# Tag Info

65

Yes. There are three mechanisms of heat loss (this applies generally, not just to the man in this example) radiation, conduction and convection. In most everyday cases radiation can be neglected so we just have conduction and convection. Conduction is just the transfer of heat along a static object. For example if you hold the end of a metal bar in a flame ...

20

First, strictly speaking a neutron star is not a nucleus since it is bound together by gravity rather than the strong force. Measuring a surface temperature for any star is deceptively simple. All that is needed is a spectrum, which gives the luminous flux (or similar quantity) as a function of photon wavelength. There will be a broad thermal peak somewhere ...

19

The blanket would help a bit in the same way it keeps the body warmer in cold air, by inhibiting convection and allowing an interface of warmer air between the blancet and the body, but water is a better heat conductor than air and it will not be very efficient. Divers who stay long in the water have water suits whose material is designed to keep the body ...

11

The heat energy being moved does not contribute to any global warming. The A/C is simply moving energy from inside to outside; the heat just turns around and flows back into the room, and the total stays the same. It's like someone trying to keep a leaky boat from sinking; you dump some water over the side, and it (or some just like it) leaks back in. A ...

7

Blankets do not "warm you." Blankets slow down heat transfer by convection, by simply restricting the movement of a medium which may carry heat. Air absorbs heat from solid objects by conduction. Air does not conduct that heat to other air molecules very well. So, if you restrict the movement of those air molecules, you will limit the ability of that air to ...

7

To figure out why this happens, you need to think about what boiling is, and how it works. As you would know, the water in the pot boils because its temperature was raised above the boiling point by the flame. This required a net transfer of heat from the flame, through the pot, to the water in the pot. Why did the heat flow in this direction? Because the ...

6

Temperature means energy. The heat energy is still here. It's just that the "object" (the Universe) grown bigger so this energy had to spread through it. The more energy in a single point, the hotter it is. That's why they say it got cooler. It's like the expanding gas from your spray deodorant is cold when it leaves the can, but it was at room temperature ...

5

A quick google search for "aurora plasma temperature" brings up several interesting results, which seem fond of reporting temperatures in electron volts. That's entirely sensible, but probably not quite what you want. While we could do some math to convert those measurements to Kelvin, Rocket measurements of plasma densities and temperatures in visual aurora ...

5

This can be computed for small changes in the pressure by considering the partial derivative of the temperature w.r.t. pressure at constant entropy. If we suddenly raise the pressure a bit, then this is to a good approximation an isentropic process as no heat is exchanged and it is not a violent process causing large irreversible effects. So, we want to ...

5

No. You wouldn't say that pair of beams has a temperature. Temperature is defined by the zeroth law of thermodynamics, which states that if $A$ is in thermal equilibrium with $B$ and $B$ is in thermal equilibrium with $C$ then $A$ is in thermal equilibrium with $C$ and $A$, $B$ and $C$ are said to have the same temperature. Temperature is fundamentally a ...

4

To start with, "water freezes faster when it starts out hot" is not terribly precise. There are lots of different experiments you could try, over a huge range of initial conditions, that could all give different results. Wikipedia quotes an article Hot Water Can Freeze Faster Than Cold by Jeng which reviews approaches to the problem up to 2006 and proposes a ...

4

This happens due to cooling affect of evapourisation. As you must be knowing, the temperature of the lquid is a factor of evapourisation. So as the temperature of hot water is more, the rate of evapourisation is also more. Now this is where thwe cooling effect of evapourisation takes place. As the water evapourates, it takes away some heat thus cooling ...

4

It's the differential relationship between internal energy and entropy: \begin{align} dU &= T\,dS + \cdots \\ \frac{\partial S}{\partial U} &= \frac 1T \end{align} As energy is added to a system, its internal entropy changes. Remember that the (total) entropy is $$S = k \ln\Omega,$$ where $\Omega$ is the number of available microscopic states that ...

4

As everyone else is saying, if you assume Newton's law of cooling: $$\dot Q = m c_p \dot T = h A \Delta T$$ The equation for how you heat or cool is an exponential $$T(t) = T_\infty + \Delta T e^{ -\frac{hA}{mc_p} t }$$ The rate constant for growth (or dying) of temperature is the same (assuming other details of the material don't change much), so ...

