# Tag Info

63

Short answer: The thermometer measures actual temperature (which is the same for both), while your hand measures the transfer of energy (heat), which is higher for the pot than the air. Long answer: Keyword: Thermal Conductivity The difference is a material-specific parameter called thermal conductivity. If you are in contact with some material (gas, ...

19

The two "no" answers you've already received are correct for all practical purposes. In real-world cases there can be a difference though. The difference depends on when the refrigerator decides to cycle on and cool. If the fridge cycles on a timer or based on heat energy then there will be a difference due to the added heat capacity. The outside of the ...

12

The heat loss (power) at a particular temperature is the same. So, No - the cooling needed to maintain the thing cold stays roughly the same. However, the empty fridge has lower total heat capacity. So, it will get warm faster in the absence of power. So, it is worthwhile to fill your fridge and freezer with bottle of water a few days before a big storm ...

12

Ways I can think of to export energy from a home that do not heat the interior: Light through windows Radio energy from transmitters (WiFi, cell phone, radio, power supply noise) Sound from stereo/TV Charge in batteries Hot items carried out (coffee in travel mug) Pressure in car/bike tires supplied from your pump Vented hot air from dryer, shower, oven, ...

10

Diamond dust (or dust of any other material) won't conduct heat anywhere close to as well as the solid material. At a molecular level the dust isn't in very good contact with other grains of dust. There is plenty of separation and air in between the particles that will retard heat conductivity. If you were to compress the dust so significantly that it did ...

9

No. The rate of cooling must simply match the rate of heating, and heating rate depends only on the temperature difference you want to establish and on the thermal conductivity and surface area of the walls. More stuff in the refrigerator would give it a higher heat capacity, so that it wouldn't warm up so much when the door is opened. However, it will take ...

8

In a microwave the EMW energy is transferred to the water molecules, but, since they are in immediate contact with other molecules (as in any food), the whole volume gets heated. You will not have a two-temperature mixture.

8

I disagree with the opinion that your skin can measure heat transfer. It can only measure temperature, or to be more precise: the surface temperature of the body you are touching. Now the thermal diffusivity comes into play: When you touch a cold piece of wood (low thermal diffusivity), you transfer heat to the wood, the boundary layer of the wood warms up ...

7

It's a steady state. If there were a pressure gradient, there would be net force on the gas (ignoring gravity). There's no net force here because the air isn't accelerating. Thus the pressure is constant. The number density varies across the box inversely to the temperature so the ideal gas law holds.

6

The metal rod will become hotter. Only not very much for a large rod. The energy will flow from your fingers to the metal until the temperature of the metal reaches the temperature of your fingers. For a large metal object this will never happen for all practical purposes. For a small object, though, it does happen. If you pick up a dime it will ...

5

In my opinion there is no physical reason. To cook food the same energy is needed, and to burn it too. It is behavioral differences because of the form of heat: One is aware of the dangers of gas and is much more careful in turning it off on time, when food just starts to smell "singed". Electric : we may turn it off and leave the pot on the still hot ...

5

Without doing the analysis, I would think that a cooling system is more effective at extracting heat from a warm container than from a cold one. For a fridge, the effectiveness (or coefficient of performance) is $Eff=Q_c/W$ is the ratio of the heat removed from the cold source (the fridge) to the energy used for the purpose. It increases with the ...

5

I can't make this a comment since I don't have enough reputation. The metal box itself can absorb some of the heat (by conduction) and then give out energy in the form of electromagnetic radiation. If you want to do some real physics with such a system, you could idealize the box as a black body and continue.

5

For metals there is a connection between the thermal conductivity and electric conductivity (Wiedemann–Franz law). However specific heat is not directly related. This is because electric and thermal conductivity are due to the electrons, however the specific heat is mostly due to the ion vibrations (phonons). Despite "classical" intuition electrons ...

5

In a liquid mixture such as ethanol-water, both components vaporize to some extent. If the combined vapor pressure of the two equals the external pressure, say 1 atm, the mixture will boil. The components DO NOT boil separately. Further, the composition of the vapor and the composition of the liquid will be different from each other. This is the basic ...

