Heat is energy transferred from one system to another by thermal interaction. In contrast to work, heat is always accompanied by a transfer of entropy. Heat flow is characteristic of macroscopic objects and systems, but its origin and properties can be understood in terms of their microscopic ...
14
votes
1answer
363 views
Is there some way to narrow down the Leidenfrost point for water?
Cooks sometimes use the Leidenfrost effect to estimate the temperature of a frying pan by flicking a few drops of water onto the heated pan. I had no idea, before looking into this, that this could be ...
8
votes
2answers
654 views
Do photons lose energy while travelling through space? Or why are planets closer to the sun warmer?
My train of thought was the following:
The Earth orbiting the Sun is at times 5 million kilometers closer to it than others, but this is almost irrelevant to the seasons.
Instead, the temperature ...
2
votes
3answers
150 views
Why is an air conditioner more efficient in a low-thermal-mass house?
Why is an air conditioner more efficient in a low-thermal-mass house?
I recently read
To get these efficiency gains it is important to use the air
conditioner as it is intended: the unit has to ...
1
vote
3answers
219 views
Can one heat up a vacuum?
I've got a question about heating a vacuum. If there were, say, a container in space, at 2.7 degrees kelvin (the typical temperature of space, if I'm not mistaken) and as empty as space (as close to ...
3
votes
0answers
77 views
Thermal imaging camera [closed]
I'm making a tea bag experiment (Make a hollow cylindrical tube from light paper e.g. from an empty tea bag. When the top end of the cylinder is lit, it takes off.) I need to know how hot air is ...
3
votes
1answer
116 views
If layers of insulation are continuously added to a heated object, will it continue to be better insulated?
If you were to keep adding layers of insulation to something, like blankets to a person, would each blanket continue to improve the insulation? Or do you reach some point at which the next blanket ...
3
votes
0answers
45 views
Photon pumping in Laser
Let's consider a ring laser where the laser must pass through the gain material before it is sent toward a partially reflective surface $\ R=1-T $. The other mirrors are perfect reflectors with $\ ...
2
votes
0answers
37 views
In a non-degenerate plasma, why are e-e collision negligible compared to e-ion for thermal conduction?
I'm trying to make some order of magnitude estimates of heat transfer in stars - to better understand 1) why conduction is said to be negligible (for non-degenerate matter) and 2) when convection ...
0
votes
2answers
100 views
Stealing heat, or not?
Say I hook a 1KW steam engine to the steam heat in my apartment, and generate 1KW of electricity (the engine is 1KW mechanical, not 1KW thermal) from it, so I don't have to pay ConEd.
That means I ...
1
vote
1answer
93 views
Is $E=mc^2$ reserved to nuclear physics?
I was wondering, while putting a log in my fireplace, how much energy the piece of wood would give. The most famous formula poped into my head: $E=m \cdot c ^ 2$!
Is this formula applicable to a ...
0
votes
0answers
134 views
Constant volume specific heat
A monatomic ideal gas is confined to move in two dimensions. What is the constant volume specific heat for this gas?
Consider a system of N independent harmonic oscillators moving in two dimensions. ...
1
vote
0answers
52 views
Can I estimate the long-term surface heat transfer coefficient based on a temperature difference between the air surface and ground surface?
I want to estimate the long-term or annual heat transfer coefficient for the earth's surface in a particular area where the mean annual air surface temperature is about 13$^\circ$C, and the mean ...
-2
votes
1answer
231 views
Why do phonons cause excellent heat conduction in diamonds?
Phonons are the quantum of lattice vibrations in crystals and are not to be confused with photons, the gauge bosons of the electromagnetic force. Apparently, they contribute to heat conduction, but I ...
5
votes
0answers
187 views
Where to place a fan in order to cool a room the fastest [closed]
Imagine I have room that is very hot and it is colder outside. I want to cool it down using a fan.
Scenario 1
There is only one door. I have one fan.
Where should I put it and which way should it ...
0
votes
1answer
73 views
What is the difference of work $W$ and thermal energy $Q$ in thermodynamic Stirling-process for ideal gas?
What is the difference of work $W$ and thermal energy $Q$ in thermodynamic Stirling-process (in simple form) for ideal gas?
I think that you need work to preserve this process and you bring thermal ...
