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 ...
16
votes
2answers
476 views
How hot does the tip of a pencil get while writing?
When writing with a pencil, there seems to be quite a lot of friction - which seems like it would induce heat. How hot would the tip of a #2 pencil get writing on normal copy paper?
2
votes
1answer
20 views
Solid in Liquid Heat Transfer
If there is a solid immersed in a large (but finite) pool of water, where the solid has temperature $T_s$ and the water has temperature $T_w$, with $T_w>T_s$, how can I calculate $T_s(t)$ and ...
-1
votes
0answers
32 views
Improve heat transfer on frying pan design [closed]
I have a design for an industrial frying pan which will have $T_{Oil} = 500 K$
So the idea is to submerge copper pipes with this dimensions:
$thickness = 5 mm$
$inside_{diameter} = 50 mm$
...
10
votes
3answers
518 views
Best way to chill a cup of coffee with cold water and 5 minutes [duplicate]
Initial data
1 x 3/4 full cup of hot coffee / tea / your favorite morning beverage
cold water
5 minutes
Considering that it's starting to get hot outside, and we all want to drink reasonably cold ...
6
votes
1answer
116 views
Why does asphalt crack along painted lines?
My elementary school playground was made of asphalt and had a track painted on with white lines. While walking on the track, I noticed that cracks in the asphalt often followed the paint.
The ...
0
votes
1answer
24 views
Electricity directly from heating a material [duplicate]
I am looking for some more information about how to obtain electricity from heat directly. This e.g. involves the Seebeck effect, as I have found it is called, where a material produces a voltage ...
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 ...
6
votes
1answer
1k views
LED Thermal Modeling (How to solve heat equation with constant heat source)
I have a mechanical design with LEDs that generate heat. I want to estimate the temperature at the LED junction vs. time, but especially at steady state.
Knowing the LED voltage drop and current, I ...
1
vote
1answer
41 views
1/2 freeze a water bottle
Is it possible to quick freeze half of an unopened bottle of water or soda by putting half of the bottle in a subfreezing solution liquid nitrogen or other solution without freezing the whole bottle?
4
votes
1answer
47 views
Fluids in thermodynamic equlibrium
I am reading about the Euler Equations of Fluid dynamics from
Leveque's numerical methods for conservation laws.
After introducing the mass, momentum and energy equations, some thermodynamic
...
-1
votes
1answer
90 views
The effects of heat on gravitational fields
In boiling soapy water, globs of soap coalesce as the temperature increases to boiling. Does this mean that temperature increases the gravitational pull of bodies?
3
votes
2answers
103 views
Why does the Sun feel hotter through a window?
I have this big window in my room that the Sun shines through every morning. When I wake up I usually notice that the Sunlight coming through my window feels hot. Much hotter than it normally does ...
3
votes
3answers
543 views
Why the heat flux vector at a point must be perpendicular to the temperature isothermal surface? Is it a definition or a deduction?
Before the question: I am working on numerical calculation of three dimension parabolic equation that based on Fourier's Law of which I am a little confused.
Here comes the law in modern mathematics ...
0
votes
3answers
140 views
How do you determine the heat transfer from a P-V diagram?
I doubt this question has been addressed properly before, but if there are similar answers, do direct them to me.
I am currently studying the First Law of Thermodynamics, which includes the p-V ...
5
votes
1answer
88 views
What exactly heats a pool of water on a sunny day
On a sunny day an outdoor swimming pool will heat up fairly quickly. My question is, what is the exact mechanism for this and can we put numerical figures on it?
Given that water is clear and ...
0
votes
1answer
49 views
How to calculate incident solar energy on earth in a given day with a spectral filter
The daily solar radiation irradiation is often quoted as being between $3.2 \text{ kW hours}/\text{m}^2$ and $7 \text{kW hours}/\text{m}^2$ per day . If you filter out a range of wavelengths, how can ...
0
votes
1answer
89 views
Is the heat required to alter the Higgs field an 'absolute heat'?
