Questions tagged [heat-conduction]

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69
votes
3answers
6k views

Why does a lot of water vapour come suddenly after the heat source of boiling water is removed? [duplicate]

I have noticed this several times. When I am boiling water, a few seconds before its boiling point, vapours are formed as usual. But if I turn the gas off before boiling, the moment it turns off, I ...
42
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7answers
5k views

Bush fires and heat waves, the real mechanics?

Like many others, I have been following the sad development of the bush/forest fires in Australia recently. A claim that gets repeated is that one of the contributors to this blaze is the ongoing >45°...
38
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4answers
17k views

Why does wrapping aluminium foil around my food help it keep warm, even though aluminium is a good conductor?

Aluminium being such a good conductor, how is it possible that it is helping me keep my food warm ?? Because ultimately it should conduct the heat that is inside to the outside for exchange and ...
19
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2answers
4k views

Why doesn't a mercury thermometer follow the rules of volume dilatation?

let's consider a classic mercury thermometer. I do not understand why it does not behave like a "normal" thermometer which exploits volume dilatation. In a normal thermometer, I'd say that ...
12
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4answers
2k views

Why would frozen sausages defrost faster in water?

My mom uses frozen sausages for a soup. She defrosts frozen sausages by submerging them in water (room temperature I believe). She claims this makes them defrost faster. She learned this from some ...
11
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5answers
833 views

Physical meaning of potential in heat equation

I'm working on the mathematical theory of parabolic equations. The prototype of such equations is heat equation given as follows : Let $\Omega$ be a bounded region of the space and $T>0$ a fixed ...
8
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2answers
2k views

Heating cup in microwave?

I heated my milk cup in the microwave today and noticed that the cup was hot but not the handle. Even if I heat it too much , cups handle temperature remains the same. How is that possible?
5
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2answers
257 views

A question about thermal conductivity and heat transfer

Put a piece of cold pizza on top of a sheet of aluminum foil and then stick it in the oven to heat up. After about 10 minutes, the pizza should be nice and hot—the aluminum foil is the approximately ...
5
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3answers
4k views

If steam and water, both at 100 °C, were exposed to a person's skin for a small time interval, which would result in a more severe burn?

Imagine that you had two boxes with of steam and water of equal mass, both at $100~^\circ\rm C$, and you decide to expose your left land to the steam and your right to the water for a short period of ...
4
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4answers
84 views

Can a heated surface heat a solid on it above the temperature of the surface itself?

Let's say for example that I have a ceramic stove-top that heats to a measured maximum of $400\ \mathrm{^\circ F}$. I have a skillet of some unknown material that should not be heated above $400\ \...
4
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1answer
187 views

General relativistic heat conduction equation

I read this wikipedia article https://en.wikipedia.org/wiki/Relativistic_heat_conduction It mentions the relativistic heat conduction equation: $$\frac{\partial \theta}{\partial t}=\Box\, \theta$$ ...
3
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4answers
459 views

Why is a temperature gradient set up in a heated rod? [duplicate]

Suppose a cylindrical rod is maintained at 100 degree Celsius and the other at 0 degree Celsius. My book says that after reaching "Steady State" the rod will have developed a constant temperature ...
3
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5answers
159 views

Pressure=Heat? Else, Heat=Radiation?

A few days ago, I came up with this paradoxical conundrum: Let's say we have a gas enclosed in a container. Supplying heat to it will cause an increase in the (magnitude of?) vibration of gas ...
3
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1answer
1k views

Will increase in flow rate increase the rate of cooling down?

Suppose a hot body is surrounded by a coil of very good thermally conducting pipe with cold water flowing inside which comes at a certain fixed low temperature in the pipe and leaves at some higher ...
3
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1answer
28 views

Stefan Boltzmann law and absorptivity

I have this really big confusion , and it is about the Stefan Boltzmann law Does this law works for absorption? Or is it only radiation I have seen my textbook replacing emissivity with absorptivity ...
3
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1answer
20 views

Proof for net heat flow law for radiation

So this has been bothering me for quite a time: $$\frac{dq}{dt}=e \sigma A(T_1^4-T_0^4)$$ Net radiation is $emission-radiation$. According to that if you observe: Consider a situation where a body is ...
3
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0answers
65 views

Dimension reducing of heat equation 2D problem

I have two metal objects of different thickness and conductivity: 1) thicker, but poorly conductive. It's properties: $k_1, H$ 2) thin and much better conductive. It's properties: $k_2, h$ ...
2
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3answers
31k views

Is heat from a stovetop, transfered through convection, radiation or conduction?

