Questions tagged [thermal-conductivity]

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Tea, Heat capacity and entropy

On a Thursday morning, one of my university friends prepares for his thermodynamics exam on the following day. He has been wondering recently that entropy has the same unit as heat capacity, but ...
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Heat Generation in Watts per cubic meter - how to apply this to a volume?

Water is running through a pipe. The pipe is heated uniformly in order to heat the water running through the pipe. I want to calculate how much heat needs to be generated by the pipe in order to heat ...
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Joule Heating, Heat generating in the middle?

I work with resistance brazing and RSW on a daily basis. However there is still one thing i can't understand and its why heat seems to be generated in the middle resistance. This is a basic diagram ...
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3answers
728 views

How can plasma torches reach $28000°\text{C}$?

According to this the plasma of a plasma torch can reach up to $28000$ degrees Centigrade. How is it possible for the plasma to reach a temperature much higher than that of The Sun's surface and not ...
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325 views

Why does a salt water solution have a higher coefficient of thermal expansion (i.e. expands more) than distilled water?

I have found several different sources of data which show that a solution with a higher concentration of salt has a higher coefficient of volumetric thermal expansion than a lower concentration salt ...
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2answers
20 views

Does aluminium foil still reflect heat if covered?

If I place a foil covered sheet of board in my window to reflect heat back into the room, will closing the shutters and pulling the curtains over it render it useless ? Will foil on the other side ...
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Calculating new temperature of an object when air temperature changes

I'm trying to calculate the new temperature of an object when the air temperature around it changes, given a period of time. Basically I get periodic readings from an air temperature sensor in a ...
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1answer
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Solid state problems (phonons conductivity)

I took a quiz where i got two answers wrong and i want to know what the correct answer is and why. Q1) In a material where both phonons and electrons contribute to the thermal conductivity, the ...
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2answers
58 views

What is the drop in temperature across a fixed length of pipe with water flowing through it?

For a project, I am looking to calculate the drop in temperature across a length, $L$ of PVC pipe of diameter, $d$. Water, initially at temperature $T_1$ enters the pipe with a constant flow rate of $...
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1answer
42 views

Which heat capacity is used in the heat equation?

The heat equation is often written as $\frac{\partial T}{\partial t} = \frac{\kappa}{c} \nabla^2T$ where $\kappa$ is the thermal conductivity and $c$ is a heat capacity per volume. I often see $c$ ...
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722 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 unit ...
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1answer
149 views

Does a crushed ice cube and a regular ice cube have different potential energies?

For a question on an heat transfer experiment on melting ice cubes in water and measuring the temperature decrease in water, there was a question that asked: Suggest the effect, if any, of crushing ...
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536 views

Convective heat transfer coefficients - how do you calculate them?

How do you calculate (or otherwise determine) $h_\text{inner}$ and $h_\text{outer}$ (convective heat transfer coefficients) for a heat exchanger? For reference, the standard equation is: $$ \text{UA}~...
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38 views

What's the best kind of a stone slab to place under electronic devices to keep them cool?

What's the best slab of stone to place under electronic devices to keep them cool? Each stone has its own thermal profile which includes its thermal diffusivity, thermal conductivity, and thermal ...
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1answer
40 views

Statistical definition of temperature

It is written in the book(Concept in Thermal Physics by Blundell) that A system will appear to choose a macroscopic configuration that maximizes the number of microstates of the system. And this ...
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1answer
43 views

Increase in internal energy and temperature of a thermodynamics system [closed]

A banquet hall ($150 m^3$) is to be used for a formal dinner for 20 persons. Each person occupies $0.075 m^3$ of space and has an average heat transfer rate of $500 \frac{kJ}{hr}$. If the AC system ...
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1answer
39 views

Stirring the Pot: A Simple Question About Heat-Transfer

I was preparing hummingbird food. To get the feeders filled and hung quickly, I placed the pot in a sink with a similar or greater volume of cool water in it. I hypothesize that absent any other ...
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2answers
37 views

