# Tag Info

39

While I agree in principle with David Lynch's answer, I think it's good to take a closer look at the phase diagram (adapted from http://upload.wikimedia.org/wikipedia/commons/4/46/Carbon_basic_phase_diagram.png): I added the arrows to show possible paths you might follow. Red path: diamond would become graphite before melting; the molten carbon becomes ...

20

@AMCDawes gives a well reasoned explaination of how the physics would play out. However in the scenario depicted by Dawes, he leaves out the part whereby we reintroduce the poor hapless subject to air (I assume 20% oxygen @ STP). If we duty cycle this quickly enough the hair would definitely reignite [citation needed]*. The issue is that flammable ...

18

For the sake of answering your question as asked, we'll assume you can in fact pump out all the air in the room instantaneously. By "all the air" we'll assume we can drop the pressure to something quite low but not ultra-high-vacuum. Two scenarios suggest that this would work: The first is the fact that humans have survived at near-vacuum conditions for up ...

18

MSalters already said "yes". I would like to expand on that by computing the change. Let's take a 10 kg cannon ball, made of lead. Heat capacity of 0.16 J/g/K means that in dropping from 1000 K to 100 K it has lost $10000\cdot 900 \cdot 0.16 \approx 1.4 MJ$. This corresponds (by $E=mc^2$) to a mass of $1.6 \cdot 10^{-11} kg$ or one part in $6\cdot 10^{11}$. ...

16

Of course, it does, since: $$\frac{\partial E}{\partial t} = \frac{\partial }{\partial t} \left(m \cdot c^2 \right)$$ Very little, though

14

What follows is certainly not a comprehensive answer addressing all of your concerns. It is an answer to the question is there a way to see something clearly pathological like superluminal signals in the heat equation? I would argue that yes, there is. The general solution to the initial value problem $T(x,0) = T_0(x)$ for the heat equation on the ...

11

If it was possible to reflect the energy back at the sun, yes, the location where the energy strikes will become hotter. If fact, if you could insulate the sun from radiating energy, then the sun would get even hotter.

11

LDC3 and Kitchi addressed your main question, but I'd like to comment on your second paragraph. Isn't this something like a machine is running, I am getting work out of it and supplying back to the machine to 'accelerate' it? I am not expert in physics, but intuitively thinks that this may not be possible. Actually, we do this all the time! Electricity ...

10

Temperature is not a property of individual particles, it's a statistical property of a collection of particles. For example, nuclear physicists have been known to do thermodynamics with as few as $10^2$ particles, but that's pretty minimal. As you get to lower and lower particle numbers, thermodynamics works more and more poorly, e.g., the second law can be ...

7

The route to the answer is somewhat anti-intuitive. By reflecting some of the Sun's energy back towards the sun at a point you are effectively reducing the flux of energy that can emerge from the photosphere and escape. The global effect of this on the Sun must be similar to that of blocking the flux at the photosphere - in other words, similar to the ...

5

I agree with Andre that it was probably a coincidence that your two pans boiled at the same time; but I want to spend a little bit of time picking away at the physics, and specifically your assertions: If the size of the hob does not matter then we would expect both pans to be receiving the same dose of heat per unit time While you did not define "dose ...

5

Think of it like this. When you have an object of mass $m$ which is held a height $h$ above some reference point, you think of it as having potential energy (considering only gravitational interactions) $U= m g h$, and gravity will exert an amount of work $W_g = m g h$ on the object. When you drop the object, it shall fall towards the ground, towards ...

5

Suppose you have a thermodynamic system in a state $A$. In this state it has a certain amount of internal energy, $U_A$, because internal energy is a state variable. You can determine the internal energy by knowing only the state. Now suppose the system undergoes some process - you don't know (or care) what - that leaves it in state $B$. Again, you can ...

5

It may be worth pointing out that blankets also (surprisingly) act as (thermal) radiation shields. This is the reason that "emergency blankets" can sometimes be found in survival kits that appear to be nothing more than thin shiny plastic. But they really make a difference in the amount of heat lost by a warm (37 °C) body on a cold night (cloudless sky - ...

4

Is the vacuum a required part of the problem? The available action time is increased if it were only oxygen suddenly removed, baring panic. The best case scenario with planning and specific conditions met is about 20 minutes, http://www.guinnessworldrecords.com/world-records/1000/longest-time-breath-held-voluntarily-(male) (Out of water the max time is ...

