Hot answers tagged

101

Energy is needed to convert water to steam. This is called the latent heat of vapourisation and for water it is 2.26MJ/kg. So to boil away 1kg (about a litre) of water at 100ºC the kettle would need to supply 2.26MJ. Assuming the kettle has a power of 1kW this would take 2260 seconds. Given the unexpected interest in this question let me expand a bit on ...


88

Thus, the air molecules contribute a small portion of their kinetic energy to the paddle, which is then expended as heat on the other side of the border, making the air molecules on the left colder, while air molecules on the right heat up. Doesn't this mean a decrease in entropy? Yes it does. However, we need to take the thermal noise of the resistor ...


72

There's two main misconceptions in your question that cause your confusion. First, pressure doesn't cause higher temperature. This misconception is probably a result of a massive oversimplification with relation to the ideal gas equation. The actual relation is "increasing the pressure of an ideal gas while volume remains constant increases the temperature ...


61

It's not so much the pressure, but rather compression that creates heat. Heat is a measure of increased kinetic energy as molecules are forced into a smaller space. Water is not very compressible, and water at the bottom of the ocean is not confined to a significantly smaller space under pressure. The kinetic energy of water molecules at the bottom of ...


48

Sort of, yes. Ice water is, in fact, a negative-calorie foodstuff and could be used to lose some weight. Fats contain about 37 kJ/gram of energy, drinking one glass of ice water will burn about 37 kJ or up to three times more if you eat some crushed ice as part of drinking the water: so that's 1 gram of fat burned per drink, up to 2-3 if you eat ice. The ...


41

I did some experimenting (playing ? :-)). The effect is "ill conditioned" and, while the result when the wicks are in close proximity is always a joined flame, the results when the separation is increased slightly is very 'time variable'. Using even quite thin candles (thicker than tapers - about 10mm od) flame proximity could not be got close enough to ...


31

Law of Thermodynamics says that two bodies eventually will have equal temperatures. That is not an absolute Law. There are conditions, and one of those conditions involves the energy input to the bodies. If this Law was absolute, then the Sun would be at the same temperature as the universe, about 2.7 K, because the universe is much larger than the ...


31

Do black holes violate the first law of thermodynamics? No. See Wikipedia re the first law of thermodynamics: "The first law of thermodynamics is a version of the law of conservation of energy, adapted for thermodynamic systems. The law of conservation of energy states that the total energy of an isolated system is constant; energy can be transformed ...


31

That is simply convection. The wick does suck molten wax, and it goes up by capillary to the middle of the flame, but that movement is way to slow to explain the fast particles in your video. Moreover they are moving in the opposite direction! Convection happens because the wick is hot, and it makes the wax around also hot, so the wax expands a little and ...


30

With the same argument, I could deduce (and I know that it's wrong) that the cold air above is denser, so it will go down, pressing the hot air away sideways. Replace your hot air with a helium balloon. You can see there's no force on the balloon to push it sideways. The buoyancy forces it to accelerate upward (and some cool air around it to ...


27

Has Musk done his homework? With regard to the basic idea of using nuclear weapons to release CO2 and thereby warm Mars, no, he hasn't. I suspect this was either Bored Elon Musk speaking, or perhaps the Elon Musk who didn't quite deny being a super villain ( 1-900-MHA-HAHA Elon Musk?) in that interview with Colbert. CO2's enthalpy of sublimation is ...


26

We can understand all of this business if we visit the statistical mechanics notion of temperature, and then connect it to experimental realities. Temperature is a Lagrange multiplier (and should have dimensions of energy) First we consider the statistical mechanics way of defining temperature. Given a physical system with some degree of freedom $X$, ...


25

I don't understand the difference between the first and the second question, but the answer is "No, you don't need air for the clothes to dry". In fact, it will dry faster if in vacuum, because the water will start to boil in zero pressure, even if the temperature is not 100º C. In fact, at zero pressure, water cannot exist in liquid, but will evaporate if ...


23

The partition function is strongly related to a very useful tool in probability theory called the moment generating function(al) of the probability distribution. For any probability distribution $p$ of some random variable $X$, the generating function $\mathcal{M}(z)$ is defined as being: \begin{equation} \mathcal{M}(z) \equiv \langle e^{zX}\rangle ...


23

Well first you have the energy in the form of kinetic energy of the spinning water. Once you let that water settle, it DOES get hotter. The only problem is that water has a high specific heat (it takes a LOT of energy to heat up water), so you don't notice the water getting hotter since the amount it's heating up is not very noticeable. Coincidentally, it ...


