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24

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 100ยบ C. In fact, at zero pressure, water cannot exist in liquid, but will evaporate if ...


21

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 ...


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.


12

Dark skin absorbs UV better than lighter skin. More specifically, melanin absorbs most of the UV radiation so that your skin cells don't have to.


8

The partition function contains so much information because it is directly related to the free energy, $$F = - k_B T \ln(Z) \, .$$ The physical assumption behind considering $F$ as a thermodynamic potential is that the statistics of the system as described by the canonical ensemble. In turn, the applicability of the canonical ensemble is a direct ...


7

I think when you say "no air" you mean "no wind" In modern greek too "air" can mean "wind" and and also the content of the atmosphere. So if you hang clothes in the same sun but with no wind to supply convection, the clothes will try slower than when a wind is blowing, due to convection. Convection replaces the saturated air close to the clothes with ...


5

Giving the value simply of $k_B T$ is generally more useful, because I can plug that into anything. Sure, I might need to know the ideal gas energy, and multiply by $3/2$. But maybe I need to put it into a partition function, and I just need $k_B T$. Maybe I'm worried about a harmonic oscillator and I just have the two degrees of freedom. The 3/2 is ...


5

Actually that's not an impossible task as long as you don't constrain the problem by not allowing energy input to the filter. The problem is the famous Maxwell 's Demon, but in the end you have to pay the demon. His efforts don't come free. The Hilsch tube, originally thought to house the demon fails the challenge as it takes excessive energy to separate hot ...


4

In some sense yes. Let me explain a little. If we were to take a sealed container of gas and put it into free space far away from other bodies so that the gravitational force on the box is negligible would you agree that there would still be some pressure in the container? If we assume we have an ideal gas then the pressure is simply given by $$P=nk_{B}T$$ ...


4

Recall two things... First that the 1st law is conservation of energy. Second that temperature is a non-decreasing function of internal energy. So if we take two identical samples of gas and add the same heat $Q$ to each (increasing their internal energies), but allow one to do work $W$ on the surrounding while the other does none, then the sample doing ...


4

Let us take the example of the Hubble primary mirror. It has a diameter of 2.4 m and a mass of 828 kg. It is actually made in a sandwich structure - glass-honeycomb-glass - making it about 30 cm thick (for stiffness) but light. The mirror is coated with an aluminum coating of thickness t = 65 nm, with a 25 nm MgF2 protective coating on top. Coefficient of ...


4

Wikipedia has a good article on hybrid photovoltaic-thermal systems. As you proposed they consist of a solar cell with a thermal collector at the rear. Solar energy conversion is a fascinating topic from a thermodynamic perspective and has been summarised beautifully by the work of De Vos, The Thermodynamics of Solar Energy Conversion, ISBN: ...


3

The gas outside the white dwarf (and actually, in the interior of the white dwarf but close to the surface) is non-degenerate. So I don't see what the presence of the white dwarf has to do with the question unless it is significantly photo-ionising the cloud? Anyway for spherically symmetric accretion onto an object, one approach is to make an assumption ...


3

The current collision energy of the restarted, upgraded LHC is 13 TeV. This is about $2 \times 10^{-6}$ Joules. Water has a specific heat capacity of about 4.2 Joules per gram per degree Kelvin. So this tiny amount of energy would heat up a gram of water (i.e. a large drop) by about $5 \times 10^{-7}$ degrees. That's for a single collision between two ...


3

There are several possible approaches to this question, but I've always been a fan of the one taken by Edwin Jaynes in his 1965 paper Gibbs vs Boltzmann Entropies. (See sections V and VI for the discussion, which I think can be read in isolation from the rest of the paper.) Here he derives the second law from the empirical fact that we as scientists and ...


3

For gravitational systems one has to be careful making statements about entropy and the second law of thermodynamics. Your example is similar to the gravitational collapse of a gas cloud if you think carefully about it. In that case and in yours, the shrinking of the gas will raise it's heat. Now even though the increase of entropy due to the increased ...