4

The key here is the air curtain. You can be certain that if it didn't save Costco money, they wouldn't bother with it! It takes a bit of power to push air that much. Two very helpful diagrams are in the youtube video Powered Aire - Cold Storage Air Curtain: In the first case, the air can mix and change temperature through convection, and it does so in a ...

4

If we want to examine gravitational collapse from a statistical mechanics point of view, we find that there's a tradeoff between the fact that a more spread-out collection of matter has more possible position states, whereas a more concentrated collection has more possible momentum states (because more of the system's potential energy has been converted to ...

4

I don't know what you define as too large, but soap bubbles can reach very large sizes. https://www.youtube.com/watch?v=5bjggctu3kw It's a balance between the surface tension being weak enough to allow the film to stretch to large size but strong enough to not tear as it flexes. The soap added to water is for weakening the surface tension.

4

You could blame the laws of thermodynamics and say that cooling is much harder in our universe because of them. However, since we're in a dark energy-dominated universe that's expanding and cooling, it seems as though cooling is generally easier for the universe on the largest scales. Even on smaller scales, cooling is usually easier (I've of course ...

4

The specific heat capacity of mercury is 140 J/kg/K. Let's suppose you have about 1cc of mercury in your thermometer; let's assume this isn't going to boil and your thermometer isn't going to melt and let's suppose the mercury is in the form of a cube which is 1cm on a side. The density of the mercury is about 7.6 g/cc, so you have 7.6$\times10^{-3}$ kg of ...

4

The aging of the body has nothing to do with entropy. As has been pointed out, the body is not a closed system. It takes in energy all during its life and the overall thermodynamic state of 2 bodies of different ages but identical everything else (such as fat content, state of hydration, and so on) are equivalent. Aging is caused by many factors such as ...

3

Flip four coins. Coins have heads $H$ and tails $T$. What are the possible results? 1: HHHH 2: HHHT, HHTH, HTHH, THHH 3: HHTT, HTHT, THHT, HTTH, THTH, TTHH 4: HTTT, THTT, TTHT, TTTH 5: TTTT There are five different outcomes. That is, five different macro-states. Macro-state 3 happens for most different micro-states. It is most probable. If the ...

3

Heat travels in three ways: Conduction, convection, and radiation. Conduction is when objects of different temp come into contact, and the vibrating molecules from the hotter object increase the vibration of the molecules in the cooler object, thereby cooling the hotter object and heating the cooler one. Convection is similar, but occurs when one of the ...

3

Suppose you compress a piston of gas, while decreasing its temperature drastically. You will find that as $V$ decreases, $P$ decreases as well, so $P$ is no longer proportional to $\frac{1}{V}$ . Example: Initial state:$V = 1~\text{L}$, $T = 300~\text{K}$, $P = 1~\text{bar}$ Final state: $V = 0.5~\text{L}$, $T = 50~\text{K}$, $P = 0.3~\text{bar}$

3

I believe it's undisputed that bigger hobs emit more power. Thus, both affirmations are wrong. As the volume of water increases, so does time it takes to boil, because, as you said in #1, more volume equals bigger heat capacity. However, a larger hob emits more heat, thus compensating a bit for that effect. It's just a coincidence. You managed to ...

3

Well, to clarify some things first In atmospheric science, or more correct: If you do the math... your only to free Variables are Density and Temperature. The equation of state which gives you the pressure, is a material property. The equations for the atmospheric variables are interconnected at any moment, it is nonsense to say P causes T or T causes P. ...

3

There won't be a violation of thermodynamics because you are not creating energy from nothing. The total energy of the system is still conserved, it is just fed back into the system. Here's what will probably happen - The concave mirror will not be perfectly reflecting, so will reflect something like $99\%$ of the incident energy. This energy (although very ...

3

I was told a long time ago that the sound is from "twinning" - this is where a metal under large stress experiences a reorientation of grains to relieve stress. However I am not convinced this is the case - typically when the engine parts (catalytic converted being probably the hottest) cools down, it will shrink - and there is some "give" in the mountings ...

2

It should be in joules. The term $\ln \frac{V_2}{V_1}$ is unitless, because the units on the top and bottom of the fraction cancel. Pressure, in pascals, is defined as Newtons per square meter. You've already put the volume into cubic meters, and the cancellation goes \text{meters}^3 \times \frac{\text{Newtons}}{\text{meters}^2}=\text{Newtons} \times ...

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