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15
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7answers
4k views

Does an empty refrigerator require more power to stay cold than a full one?

Given that everything else is equal (model of fridge, temperature settings, external temperature, altitude), over a given duration of having the door closed, does it require more electricity to cool ...
9
votes
4answers
5k views

What happens when you heat vodka in a microwave?

Since ethanol has a lower dielectric constant than water would the water heat up and boil before the ethanol? Would the water transfer heat to the ethanol and, since ethanol has a lower boiling point, ...
8
votes
1answer
672 views

Are two thin blankets significantly warmer than a single thick blanket?

Almost every source I can find online maintains that two 0.5 cm blankets are significantly warmer than a single 1cm blanket due to air trapped between the thin blankets. However, the thermal ...
8
votes
1answer
2k 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 ...
7
votes
6answers
461 views

What's the best strategy to fully fill the fridge with beer bottles and have them all cooled?

I'm having a party. Suppose I'd like to have a fridge full of cold ($6~^\circ\text{C}$ or below) beer bottles, in as short a time frame as possible. The fridge indicates that it is targeting (and ...
7
votes
4answers
6k views

Are specific heat and thermal conductivity related?

Are there any logical relationship between specific heat capacity and thermal conductivity ? I was wondering about this when I was reading an article on whether to choose cast iron or aluminium ...
6
votes
2answers
442 views

When should I take wine out of the fridge - transient heat transfer problem

I am hosting a dinner tonight for which I'll be serving white wine (Riesling to be more specific). Generally white wine is best served chilled (not COLD!) at around ...
5
votes
2answers
192 views

Does diamond dust conduct heat as well as diamond?

Diamond is one of the best thermal conductors you can get. If the diamond is crushed into dust and spread out over a flat surface, but still held fairly compact (for instance in a small petri dish), ...
5
votes
1answer
187 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 ...
5
votes
1answer
237 views

How do I measure the temperature of a tiny water droplet?

How do I accurately (+/- 0.1 degrees Celsius or better) measure the temperature of a small (5 to 50 microliter) water droplet without noticeably affecting its temperature? The mass of a thermistor or ...
4
votes
2answers
163 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 ...
4
votes
1answer
97 views

Thermal conductivity affected by electrical current

Does anyone know of any materials whereby the thermal conductivity can be changed by passing an electrical current through the material?
3
votes
4answers
1k views

Why water inside a vessel placed in a body of boiling water does not boil?

Imagine a plastic sac filled with water placed in a pan of boiling water. I wonder why the water inside the sac does not boil. The temperature of the water inside the sac should be the same as that of ...
3
votes
2answers
496 views

Performance of a thermos bottle relative to contents

I'm not a physicist but I majored it at high school (a long time ago) and I study university math. Me and my roommate discussed whether the performance of a Thermos bottle is influenced by how full ...
3
votes
4answers
1k views

why does a larger thermal conductivity provide a smaller temperature gradient?

I was thinking about Fourier's Law in heat transfer today and for some reason I am just not understanding the relationships it gives us. Fourier's tells us that if the heat transfer rate is kept ...
3
votes
3answers
2k 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 ...
3
votes
1answer
273 views

What is the time correlation function in the Green-Kubo formulation of ionic current?

I am reading a paper, and I came across the Green-Kubo formulation, where the conductivity $\sigma$ of charged particles is related to the time correlation function of the $z$-component of the ...
3
votes
2answers
1k views

Why is oil a poor conductor of heat?

Why is oil a poor conductor of heat? Does it help in insulation?
3
votes
1answer
2k views

Why is the lid of the cookware kept on induction cooker not hot?

Induction cookware cooks food by inducing an electro magnetic field in the ferro-magnetic cookware. Since iron offers a lot of resistance to the current, the current is converted into heat in the ...
3
votes
1answer
804 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 ...
3
votes
2answers
856 views

How fast is heat transferred by conduction?

How fast is heat transferred by conduction? Is there some simple, but quantitative way that starts from some properties of the material (e.g. its thermal conductivity) and makes rough predictions, for ...
3
votes
2answers
247 views

What devices could be taken round dwellings to measure the thermal conductivities of buildings in situ?

Your lateral thinking and knowledge of lab kit could help us solve a tricky measuring problem in building-physics. One of the problems we get in modelling the heat demands of buildings, is getting an ...
3
votes
1answer
133 views

How thermally conductive is a black body?

Does a black body (imagine like a square of solid black-body-matter) thermally conduct well at room temperature?
2
votes
2answers
57 views

Energy loss of Earth

How does the Earth lose the energy that it gains from the sun's radiation if it is surrounded by - and in contact with - nothing?
2
votes
1answer
1k views

Confused with stress, strain and linear thermal expansion

Four rods A, B, C, D of same length and material but of different radii r, 2r , 3r and 4r respectively are held between two rigid walls. The temperature of all rods is increased by same ...
2
votes
2answers
395 views

Can any solid of low heat capacity release or gain energy slower than another solid of higher heat capacity?

