# Tag Info

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 ...

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 ...

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 $$P=nk_{B}T$$ ...

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.

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 ...

2

Check out the description of Charles' law on wikipedia. Charles' law relates the temperature and volume of a single body of gas at constant pressure. If you're mixing two bodies of air at different temperatures by punching a hole in the wall between them, you're not changing the volume of a body of gas at constant pressure, you're combining two bodies of ...

2

But if heat consists of the speed of the molecule (which is an if) then shouldn't there be an Absolute Infinity as well as an Absolute Zero? This question's "if" is not correct. Temperature (not "heat", as we use this word in a specific technical way) consists of the energy, not the speed, of particles. While these two are obviously related, it's ...

2

Burning the fuel in car produces less heat than just burning the fuel in an open container because in an engine some of the energy produced by combustion goes into doing work on the car. In an open container all the energy appears as heat. However the energy that goes into doing work on the car ends up as heat eventually because the car dissipates the ...

2

When you put the pot on the stove, the heat from the stove is somehow getting to the pot, which gets hot. The pot and the stove are obviously in contact with each other. Therefore conduction plays a role here. If you have an old pot, with a warped bottom, it will heat up slower, because the contact surface between pot and stove is smaller. When you hold ...

1

Air pressure exists because if we place something in a gas, then the molecules/atoms flying around will keep banging into it, and in this way produce a net constant force per unit area. As explained by @Chris2807 in the neat formula $P=n k_{B} T$, this is proportional to how many particles there are (since this is proportional to the amount of "banging" in ...

1

If you have a path on $p-V$ diagram that is $p=F(V)$, then using $$dU=\delta Q-pdV \implies \delta Q=dU+pdV$$ NOTE MINUS SIGN as $pdV$ is work done BY the system. $Q$ is the total heat received by the system (it is negative if system releases heat). Assume we are dealing with an ideal gas with $f$ degrees of freedom per particle ($f=3$ for monatomic gas). ...

1

Here's a way to get a decent distance estimate for solids, liquids or gases.: for solids or liquids, you can get the number density, $n$, of atoms or molecules (as needed), from the density, Avogadro's number ($6.02E23$) and the molecular weight ($\rho,N_a,W$: $$n = \frac{\rho N_a}{W}$$ for a gas with pressure, temperature and the boltzmann constant ...

1

Let us understand Brownian motion in liquids before we look at the motion in solids. If you observe a glass of water at rest on a table, it "appears" to be motionless. However, all we need is a magnifying glass to observe the random, incessant motion of water on the surface. This random motion is a manifestation of heat. The same thing happens in a solid. A ...

1

The radiation of a body is given by the Stefan-Boltzmann equation: $$E= \epsilon \sigma T^4$$ Which means that if you can measure the radiated power $E$, the temperature is $$T=\sqrt[4]{\frac{E}{\epsilon \sigma}}$$ If the emissivity is lower than you think, it follows that you see less emission that you would expect for a given temperature and you would ...

1

The Sun heats up the surface that it's light touches and the heat created causes the air to rise and the light hitting the hot air causes a mirage. http://en.wikipedia.org/wiki/Mirage

1

The temperature at the boiling or condensation point remains the same for gas and liquid. 100 degrees Celsius for water at standard conditions. L called the Latent (meaning hidden) Heat of fusion times the mass of Water vapor, m, equals Q the Heat released.

1

For a reversible process, the overall change in entropy is zero (i.e. no entropy is created). However, entropy can be transferred between different systems and the environment through reversible processes. For a system transferring heat/work to/from its environment - as I understand it, for an internally reversible process, no irreversibilities exist within ...

1

Let me clear a few things up first; the latent heat of fusion is the energy required to convert a substance from solid form to a liquid form. Since water is liquid at room temperature, the latent heat of fusion is positive as energy is absorbed to convert ice to water, just as energy is released when water is converted to ice. It sounds counter-intuitive, ...

1

I was simply confused by the sign convention in the first law of thermodynamics. The most convenient way to write it is $Q=\Delta U + W$ where $W$ is the work done by the system. Hence the answer to my question becomes clear after this.

1

In thermodynamics, any well-behaved quantity of matter generally has a temperature (a measure of average internal energy) associated with it. Therefore, the sole addition or removal of matter from a volume is not accompanied by a change in the energy contained in that volume if and only if the matter transferred has zero temperature; transferring matter at ...

1

Stoves and other hot objects heat up, but don't burn. Burning is very different. Burning is a chemical reaction. In the example of stoves, they work by conduction.

1

You state the second law as : The entropy of the universe always increases. In my college textbook it is stated as : Processes in which the entropy of an isolated system would decrease do not occur, or, in every process taking place in an isolated system, the entropy of the system either increases or remains constant.( F.W.Sears an introduction ...

1

Here's a simple mental picture to have of a how a burner on a stove heats up water in a pot (which is sitting on the burner). (In what follows, I will use the term "molecules" for both molecules and atoms.) Also, keep in mind that thermal conduction is different than electrical conduction. One (electrical conduction) concerns the flow of charge, so in this ...

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