Questions tagged [heat-conduction]

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1answer
38 views

How does a light source / laser transfer energy to a material?

I am trying to understand the physics of how a laser is able to heat a solid. My understanding so far is that when photons are incident upon a body (e.g. a metal) the momentum of the photons is ...
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2answers
56 views

Difficulty in theoretical understanding of Newton's law of cooling

Newton’s law of cooling states that the rate of heat loss of a body is directly proportional to the difference between the body’s temperature and that of the surroundings when there is a breeze. I can ...
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1answer
40 views

Heat Transfer on boiling water

Let's say I'll boil water on the stove from 25°C to 100°C, what heat transfer happens? Is it conduction (heat warms up the pot), or is it convection (causes water molecules to move away from the ...
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1answer
30 views

Is there convection between two volumes of air at different temperatures?

As I have learned in my engineering classes convection takes place when a fluid meets a surface of both different temperatures. My question: can this also be evaluated for two volumes of air at ...
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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 ...
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1answer
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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 ...
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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 ...
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39 views

Transient heating of semi-infinite solid - time it takes for given depth to reach a specific temperature [closed]

Given a heat flux and the thermophysical properties of an object, how is the time to reach a specific temperature at a given depth determined for the case of no convective loss and with convective ...
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1answer
27 views

What is the temperature profile of hot and cold fluids for a double pipe parallel flow heat exchanger?

In books, they just mention the temperature profile is exponential. Can someone help me come up with the exact temperature profile (the relation) for the hot and cold fluids in a double pipe parallel ...
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1answer
43 views

Is it wrong to derive Log mean temperature difference (LMTD) of a Heat Exchanger, this way?

I tried deriving the LMTD of a parallel flow double pipe heat exchanger by first finding the temperature profiles of the hot and cold fluid and then averaging them over the entire length of the Heat ...
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1answer
27 views

While determining LMTD for a heat exchanger do we assume, the tube wall to be at a constant temperature (no variation along radial direction)?

The derivation of Log Mean Temperature Difference (LMTD) for a Heat exchanger at preliminary level makes some assumptions like, constant specific heats, steady state operation, no changes in kinetic ...
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1answer
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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 ...
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Could this imaginary cooling solution work?

Imagine a perfectly insulated ideal closed cylinder. Inside, at the two ends there are two bodies, one with the temperature of T the other with temperature -T. Between the almost touching bodies with ...
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1answer
50 views

Heat diffusion in an oven

I have this problem where I am preheating an oven to reach $n^\circ$. So lets say within the oven there's a heating element at the bottom, which in turn heats up the rest of the oven. Assuming that ...
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2answers
86 views

Deriving the time needed until Thermal Equilibrium

Two materials of mass $m_1$ and $m_1$, specific heat capacity $C_1$ and $C_2$ and temperature $T_1 > T_2$ are separated by a conductive solid with thickness $L$, area $A$ and heat conductivity $k$ ...
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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 $\...
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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} = ...
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2answers
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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 ...
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0answers
60 views

What is the ideal gap between two panes of glass for insulation? [closed]

I have two panes of glass, about a 3/8" thick (.95 cm). Space efficiency and aesthetics ignored, how far apart should they be spaced to maximize heat insulation, assuming the gap contains regular ...
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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 ...
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1answer
48 views

Estimate room energy loss from temperature plots [closed]

What is the method for determining the heat/energy loss from a room, given a temperature plot/delta and a measure of the energy input over time ? I'm building an extension to our house, and have ...
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Why doesn't ice in a glacier feel as cold when I touch it as the ice I get from my freezer?

When I was in Iceland, I went to a tour in a glacier and I remember the ice that was there. It looked and felt like a huge amorphous block of glass: hard, smooth, not wet, and not that cold. Not as ...
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1answer
49 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 ...
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27 views

Why does bologna inflate radially when on stove?

I noticed that when heating up bologna in any way it inflates at the center, while other stuff tends to let the air escape sideways, or inflates in different places, like an egg or sausage. Nothing so ...
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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,...
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Through which medium is most of the heat transferred down (downwards) a glass tube filled with water: the glass walls or the water column?