3

Disclaimer before I get started: A perfect vacuum is impossible. As I answer your question, I will take your use of the word "vacuum" to mean "a chamber with an air pressure arbitrarily close to 0 Pa." When I use the word "vacuum" in my response, I mean the same. Your clothes don't need the air in order to dry, and in fact, will dry more quickly. ...


3

I think one way to understand why this works is that the spectrum of energy levels $E_i$ has undergone a sort of transform (analogous to Laplace transform) which results in the partition function $Z(T)$. In principle if you know the function $Z(T)$ you can reverse the process, and reconstruct the original spectrum of energy levels. As such, all information ...


3

Cost is determined by supply and demand, and I am tempted to suggest moving this to some economics (stackexchange?) site. For Helium, our defacto only supply is fossil fuels as nothing else seems to contain any concentrations worth mentioning. Because of its potential strategic importance (for 1920s airships and 1950s rocket technology), the USA is maintaing ...


3

The thermal energy $k_{B} T$ is really referring to the probability of finding a system in a state of energy $E$, given that it is in a surrounding enviroment at temperature $T$. This probability is proportional to $e^{-E/(k_{B} T)}$. Using this you can derive a great many things, including the Boltzmann/Fermi distributions. The proportionality constant is ...


3

Light like sunlight is an electromagnetic wave with components with different frequencies. These components follow a particular distribution of intensities. One portion of the energy of this light resides in what is called infrared radiation and most materials absorb in that range (link provides some more extra information regarding this radiation and heat). ...


3

At constant pressure the volume of an ideal gas is given by Charles' law: $$ V \propto T $$ and this law tells us that when the temperature $T$ falls to zero the volume $V$ also becomes zero. But no gas is ideal and real gases show all sorts of non-ideal behaviour. For example real gases liquify then solidify as the temperatue falls. Real gases deviate ...


3

The filament will be a reasonable approximation to a black body emitter, so it's spectrum will be given by Planck's law: $$ B = \frac{2hc^2}{\lambda^5} \frac{1}{e^{\frac{hc}{k\lambda T}} - 1} $$ So just measure the radiance of the light from the filament for a range of wavelengths and do a fit to Planck's law by varying $T$. This will give you an excellent ...


3

In general, air pressure in the Earth's atmosphere is hydrostatic pressure, caused by the Earth's gravitational field. If there was no gravity then there wouldn't be any centripetal force and all the air molecules would just float away into space. This is why there is no atmosphere on the moon - because it doesn't have enough gravity to sustain one.


2

You really are asking two questions. First - how do we calculate the temperature: At the typical temperatures of a halogen bulb, the large majority of heat loss is due to thermal radiation (although there is some conductive loss in a halogen bulb as the bulb is not evacuated). Because of this, the most important factor is the "apparent size" of the ...


2

Heat if you remember from 8th grade science transfers by convection, conduction and radiation. Convection is by the flow of a fluid which cannot go faster than light, conduction is caused by molecules colliding with neighboring molecules, conductive heat equations are only for after a steady flow has been established and do not treat transient effects so can ...


2

Planck published several works on the theory of blackbody radiation based on different ideas, but generally the use of integer counting of energy he meant to be used for the energy of material oscillators. He did not believe the quantization applied to light itself - he assumed Maxwell's theory with its differential equations and derived his spectral ...


2

The one that absorbs more heat from you will cool you more, and seem colder. But it isn't entirely straightforward. If you pour water in your hand, water will flow to fit you. An ice cube will not make as good contact. Water in contact with you will warm. It can then flow away and be replaced by fresh cold water. Ice doesn't flow On the other hand, Ice ...


2

Consider a container containing n moles of an ideal gas. The gas exerts a pressure P on the container and the piston. If P equals the atmospheric pressure, then the piston does not move, as it experiences equal forces from in and out of the container. When you increase the external pressure, the gas in the container is compressed. If the compression of the ...



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