3
votes
1answer
149 views
Rayleigh-Benard Convection
I found this nice paper about RB convection. However I am confused by what is going on page 6. In particular why we are suddenly using Helmholtz equation to find spatially periodic solutions. Aren't ...
-2
votes
1answer
245 views
Where is the candle hottest?
Where is the hottest spot above a lit candle? In the flame or just above the flame tip or some cm above the flame or other?
Update
My question more precisely concerns the heat energy I can get from ...
3
votes
2answers
653 views
How air humidity affects how much time is needed for heating the air?
In cold weathers it is suggested to put a humidifier since the air gets too dry. I wonder how the humidity affects how much time is needed to get the air at a temperature of 20 Celsius degrees? I mean ...
0
votes
3answers
783 views
Does closing curtains 'make your home warmer'?
I mean, in the sense that the act of closing curtains would somehow reduce the amount of heat loss of the house to the outside, thus making it warmer for a given supply of heating.
4
votes
0answers
124 views
Thermodynamic relations from Gibbs-Duhem
Given the Gibbs-Duhem relation
$V dp = S dT - N d \mu$, I am having trouble deriving the following identity:
$\ (\frac{\partial N}{\partial \mu})_{V,T} = N (\frac{\partial \rho}{\partial p})_T$
...
3
votes
1answer
332 views
Why do some metal containers not conduct heat, while some do?
Some metal containers such as the Nissan Thermos ones, even if 100 C water is filled inside, the container is still cold to the touch on the outside. It won't be even warm:
At the same time, some ...
2
votes
2answers
353 views
Possible colors of fire?
I have learnt that depending on the various gases those are involved in the reaction that produces fire, different colors (yellow, red or blue) of flames become visible.
I have a question .. what are ...
5
votes
4answers
201 views
what is the basic form of the 'fire'? [duplicate]
Possible Duplicate:
Is fire matter or energy?
What is the basic form of fire? physics defines every entity by a basic form either solid or liquid or as a gas,
example: water is liquid,
ice ...
4
votes
3answers
369 views
Can I take heat from the air and convert it to electricity?
Its a summer day and the air in my house has been heated up. I could switch on my air conditioning, but then I'd be using energy from the grid in order to reduce the amount of energy in my house.
...
0
votes
0answers
79 views
How can I measure the calories consumed in daily activity? [closed]
From a nice question here:
How are the calories in food calculated?
I can roughly understand how people provide the data of calories in certain food.
However, I am still confused about 2 things.
1) ...
0
votes
2answers
2k views
What is the characteristic length of a cylinder
I have a cold cylinder that is submerged in hot water and I need to find the convective heat transfer coefficient. I can do the whole process but I am stuck finding the characteristic length. I found ...
2
votes
2answers
129 views
How cold should it be outside for a hot coffee mug to break?
So I like to go outside for a morning coffee with a cigarette. In winter here it's usually between -5C to -25C, and sometimes it gets down to -30C and colder. Assuming that my coffee is about 75-80C, ...
5
votes
4answers
1k views
After what speed air friction starts to heat up an object?
I understand that air friction cools off an object at low speeds. For example, if you blow on a spoon of hot soup, it cools off. Or if you swing a hot frying pan in the air, it cools off faster.
But ...
7
votes
4answers
738 views
Why does pizza cheese seem hotter than the crust?
When I eat hot pizza or a melted cheese sandwich, the cheese feels a lot hotter than the crust or bread: in particular, the cheese might scald the roof of my mouth. but the crust will not. Is this
...
5
votes
1answer
105 views
Which came first, movement or heat?
According to my measly understanding of the universe, when particles hit one another, some of their kinetic energy is transformed into heat. But when we heat particles (for instance, putting a bucket ...
0
votes
1answer
501 views
Heat Exchanger Calculation
I have a tank of oil at 55 degrees c. I plan to run a copper pipe 8mm in diameter (1mm thickness) into a coil 15m long inside the tank. For all purposes of assumption, the copper pipe is perfectly ...
0
votes
1answer
139 views
What is the ion drag mechanism in dielectric heating?
While reading about dielectric heating on Wikipedia, I read about the ion drag mechanism but there wasn't enough information about.
I know there is another Phys.SE question talking about the ion drag ...
3
votes
1answer
196 views
Why do non-stick frying pans work?