Why can't information be conducted out of a black hole? [closed]

Why can't information be conducted out of a black hole? Does gravity affect conduction? What is the temperature profile in equilibrium of a metal pole half in, half out of a very large BH such that ...
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1answer
36 views

How to prove that a set of derived closed-form equations for a heat exchanger do not violate the second law of thermodynamics? [closed]

Recently I have been working on a problem involving a cross-flow heat exchanger (Two fluids flow orthogonal to each other on either side of a separating wall and exchange heat with each other through ...
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2answers
658 views

Wiedemann-Franz law

The Wiedemann-Franz law states that the ratio of thermal conductivity $\kappa$ and electrical conductivity $\sigma$ for metals fairly accurately obeys $\kappa/\sigma = LT$, where $T$ is the ...
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Why does holding a hot object with a cloth make it feel less hot?

Let's say that I held a hot object with a warm cloth. It instantly feels less hot and only warm to the touch. This is because the cloth is an insulator and doesn't allow as large a heat transfer as if ...
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1answer
461 views

Material thickness, thermal conductivity and electrical resistivity

I have a plastic housing (for a printed circuit board- PCB) in the shape of a box (a *b *c) with L (thickness). Its material properties are: *thermal conductivity: $k_h~[...
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1answer
23 views

Why heat flux is not related with specific heat capacity?

Rate of heat transfer across a solid material kept between two environments of different temperature (Assume a wall of a house) is mentioned to be dependent on the temperature gradient, length, and ...
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3answers
47 views

A thermally conductive gap filler does not conduct the heat as expected. Where does the heat go?

I have a mug warmer, which is a hot plate that I've measured heats up to 56C. It came with a mug with a flat bottom, which gets its contents warmed up to about 53C after some minutes. My understanding ...
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23 views

Heat relation of a isobaric expansion

A piston closed at the top contains $n=0,2$ $mol$ of a mono-atomic ideal gas at the temperature of $T_0$. The pressure of the gas is constant during the transformation and we don't have to consider ...
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32 views

What are operators for electric and thermal current in quantum mechanics?

Using Green's function formalism, I have derived a formula that can measure the expectation value of any operator $\hat{O}$ under temperature gradient $\nabla T$. The formula is more or less as ...
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1answer
46 views

Temperature of air coming out of a tyre due to a burst

I saw several questions where I had to calculate the temperature of the air coming out of a tyre due to a burst, given the external atmospheric pressure and the initial state of the gas present in the ...
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3answers
121 views

Are there thermodynamic obstacles to thermoelectric generation on the surface of Venus?

There has recently been a NASA design challenge for a clockwork Venusian terrain navigation system which uses no sensitive electronics or hydraulics. I was thinking that rovers and probes on Venus ...
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Convection/conduction simulation run from equal temp, but losing heat

I was investigating the effects of heated wall materials on outdoor air temperature and indoor air temperature with a certain software. The system is as the following: 1x1x1 meter cube of outdoor air ...
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1answer
362 views

Air gap in a radiant barrier insulation system: why?

It's generally accepted that a reflective surface, when used to insulate a system by reflecting radiant energy back to it, must have an air gap in front of it. Why exactly is this, and what determines ...
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How to calculate vapor quality in a heated annular channel

Say I have a pipe of length L and an inner diameter of ID. Inside is a heated rod of diameter OD that is uniformly heated with power P. I have water flowing through the pipe with mass flow rate G ...
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1answer
50 views

Ideal gas equation for adiabatic process

In an adiabatic expansion or compression of gases, does the ideal gas equation change from $PV=nRT$ to $PV^{\gamma} = nRT$? If so, then why as pressure and volume are inversely related in both the ...
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Does the Mpemba Effect affect none-water substances?

Does the Mpemba Effect work on liquids other than water? If so, what are the similarities/differences, and what contributes to it(mainly the differences)?
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1answer
32 views

Why does thermal conductivity decrease from solids to gases?