5

Thermal conductivity relates to the propagation of heat, whereas electrical conductivity relates to the effective propagation of electric charge. In the case of thermal conductivity, not only the electrons play a role in the conduction but also phonons or magnons contribute to it. The electrons only play a significant role in heat conductivity in metallic ...

5

Your confusion lies within your perception of natural length in the Young's modulus formula. When we say strain=$\Delta L/L$, the $L$ refers to the natural length of the rod at a given temperature. So, if the rod is not clamped, and we increase the temperature, there is no deviation from natural length at that temperature (as we can define natural length of ...

5

By conductor of heat, do you mean that it is bad at transferring heat via conduction? Or that it is just bad at transferring heat? First, a picture of the molecular structure of an oil: Conduction Oil is a liquid. Heat transfer by conduction requires strong bonds between the molecules, so that a vibration(heat) travels down the line. With liquids, this ...

4

Your first answer is the correct one. The temporal auto-correlation function (or the time correlation function) of a fluctuating quantity $A(t)$ is $$C_{AA}(\tau) = \langle A(t) A(t+\tau)\rangle$$ This is a measure of how correlated $A(t)$ is to its value at another time $t+\tau$. The Green-Kubo formula that connects a response function (the electrical ...

4

conductivity is the inverse of resistivity which is resistance times cross-section area divided by length. So measure the resistance using a micro-ohmmeter and measure the length and area using rulers and calipers.

4

Your first assumption is not really correct. The induction cooker creates an electromagnetic field with a frequency in the range of 10 kHz to 100 kHz. The coil used in the cooker looks as follows: The coil and the frequency are choosen in such a way that the field does not extend more than a few centimeters along the axis of the setup. Now when you put a ...

4

Water boils at 100° C and atmospheric pressure. If you try to boil water in a closed container, a small amount of steam will form; this will increase the pressure, and the rest of the water will not boil (unless the container bursts or leaks). This explains why water will not boil inside a plastic sac. I don't completely understand the set-up with the ...

4

Both bodies of water will heat till 100 C (as well as the vessel). Then, the outside one will start boiling by taking latent heat from the surroundings, Now, we only have heat transfer due to conduction/radiation when there is a temperature difference (or an emf, but that's irrelevant). It's mainly conduction we need to consider, anyways. Convection won't ...

4

Heat is the thermal motion of particles. Hot object's atoms vibrate more than cold object's atoms. Heat is transfered by 3 main ways: Conduction: Heat flows from hot objects to cold objects. If you have an electric stove, the heat flows from the coils to the pan. Convection: Heat flows by bulk motion of a fluid. If you heard "hot air rises" this is the ...

4

Heat flux is a vector beceause it has a magnitude and a direction. Furthermore it has these properties in every point in space, which makes it a vector field. You can think of an analogy with the mass flux in a medium with inhomogenius density; diffusion will tend to equalize the denisty everywhere, so there will be specific motion of mass at every point ...

4

I don't have a thermos flask to hand, otherwise I'd do the experiment (the only sure way to answer :-). In the absence of experimental data I'd guess that the half full flask will cool faster. The heat flow will be roughly proportional to the temperature difference between the inside of the flask and the ambient temperature outside. The constant of ...

4

Given limited time I believe the best strategy would be to get a tub of ice water and put as many beers as you can in there. Those should cool sufficiently between 10-20 min. Put the rest of the beer in the fridge while it's chilling. After the ice beer has chilled, put it in the fridge. The large mass of cold beer will keep the temperature from dropping as ...

3

My electric heat-pump removes more sensible heat from my house than it adds into it - and the electric bill comes to the house. So perhaps the answer should be no. The point is the temperature of my house, may not be proportional to how much electricity we use that day.

3

The main problem with your approach is that you are using the wrong area for the radiation "window". The area over which the exchange of radiative energy can take place is just the window in the door - the walls "see each other" and that part of the radiation has no net effect. So you want to use just $H\cdot W$ for the area. Secondly the window is ...

3

The second law of thermodynamics forbids materials that conduct better in one (forward) direction than the reverse direction - such a material placed between two containers at thermal equilibrium would drive the temperature away from equilibrium, decreasing the entropy of the whole system and paving the way for a perpetuum mobile... Reflectance and ...

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