I have read and heard that manipulating the Higgs field would require heating up a local geometry to ridiculous temperature. I am trying to understand if there are stars or places in the universe ...
4
votes
5answers
213 views
Having a problem about entropy, thermodynamics
I am a high school student. So, while studying about thermodynamics, I got a little curious about entropy. As I read, entropy is the rate of change of chaos. So, if the entropy change of a system is ...
1
vote
1answer
44 views
Is heat flux density and heat flux the same thing?
Heat flux and heat flux density is the same thing, while electric flux density and electric flux is not the same thing?
It makes me confused since we compare Fourier's law with Ohm's law.
Here is a ...
4
votes
1answer
68 views
How can I understand a Vortex Tube and its efficiency?
A Vortex Tube takes a pressurized input stream, most typically of a gas, and creates two output streams with a temperature differential. Apparently, it has been described as a Maxwell's Demon.
Both ...
0
votes
1answer
55 views
A really simple question about heat transfer? [closed]
In rectangular coordinates, the heat conduction vector can be expressed in terms of its components
$$\vec{\dot{Q_n}}=\dot{Q_x}\vec{i}+\dot{Q_y}\vec{j}+\dot{Q_z}\vec{k}$$
Please explain the meaning of ...
1
vote
1answer
44 views
Optimal way to heat liquid from the sun?
I understand that one way to tell how much a liquid will be heated up by a remote heat source is by looking at its color. I assume clear and colorless heats up least, then white, other colors and last ...
2
votes
0answers
51 views
Why does the water diffuse at right angles in my coffee? [closed]
I purchase an espresso shot each morning at my local coffee shop, and I usually water it down slightly to take the edge off as well as to cool it. Recently, I have noticed that when I pour the cold ...
3
votes
2answers
68 views
Do Microwave oven cook times grow linearly with Wattage? Calculating optimal cook time
So this is a completely random and trivial question that was prompted by looking at my microwave oven and the back of a TV dinner and my google searching failed to produce a meaningful answer so ...
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 ...
2
votes
1answer
138 views
Optical element is heated by laser: is it possible to get oscillating heat distribution?
Imagine that we have thin optical element, which is irradiated by laser. Laser heats element, so there is some heat distribution in element. There is a heat sink through upper and lower element faces ...
0
votes
1answer
61 views
Indicators on how even the heat is distributed?
I'm wondering if there are any good indicators on how even the heat is distributed on an object (for simplicity, a flat object maybe)? What are the possibly reasonable ways to maximize the evenness if ...
0
votes
1answer
23 views
heat spread from hot coil: diffusion, radiation?
When modeling the heat spread from a hot object through a vacuum (or a gas), what is the dominant physical process for heat energy transportation? Thermal diffusion, thermal radiation, a mix of the ...
4
votes
3answers
133 views
Room temperature and fan orientation
So I'm in a tiny dorm room and I normally point my fan blowing outside the window to cool my room off. I've been in some debates on blowing air out or in is more effective, so I'm hoping to get some ...
-2
votes
0answers
20 views
Numerical on heat [closed]
Q. A 120 watt heater is switched on for 44 seconds and provides heat to 10g of ice placed in a metallic container of mass 50g. Find the specific heat capacity of the metallic container.
This is what ...
2
votes
1answer
71 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?
0
votes
1answer
86 views
Calculating the coefficient of thermal expansion in liquid
I am trying to write a matlab function that calculates the coefficient of thermal expansion of water from a given temperature. From what I understand the thermal expansion coefficient is calculated as ...
4
votes
1answer
122 views
Mechanical Equivalent of Heat
Recently I have been looking up James Joule's experiment regarding the mechanical equivalent of heat. After viewing some drawings of the apparatus, I assumed that the lines holding the weights would ...
1
vote
0answers
30 views
Heat transfer in a pipe
I have a gas (assuming air) at T = 500 K that enters a cylindrical pipe. The outlet target temperature is 330K.
There will be heat transfer via: Forced convection from the gas to the inside of the ...
0
votes
2answers
77 views
Conceptual difficulty in understanding Fourier's Law of heat conduction
I am having a bit of difficulty understanding the statement of Fourier's law of heat conduction.
