H+T_C)/2$. So the entropy changes of each of the two ...
- 32.8k
2
votes
Cup of warm water suspended in a pot of water held at a steady boil
Indeed, the boiling point of water is 100°C, thus to clarify, the reason isn't that the water inside the cup is salt water.
Firstly, you have to understand that extra energy is needed for water to ...
- 193
2
votes
Accepted
Heat equation and discontinuous thermal conductivity in the finite volume method
Put a grid point $x_j$ at the interface between the two regions. Then $$k^+\frac{T_{j+1}-T_j}{\Delta x}=k^-\frac{T_j-T_{j-1}}{\Delta x}$$That is, the heat flux is continuous at the interface.
- 32.8k
2
votes
Accepted
How does wetting cotton increase its UV transparency?
Cotton fibers are somewhat transparent, and much of the interaction of light
with those fibers is scattering at the air/cotton interface.
A wet cotton fiber has a water/cotton interface instead of
air/...
- 9,518
1
vote
Accepted
Thermodynamics: A piston can freely move inside a horizontal cylinder which is closed from both ends
I assume that the two subvolumes separated by the piston are a single closed, isolated system. The answer is affirmative. However, it requires different justifications according to the nature of the ...
1
vote
Rate of Thermal Conduction between different materials
You could use a formula known as Fourier's Law, which essentially states that
$Q = -k \cdot A \cdot \frac{dT}{dx}$
Where:
Q is the rate of heat transfer (in watts, W).
k is the thermal conductivity ...
- 258
1
vote
Accepted
How does a counterflow heat exchanger achieve efficiency of 0.5 and greater?
This is the trick of a counter flow heat exchanger, the air streams arent in thermal contact when both are at 20°C.
In a parallel flow, 20°C would be the temperature both streams approach as they ...
- 15.2k
1
vote
Heat being transferred from colder object to hotter object
What doesn't happen is a spontaneous flow from a colder to a hotter body, and it is an experimental fact. For example, a cold beer and warm beef on a table tends to evolve to the ambient temperature. ...
- 15.8k
1
vote
Heat being transferred from colder object to hotter object
Heat is energy transfer due solely to temperature difference.
The kinetic temperature of an object can be considered a measure of the average translational kinetic energy (KE) of the atoms and ...
- 69k
1
vote
Heat being transferred from colder object to hotter object
In the realm of macroscopic systems used for measuring heat flow, the likelihood of heat moving from a colder object to a hotter one is incredibly slim, practically approaching zero. As a result, we ...
- 11
1
vote
Heat equation and discontinuous thermal conductivity in the finite volume method
The scheme appears applicable, as discussed here. First, from Chet Miller's answer and comments the divergence theorem must hold and the flux be cotinuously differentiable, so at boundary the quantity ...
1
vote
Heat flux in the negative radial direction in a hollow cylinder
If the temperature outside is higher than inside, then the hear flux will be in the negative r direction: $$\mathbf{q}=-k\frac{dT}{dr}\mathbf{i}_r$$And, since the total heat flow Q is constant within ...
- 32.8k
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