If a much hotter object is attached to the tube from the outside, and the heat in question is transferred to the part of the tube that is below the source of heat, so there is no convection. ¨ The ...
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1answer
38 views

What is the difference between rate of cooling and rate of heat loss?

I had a perception that rate of cooling and rate of heat loss are synonymous because the rate at which it looses heat indicates the rate at which it is cooling but this question changed my view There ...
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1answer
32 views

Steady State conduction of cylindrical pipeline

I have this problem involving conduction of heat: Suppose that $z = 0$ represents the ground level on a street where an electric cable is buried at $x = 0$ and $z = −D$. The ground is kept at a fixed ...
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2answers
52 views

Microscopic description of thermal conduction in steady-state

In the steady-state condition of thermal conduction the temperature at each point of a rod becomes constant w.r.t time but there is a temperature gradient! I can understand it from the mathematical ...
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1answer
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Will the temperature reading of a thermostat change as a result of direct skin contact? [closed]

If I was to place my hand on a thermostat for an extended period of time, would this eventually result in the temperature reading rising after a period of time - or would the reading not be impacted ...
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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 ...
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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 ...
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29 views

Condensation of saturated vapor on subcooled droplet

Consider the process of condensation of vapor on falling subcooled droplet in the stagnant saturated vapor (neglect the vapor drag on droplet). What can be said about the relation between the droplet ...
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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?
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1answer
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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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50 views

How does the change in temperature of water in a tube depend on the angle that the tube makes with the gravitational field/the horizontal?

Let's imagine a tube that is open at one end and is filled with water. A constant source of heat is applied to a specific spot somewhere at the middle of the tube, for a example a preheated soldering ...
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0answers
35 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
54 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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67 views

How to obtain a solution for the following IBVP?

I am trying to solve the following advection-diffusion equation for transient flow conditions for radial flow. The governing equation is as follows. $$\frac{\partial T}{\partial t} = \frac{\partial^2 ...
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Coupled solid-fluid heat transfer over a rectangular plate heated from the bottom (Boundary value problem)

I have the two-dimensional temperature Laplacian $(\nabla^2 T(x,y)=0)$ coupled with another fluid equation (which is one-dimensional). The Laplacian is defined over $x\in[0,L], y\in[0,l]$. On ...
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2answers
77 views

After sweating ,why do our body cool down faster under a fan?

The sweat on our body evaporate taking energy from our body in the form of heat.the temperature of our body decreases and we feel cool. Thus sweating cools down our body. Now why does our body cool ...
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1answer
143 views

Calculate rate of heat gain of a steel chain in a campfire [closed]

Up front: I'm trying to be scientifically accurate about heating up a length of steel chain in a campfire to use as a surprise attack tactic in a D&D campaign, but I'm not sure I'm using the ...
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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 ...
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2answers
40 views

Will “stationary” air sorrounding a hot pipe from a wood stove act as insulation with a given time?

The inner pipe (colored in red) radiates considerable more heat than the current one so I'm planning on extending the more thermally conductive one so that it heats better. Will removing the outer ...
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1answer
36 views

Physical interpretation of the heat equation with variable coefficient

I would want to know what is the physical interpretation of the heat equation with variable coefficients such that: $$u_{t}-\frac{1}{1+t^2}u_{xx}=0$$ well, I think I got it, it means that the ...
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1answer
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Heat transfer in glass top ranges

The "glass" in glass top ranges apparently has quite low heat conductivity as areas just a short distance from the heating element don't get so hot - especially compared to a metal surface which has ...
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0answers
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Should I determine the wattage of my heater based on the heat conduction equation, or the specific heat capacity equation?

I know of the equation equating rate of heat flow to the product of a materials thermal conductivity, area of contact, temperature difference, divided by the thickness of the material. However, I ...
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2answers
61 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 ...
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1answer
75 views

Definition of the rate of heat flow

According to Wikipedia the rate of heat flow is:$$\frac{\Delta Q}{\Delta t}=-kA\frac{\Delta T}{\Delta x}$$ Where to evaluate? I can't understand where we must evaluate this rate. In the hottest ...
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1answer
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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 ...