As mentioned in Georg Joos , Theoretical Physics
The quantity of heat passing per second through ...
1
vote
1answer
35 views
Do metals generally lose more heat by convection than non-metals?
I've been experimenting with radiative cooling lately and in my mind convection is always an enemy (especially forced convection since the modules are outdoors).
Looking at Newton's law of cooling, ...
2
votes
2answers
59 views
Why increase pressure to go from liquid to steam?
Assume the following phase diagram $T-H$ of water
At a nuclear engineering course, I was told that in order to increase the performance of a pressurized water reactor, one has to increase the ...
4
votes
1answer
92 views
Physical interpretation of different boundary conditions for heat equation
When solving the heat equation,
$$
\partial_t u -\Delta u = f \text{ on } \Omega
$$
what physical situations are represented by the following boundary conditions (on $\partial \Omega$)?
$u=g$ ...
1
vote
1answer
35 views
Adding heat to a system doesn't equal the work done by gas?
So, I answered a physics question for a class that goes as follows:
A gas in a cylinder is kept at a constant pressure of $250000\: \mathrm{Pa}$ while $300\: \mathrm{kJ}$ of heat are added to it, ...
0
votes
1answer
39 views
How long does the 2nd pot of water take to boil right after the 1st one finishes?
Say I have a pot of water that boils in 20 minutes, at whatever temperature.
If I leave the fire on, take the pot off, pour the hot water into a container, refill the pot with tap water and put it ...
0
votes
0answers
28 views
Constant Pressure lines in evaporation diagram
Assume that there is the following $T-H$ phase diagram with the constant pressure lines.
Which is greater preasure? The preasure drops as we move to lower temperatures (i.e. from top to bottom)?
...
6
votes
2answers
525 views
Why isn't the Earth's core temperature the average of its surface temperatures?
Assuming that the earth is spherical, that its temperature is continuous, and that some other more or less realistic conditions hold, we might think that the Earth's core temperature should be about ...
0
votes
1answer
45 views
Finding cooling time using an impulse of heat energy
A chip has a thermal resistance of 0.007 K/J s and thermal capacitance
of 1.6 J/K. You give it an impulse of heat equal to 100 J. How long
does it take for the chip to cool to within 10% of the ...
1
vote
0answers
42 views
Calculate heat loss of aquifer with unknown shape
I am trying to calculate the heat loss of an aquifer. An aquifer is basically a hot-cold storage $80\: \mathrm{meters}$ under the ground. It is an open system and utilizes groundwater. By pumping for ...
0
votes
2answers
173 views
Does more reflective aluminum foil make a room cooler compared to less reflective foil?
Aluminum foil is said to be not absorbing light at all.
It reflects light. So, does it mean that a more shiny aluminum foil will reflect more light and thus make the room more cooler as compared to ...
0
votes
1answer
66 views
Time constant of ice melt
I'm familiar with problems of "how much ice can you melt given some amount of energy", but I'm writing to get some clarification on the time constant of this event. This question might be somewhat ...
3
votes
1answer
58 views
Microwave oven heating time
It's logical to think that the time it takes a microwave to heat the food would be proportional to the mass heated. But since a microwave is based on dielectric heating, I think that if you increase ...
0
votes
2answers
56 views
Dividing values with units
I'm reading about the subject of heat in a basic physics book. If I am not mistaken the formula to work out how much energy is required to increase the temperature of water is
...
2
votes
3answers
90 views
Question about the proof that heat capacity goes to zero if temperature approaches $0K$
I don't completely understand the proof that is given for the claim that the heat capacity goes to zero, if the temperature approaches $0K$.
They do it as follows, if $C_x$ is the heat capacity where ...
1
vote
1answer
473 views
Finding electric power generated using heat transfer
I'm working through an example I have been given to study. Suppose I have a 2m X 4m photovoltaic panel on my roof that is irradiated with a solar flux of $G_s = 700W/m^2$.
Given:
$\alpha_s = 0.83$
...