I am learning in my heat transfer class that the thermal conductivity decreases from solids to liquids to gases i.e. $$K_{solid} > K_{liquid}>K_{gas}$$ The reason cited is that (based on my ...
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28 views

Increasing boiling point without change in physical conditions

Recently while going through the chapter calorimetry of class 10 I came across a passage saying "if we add impurities to water such as salt in proper proportion the boiling point of water ...
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45 views

Lagrange Equation for thermal system

I am working on modeling of a multi-physical system using Lagrange based modeling and we know that Lagrange function, $L$, can be defined as below; $L= T-V$ where: $T=$Kinetic Energy $V=$Potential ...
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32 views

Best color for pool cover [closed]

I couldn't find a stack* forum for pools, so I hope this is the next best place to post. I have an 18 foot diameter above ground pool in the north west, and I don't like the pool cover that came with ...
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1answer
337 views

What is the overall rate of heat conduction through two pipes of different dimensions if the thermal conductivity is not constant?

I am trying to work out how I would find the rate of heat transfer through two pipes made of the same material that are joined at their ends but which have different cross-sectional areas and lengths. ...
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How to protect glass from direct, excessive hot air?

I need to use a hot air gun directly next to a one-way mirror glass window which is embedded in a wooden frame. I've read that a heat gun can shatter the glass easily because the excessive direct heat,...
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30 views

What is the physical meaning of ΔG = q(irr) - q(rev)?

Why is change in enthalpy ΔH equal to heat exchanged following whether reversible or irreversible path whereas TΔS is equal to the heat that would be exchanged if the process was reversible? What is ...
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3answers
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Which substance cools faster a glass bottle or the liquid inside it?

I had an argument with a friend who says that the temperature of the glass of a beer decreases faster than the liquid inside. for example if there's a beer bottle in the fridge for 10 minutes the ...
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159 views

Is thermal transfer in a vacuum proportional to temperature difference?

I would have thought that heat dissipation was proportional to temperature difference, especially in a vacuum. Is this true?
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Does paint affect the thermal conductivity of a metal?

We noticed that in our experiment that painting a metal increases its thermal conductivity; is this true? If so, can you guys send me a link to a research paper to support this claim.
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Materials to Insulate Against Convection, But Not Radiation

Are there any materials which could effectively insulate against convection (and/or conduction), that don't insulate against radiation? Specifically, I want to make a water-heater that uses solar ...
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How many dry ice produced by a 5lb co2 cylinder at 860 psi and room temperature (30 C)?

just wonder how many lb dry ice could be produced from $5$ lb $CO_2$ cylinder at $860$ psi and $30$ C , assuming no recyle co2 stream) ?
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1answer
40 views

Heat absorbed from a reservoir by a moving object [closed]

Imagine two bodies, say a body A with an infinite heat capacity(reservoir) and the another body B with some finite heat capacity($C = \alpha \ T_{B}(t) $). They come into direct contact with each ...
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2answers
50 views

Melting ice under pressure

If we put a cube of metal on ice (ice with a certain thickness), under which conditions the cube will penetrate the ice? and how can we calculate the velocity of penetration?
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Nusselt number for a rectangular fin under forced convention

I am looking for the correct Nusselt Equation to calculate the heat transfer coefficient for a rectangular fin acted upon by air with a certain speed
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21k views

How long can a thermal flask hold heat?

After reading this question asking why kettles aren't insulated, (a question I had myself) I questioned, "What about people who boil the kettle 2~5 times a day?". So, my question is, if we were to ...
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45 views

Heat equation with a source and homogenous boundary and initial conditions

I am trying to solve the following: $$\frac{\partial^2u}{\partial x^2}-3\frac{\partial u}{\partial t}=-9$$ $$u(0,t)=0=u(\pi,t)$$ $$u(x,0)=0$$ So solving the homogenous case first by separation of ...

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