4

Have a look at this black body radiation spectrum, which is approximately the spectrum of "light", electromagnetic radiation, that a body radiates because of the intrinsic kinetic degrees of freedom of the molecules. Look at the frequency spectrum for 300K, about room temperature. Acoustic frequencies are of the order of a few thousand Hertz, infrared is ...

3

We have a perfectly unambiguous definition of temperature for canonical ensembles, and this temperature may be negative in bounded-energy systems. This kind of negative temperature is indisputable, and some would argue it has been realized in spin-inversion experiments. The problem is that there are two decent but imperfect definitions for the entropy of a ...

3

What I am asking, then, is whether someone on StackExchange might be able to shed some light on the matter as to how there can be a disagreement about something that seems should be a mathematical fact. The main disagreement seems to be about which definition of the word "entropy" in the context of statistical physics is "correct". Definition is an ...

3

This page gives a chart of Young's modulus over temperature for various metals. Taking the top line of the table, the modulus drops from 31.4 Msi at -325F (-200C) to 24.2 Msi at 800F (427C). Due to thermal expansion, you will have more square inches. Using a linear expansion of $12E-6 K^{-1}$ the area of a bar will increase $1.5\%$ The longitudinal ...

3

I believe that in the top you'll find it's the saturated vapor pressure of water. http://en.wikipedia.org/wiki/Vapour_pressure_of_water (80 F ~27 C vapor pressure of about 27 mmHg or 27mm/760 mm * 14.7 = 0.52 psi.. not bad.

3

Taking an intuitive guess here: The pressure above the water column is indeed very low, and water molecules at the surface may escape - but they are also held back by the surface tension of the water (is your meniscus concave or convex?). There is an equilibrium here, and the temperature is low enough that the water won't boil off all at once. So at room ...

2

I would use a very simple model, and assume that the item has only one temperature. Also i don't think that this will change the result, the real thing is more complicated. As far as i know, the thermal energy flux is dependent on the thermal energy difference. So the time-dependent solution is something like an exponential function. That means that the ...

2

In general, yes, for a given volume a sphere has the least surface area and therefore the least heat loss to the environment. But we have the complication that the top of the vessel must be open and uninsulated. For a fixed temperature of the liquid and the ambient air, a simple model would say the three types of surfaces on a cylinder each have a fixed ...

2

Coffee would cool via heat being lost to the surroundings. This heat loss would be larger when the exposed surface area is larger. Now, whatever material makes up the cup, it would insulate heat somewhat, as against being held completely open, like the top surface. (That's one reason why ceramic cups are so common - if the cup doesn't insulate, its like ...

2

You'll want a much bigger heatsink!! (and maybe just one TEC) If it's being cooled only by convection then maybe a heat sink area* that is 10 times that of the TEC. (maybe bigger) The classic mistake with a TEC is to make the heat sink too small. With too small a heatsink the hot side of the TEC gets hotter, more thermal leakage through the TEC, it has ...

2

If efficiency is the issue, then definitely parallel TECs (or use a single unit rated for twice the power, same thing). The only reason for stacking TECs is to get a lower temperature. However that comes at great expense to efficiency and overall power consumption. Another point is that paralleling TECs is actually more efficient overall. The reason is ...

2

When the universe expands, it is important to understand that how its energy content evolves depends on the form of energy involved. If all that energy is locked up in the form of mass energy, then the density of that matter will decrease proportionally to the relative increase of any arbitrary volume of the universe (i.e. if expansion doubles the size of ...

2

Simply the thermodynamical quantities used in the original paper were not suitable for that problem. They in particular calculated $T=\frac{\partial U}{\partial S}$ where $U$ is the internal energy and $S$ the entropy. However a wrong definition of entropy has been used. Mathematicians has proved that the use of that specific entropy was wrong and that ...

1

Using your playground example.... Imagine if you had to pass a message (electricity) across the playground, when cold you would have to stretch between each fixed person to pass this message. When hot, more people fill the gaps, the message is easier to pass. Hope this helps :)

1

Taking out your last analogy about the speedometer (which I don't find useful but it might work for you), I would add that in a sealed thermometer, thermal equilibrium between the external media and the alcohol is mostly reached by exchange of electromagnetic radiation (photons). But heating or cooling or the glass molecules by atmospheric gas and then from ...

1

We can do that using states of Matter. If temperature is frame dependent, the observers in different frames should observe different states of matter near Melting and Boiling points which is not the case. This was the easiest explanation I could think of.

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