22

I think you are right. A perhaps more precise relation between temperature and velocity is the Maxwell–Boltzmann distribution: \begin{equation*} P(\textbf{v}) = \left( \frac{m}{2\pi k_B T} \right)^{3/2} \text{exp} \left[-\frac{m ( \textbf{v} - \textbf{v}_0)^2}{2 k_B T} \right]. \end{equation*} where you see that the mean velocity $\textbf{v}_0$ and the ...


22

As far as the theory goes, you are absolutely correct, the (negative) binding energy between atoms in a molecule contributes to the total mass of that molecule, so a stable molecule is less massive than the sum of the masses of its constituent atoms. However (as you yourself calculated), the mass difference is absolutely tiny, and as far as I know, it has ...


20

Colder water is denser until it reaches a temperature a couple degrees above freezing, then it gets lighter again. So the water at the bottom is at the specific temperature where it is densest: any heating makes it rise. Any further cooling makes it rise. See Why does the ocean get colder at depth? This further points out that without ocean circulation ...


19

In practice, no. In theory, also no. The Universe is filled with photons with an energy distribution corresponding to 2.73 K. Every cm$^3$ of space holds around 400-500 of them. That means that if you place your "stable body" in an ever-so-isolated box, the box itself will never come below 2.73 K, and neither will the body inside. It will asymptotically go ...


18

Mammalian sense of smell is in general exquisitely keen: even though we think of ourselves as an animal having a dull smell sense comapared to that of, say, a dog, a pig or a rat, receptors for certain scents are still triggered by molecules counted in the tens. So the outgassing of volatile wood oils from, say, a table, can still be miniscule and well ...


17

You are looking at this incorrectly. Pale skin allows the UV to penetrate more deeply than dark skin (that has the melanin in the dead skin cells). Since dark skin individuals absorb the UV in the dead skin layer, it make no difference if it causes DNA damage.


17

Microscopically, both the water molecules in the air and the water molecules on the clothing are rapidly moving around due to their thermal energy. Every once in a while, a molecule on the clothing will have enough energy to break free; every once in a while, a molecule in the air will stick to your clothes. Because the humidity in your room is less than ...


16

In a sense, there is only one fixed point. Look to the definition (http://www.bipm.org/en/CGPM/db/13/4): The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. There's a lot hiding in that simple definition. One key concept is that of thermodynamic temperature. A ...


16

updated calculations - based on neutrino energy escaping and vapor inhalation risk Your math is close but not quite right. First - the number of tritium atoms. There are 1000/(16+3+3) = 45 moles (as you said) This means there are 45*2*$N_A$ = $5.5 \cdot 10^{25}$ atoms of Tritium Now the half life is 12.3 years or 4500 days, that is $3.9\cdot 10^8 $s. ...


15

The energy levels of a diatomic molecule are $E = 2B, 6B, 12B$ and so on, where $B$ is: $$ B = \frac{\hbar^2}{2I} $$ Most of the mass of the molecule is in the nuclei, so when calculating the moment of inertia $I$ we can ignore the electrons and just use the nuclei. But the size of the nuclei is around $10^{-5}$ times smaller than the bond length. This ...


15

I would suggest that the bottom penny does not have the same composition. A zinc penny is mentioned in the comments (BTW, what a tedious video) and zinc has a much lower melting point than copper (420 C vs 1085C). Copper plated zinc pennies were introduced in 1982. Given that it started to melt almost straight away, I think that is your answer. At about 10 ...


15

There's two possibilities that are immediately obvious. The first is that the pressure inside the house is slightly lower than the pressure outside the house before you light the fireplace. This would cause air to flow down the chimney into the house which would keep push the flame into the house instead of up the chimney. This would be easily testable if ...


15

From a thermodynamical point of view, living beings are able to reduce their entropy by exporting entropy to the external world. This does not contradict the 2nd principle, since living beings are open systems. For this reason, in a thermodynamically homogeneous universe (heat death), no change in the entropy can occur, and consequently no living beings (nor ...


14

TL; DR The material with the greater effusivity will be more likely to burn you upon contact. Analysis of a simplified case First consider the case where your palm comes into contact constant temperature wall. Often, we can consider your palm as a semi-infinite solid. The requirement for the semi-infinite approximation is that $T(x \rightarrow \infty, t) = ...


13

I personally find the terms consistent. Think of the entropy as Boltzman proposes: $S=k \, \ln W$ Meaning high entropy states can be realized via many different configurations. Truly ordered state (assume you arrange a sculpture from atoms) can be realized via much smaller number of microscopic states. So again, equilibrium is not order - it is a mess.



Only top voted, non community-wiki answers of a minimum length are eligible