If our sense of contact temperature is influenced by the thermal conductivity of the material: Can any solid material with a low heat capacity exist that feels closer to human body temperature than ...
2
votes
1answer
187 views

Abacus about the thermal expansion of copper

I am currently working on the measurement of the coefficient of thermal expansion of copper. In order to compare my experimental results with theory or experiments made by others, I am looking for ...
2
votes
3answers
341 views

Glass pipe cutting

I want to know how to quickly create the straightest possible breaks in glass pipes I apologise if this is only borderline suitable for a physics forum - I just hope experts with a lot of experience ...
2
votes
1answer
174 views

What is the difference between contact-limited and space-charge-limited charge transport?

I am reading a paper ("Tunable Electrical Conductivity of Individual Graphene Oxide Sheets Reduced at 'Low' Temperatures," Jung, et al. Nano Lett. 2008, 8, 4283-4287) about electrical conductivity in ...
2
votes
2answers
288 views

How would a change in ambient temp affect a radiator?

I'm curious if you have a radiator or say a block of metal (lets say it's copper since it has the highest thermal conductivity) and on one side is a processor producing heat. At idle the processor ...
2
votes
3answers
286 views

Putting a spoon in boiling tea brings the froth down. Why?

Two questions really: 1. Why does putting a spoon in boiling tea bring the froth down? 2. Also, the tip of spoon (outside tea) never heats up. I cannot imagine that normal air can have so much of a ...
2
votes
0answers
43 views

Is my method of calculating thermal conductivity of a metal wrong?

I am trying to determine the thermal conductivity of pure copper at various temperatures using the Weidemann-Franz Law but for some reason it is not matching up to textbook data. I used the Bloch ...
2
votes
3answers
368 views

Improving our homemade hot tub design?

My brother and I built a wood burning, convection based, thermal circulating hot-tub. (With and oxyacetylene torch, lots of 1.5" pipe, a brake drum, and a thirty year old jacuzzi). Our design is ...
2
votes
1answer
138 views

What are the largest thermal gradients achievable in a lab environment?

I am looking for a system capable of creating a gradient of $100\, \mathrm{K}/\mathrm{\mu \textrm{m}}$ on a $30\, \mathrm{\mu}\textrm{m}$ spacing of a system mounted on a Si-N membrane. My so-called ...
2
votes
0answers
56 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 ...
2
votes
3answers
451 views

Finite difference formulation of the heat equation with thermal conductivity in 1D

This may seem trivial, but I'm having some trouble deriving the finite difference form of the heat equation with a thermal conductivity function $a(x)$ depending on $x$: $$\frac{\partial u(x, ...
1
vote
4answers
127 views

Heat transfer by conduction is not possible for gases?

Today one of my instructor told me that gases cannot have heat transfer thru conduction because molecules are far apart and so it cannot transfer heat,infact the diffusion process of gases transfers ...
1
vote
1answer
843 views

Do materials cool down in the vacuum of space?

Do materials cool down in the vacuum of space? If yes, how does it really work?
1
vote
3answers
188 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
3answers
719 views

Maximum efficiency for a counter-current heat exchanger (double flux controlled motorized ventilation)

I am not sure if I can explain the question correctly because I don't know the name of this mechanism in English. This is my explanation attempt: In a house, a tube is expelling the air from the ...
1
vote
1answer
289 views

Per unit area, is there more heat transfer through the open top of a mug or the side walls?

I had a mug of hot coffee cool too rapidly for my liking the other day, which made me wonder what was the greater contributor to heat loss for a typical open-top ceramic mug: the open top, the walls, ...
1
vote
1answer
117 views

Temperature rate of change inside a cylinder

I was wondering about this today, mainly for baking a cylindrical cake. Say you have a solid cylinder which is at temperature T0 inside. Say you then move the ...
1
vote
3answers
303 views

What is the Units for Thermal conductivity?

What are the units for thermal conductivity and why?
1
vote
1answer
705 views

When a color LCD/LED display is off it is black, when on it is colored. What color to light is an LCD/LED display

When a modern display using pixels is turned on its colors are different than the black that the screen is actually made up of. When put in light, such as sunlight, does the screen react to the color ...
1
vote
1answer
20 views

Measurements for thermal diffusivity of graphene?

We have known for a long time that graphene has in-plane thermal conductivity ranging between 2000 and 4000 $W m^{-1} K^{-1}$. But in order to model heat transport on a sheet of graphene, we need more ...
1
vote
1answer
44 views

Refrigerator cooling power usage

I have a refrigerator, that makes a lot of noise, and we have therefore set a time on it will limit the run time to when we are not at the office. But this is where our discussion begins, should we ...
1
vote
1answer
167 views

Is the convective heat coefficient always dependent on heat flux and delta T?

I'm having a difficult discussion with a colleague because I'm running heat flow simulations for a small heater (5 mm^2 area), and I insist that if we're going to run more electrical power through the ...
1
vote
2answers
35 views

Should water cool faster if is inserted metal canister with ice inside either mix only with ice?

Let's say that we have two canister first bigger (metal canister 2l) with 1l of water at 100C, and second smaller (metal canister 1l) with 1l of ice. And we want to cool down boiled water to 50C. So ...
1
vote
0answers
60 views

Conduction– conservation of energy

So I know that conduction is transfer of thermal energy by direct contact via molecular collisions. For conduction, $Q/t$ is the rate of heat flow and is heat current ($I$). My textbook says that ...
1
vote
0answers
99 views

Thermal Conductivity Graph

This is our first time (as an engineer) seeing this type of graph that we can't interpret. This is a rough comparision of thermal conductivity from the Wiki page. ...