It doesn't appear to be convection, as there are no moving objects (or are there); probably not radiation, so it is conduction? I am confused because it seems that the molecules in the stove does not ...
2
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3answers
485 views

In an adiabatic process, why is it that if work is done very quickly then there is no time for energy transfer to the surrounding?

I don't seem to understand the logic in adiabatic process of how 'if work is done on the system very quickly e.g. compress a piston containing n number of gas particles, there is no time for energy to ...
2
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2answers
38 views

How does Stefan Boltzmann law work for absorption?

I have come across the Stefan Boltzmann law and I have a couple of doubts based on its use in the net heat flow expression $$dq/dt = e A\sigma T^4$$ Now according to my textbook If the temperature of ...
2
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2answers
86 views

Why does porridge start steaming more after the hob is switched off?

I have noticed that when I cook porridge in a saucepan, and stir it with a spatula, it only steams a little bit. But the moment I turn off the (gas) hob underneath, and keep stirring it with a spatula,...
2
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2answers
175 views

At what speed does heat flow in materials?

When we heat one end of an iron rod on the stove, it takes a considerable time for the other end to be heated to the same temperature as the first end. On the other hand, as we know, the atomic ...
2
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1answer
57 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 $c$ ...
2
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2answers
1k views

Calculating heat loss to surroundings from a cube suspended in air?

If you have a cube that is suspended in the middle of the air, how would you calculate the heat loss to its surroundings? For this example, I am assuming the following: Dimensions: 1m×1m×1m ...
2
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1answer
105 views

Is the randomized motion of particles produce heat?

In the show on factomania from gregg foot I've heard that when sun light comes then some gas particles of air get more energy and velocity and as the average kinetic energy of molecules is Temperature ...
2
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1answer
127 views

What causes burns when in contact with hot water?

As I understand it thermal energy (heat) is simply a measure of the kinetic energy of an object (For example : water).Hot water is simply water with a larger kinetic energy in its molecules, right ? ...
2
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1answer
19 views

Different colors and their heat absorption capabilities

I did an experiment in which small canisters (image attached) containing water, were wrapped around with colour papers of different colours were kept under the Sun (image attached) . The different ...
2
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1answer
50 views

Why doesnt equipment melt during manufacturing? [closed]

During many manufacturing process, the materials worked need to reach temps above the melting point of many common materials used in equipment. How is the issue of equipment melting avoided? Ex, how ...
2
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1answer
76 views

Why did my water boil “differently” this time?

Yesterday, I put a pot of water on the stove and set it to boil to make some pasta. I have done this many times but this time the boiling was qualitatively different than normal. Why would it be ...
2
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2answers
228 views

More efficient way to heat liquids

If we consider two processes to heat up a bucket of water as follows:- Taking one bucket and heating it to our desired temperature for V volume. Taking two buckets of half the volume and heating one ...
2
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1answer
38 views

Why does glass break into pieces rather than just melting or cracking

In the given link of a video. The maker puts hot lava on different kind of types of glass . https://youtu.be/wLNBLUm3CZQ my question is why does glass break into innumerable pieces rather than just ...
2
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1answer
94 views

Transient heat conductivity with variable heat flux in 1D

I have a metallic block heated by a heat flux variable in time on one of its faces (let's assume that it is infinite in the other dimensions). I would like to compute the temperature over time at a ...
2
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1answer
125 views

Derivation of heat equation

What are the main physical laws to derive the following heat equation: $$u_t -\Delta u=f(t,x)?$$ I'm wondering about the interpretation of the Laplacian $\Delta$ and its role in heat process.
2
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2answers
2k views

What is temperature as a function of time in Fourier's Law?