Modern non-stick frying pans use a mixture of titanium and ceramic that is sandblasted onto the pan surface, and then fired to 2,000 °C (according to Wikipedia).
Can anyone explain (at the molecular ...
1
vote
1answer
164 views
What arrangement of sound waves would be needed to heat air in a typical sized room?
From what I understand, sound is simply the jostling of the molecules that make up the air in a specific pattern, widely known as waves. I also know that these are longitudinal waves. If we were to ...
-1
votes
1answer
104 views
I need help with this question on Heat Capacity
A calorimeter has a Heat Capacity of $70 J/K$. There is $150g$ water with a temperature of $20^oC$ in this calorimeter. In this, you put a metal cube of $60g$ with a temperature of $100^oC$. The ...
4
votes
1answer
140 views
What would jumping into a pool and feeling cold be called? Conduction, or convection?
This was another question from my son's workbook. It said:
...
1
vote
0answers
169 views
Simple heat transfer question [closed]
You add an unknown volume of milk of $5.2 ^\circ C$ to a cup of coffee ($40 mL$ of water, temperature: $80.3 ^\circ C$). After a while of stirring the temperature reaches $73.2 ^\circ C$. The ...
4
votes
4answers
4k views
The difference between heat and temperature
So as I understand it, heat energy of an object is the SUM of all the kinetic
energies of the molecules of the object (upto constant factor).
The temperature on the other hand is the AVERAGE of the ...
1
vote
2answers
165 views
How does blow-drying a mirror keep it from steaming up again?
After a hot shower, the mirror in my bathroom steams up. When I try to clear it with a towel, it immediately refogs. Yet once I use my hair-dryer, it will clear the fog and the mirror will stay clear.
...
2
votes
1answer
72 views
Could we really charge metal plates using microwaves?
While skimming through Dielectric heating, I read that they use microwaves to charge the plates. How do they do that?
1
vote
2answers
404 views
What causes the vacuum in my bento box?
I can't think of a good title for my post, sorry about that.
I've got a lunch box (called a bento box) Basically it's a plastic box with a plastic lid with a rubber rim around the lid to create an ...
1
vote
4answers
353 views
Physics of a burning log of firewood
According to my knowledge, heat is nothing but the result of the vibrations of atoms and molecules. I guess this mean that in heating up a gas or liquid, we are increasing the rate at which the ...
21
votes
1answer
731 views
Energy in bolognese reduction - lid on or off?
Generally, to let my bolognese thicken, I leave the lid off in order to "let water vapor escape." I am however distracted from enjoying the taste because I'm having doubts that my physical reasoning ...
0
votes
0answers
56 views
Experimental Physics [closed]
A heater and thermocouple are used to measure and control the temp. T of a sample at $T_{0}=250^{o}C$. A feedback circuit supplies power P to the heater according to the equation $P=P_{0}+G(T_{0}-T)-D ...
-3
votes
3answers
181 views
What forces are at play when molecules wiggle (due to heat)?
What forces are at play when molecules wiggle (due to heat)?
Or in other words, What makes them move?
3
votes
1answer
172 views
How to properly bake a ultra high vacuum chamber?
I need to get rid of water excess in my vacuum chamber, and for that there is the procedure of baking. In order to do that there are several things that one needs to consider, the power, heat load, ...
0
votes
1answer
175 views
Does a concave or flat bottom pan use heat more efficiently?
This may be anecdotal.
Playing in the kitchen I realized the frying pan comes with both a flat, and a concave bottom. So here's the question -
Given two pans made of brass, one has a concave base ...
0
votes
0answers
149 views
Air condition and net heat [closed]
I have attempted to solve a problem, but my answer is not quite the same as in the book. Thus, I would really appreciate it if someone could confirm/disconfirm whether or not my solution is correct.
...
1
vote
1answer
94 views
Physics of homebrewing heat exchangers
In homebrewing on of the key steps when brewing with extract is to rapidly chill the wort from boiling temperatures to about 80F in 30 min. This is needed to reduce risk of environmental ...
3
votes
1answer
94 views
Which heated, partially filled bottle will explode first?
This is in reference to a pasteurization discussion on a homebrewing forum.
I have four closed bottles which will explode if containing too much pressure. Two of them are 50% full (A and B), and two ...