Can we find temperature as a function of time using Fourier's Law of thermal conduction? For example: If we have two square parallel plates of a given length, width, and distance apart with one ...
2
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3answers
128 views

Focusing Sunlight with Flat Mirrors- solar barbecue

Say somebody had 48 1sqft glass mirrors (almost 4.5 sq.m total), because these tiles cost about $1 each. If mirrors are all tediously aligned to redirect sunlight onto a Weber BBQ, the air temp inside ...
2
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0answers
24 views

What does it mean for resistance, as it appears in Ohm's law, to be an integral, evaluated over the body as a whole?

In literature I read: The three linear flux laws mentioned are: As seen, a correspondence exist between the hydraulic conductivity $K$, thermal conductivity $\lambda$, and electrical conductivity $\...
2
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1answer
42 views

Diffusion equation in terms of heat equation

I am solving a problem which goes like this: Solve the diffusion equation:$$\frac{\partial u}{\partial t}=\frac{\partial^{2} u}{\partial x^{2}}$$ for $t>0$ and $x \in > \mathbb{R}$, with ...
2
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2answers
62 views

Does heat flow always in the direction of maximum decrease?

I was wondering if at a given gradient temperature heat could flow in any direction satisfying moving from a higher to a lower temperature. Or it can only flow in the direction of gradient? Consider a ...
2
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0answers
182 views

How would one calculate the time it takes for an object immersed in liquid nitrogen to reach a specific temperature?

I have an aluminium object which will be immersed in liquid nitrogen for a specified amount of time. I would like to know, if the object is initially at room temperature, how long it would take for it ...
2
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0answers
146 views

What is the physical limit of thermal insulation?

If we consider a container (at very low) temperature $T_0$ surrounded by some passive structure, in turn surrounded by an environment at temperature $T_1>T_0$, there will be an inward heat flow ...
2
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1answer
295 views

What would the surface temperature of the Earth be without the Sun? [closed]

If the Sun would not heat the earth, and never had heated it in the past, its surface would only be heated from the inside. What would be the current temperature of the surface?
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3answers
87 views

Entropy Change - which formula to use?

I am looking at a system of two solids A and B, that is isolated. A is at a higher temperature; thus, we have heat transfer from A to B. If I wished to calculate the entropy change: $$\Delta S_{tot} = ...
1
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1answer
282 views

How is the 1D transient heat conduction equation derived? [closed]

From my book: $$\frac{\partial T}{\partial t}=\alpha\frac{\partial^2 T}{\partial^2 t}$$ with an initial condition and boundary conditions $$T(x,0)=T_0$$ $$T(L,t)=T_0$$ $$-k\left.\frac{\...
1
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2answers
165 views

Can thick fur isolate against heat?

I started writing this question on pet.SE but realised after a while that I think it has more to do with physics than pets but if some admin disagrees, please move it back to pets. I have heard ...
1
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1answer
50 views

Does Earth's core have a heat sink and does it need one? [closed]

Pardon a question from a complete physics noob. I know only very basic thermodynamics and if my question is completely ridiculous, I apologize. I've heard a very interesting, yet very questionable "...
1
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3answers
148 views

What will happen if we boil water in the absence of gravity?

We know that, Convection is the mode of heat transfer due to the motion of individual molecules of a fluid (liquid or gas). The motion of the constituent particles arises due to difference in ...
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3answers
1k views

Conduction, convection, radiation: Does evaporation count as one of those?

The forms of heat transfer are traditionally described as conduction, convection, and radiation. Is evaporation (or any other change of state) counted as one of those forms? Or is it considered its ...
1
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3answers
102 views

Thermal Energy in a Conductor

We know that thermal energy developed in a current-carrying resistor is given by $U=I^2Rt$ and also $U=VIt$. So my question is- Should we say that $U$ is proportional to $I$ or $I^2$
1
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1answer
75 views

How do I set up the differential equations for the finite length iron stick heat diffusion?

Consider an iron cylinder. The heat doesn't go through the cylinder surface, but at the circular cross-section in the endpoints. The temperature of the surroundings is $15\ \mathrm{^\circ C}$. The ...
1
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2answers
40 views

A question on heat conduction

I was looking up the definition of heat conduction, but every website has a different answer. In addition to several different answers, some seemed wrong, or at least oversimplified. From ...