# Tag Info

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

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

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 ...

0

To keep a constant pressure, halving the volume would halve the temperature (relative to absolute zero) - but only for a fixed mass of gas. This would be the case if you had a sealed volume and a moving wall, for example. By just adding on another volume, you wouldn't really be applying Charles' Law since you've increased the amount of gas. The temperature ...

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 ...

2

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.

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 ...

0

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

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 ...

0

No, I don't think you have the correct entropy. I think the temperature is incorrect in the numerators, it should be (328 - 313) and (313 - 283).

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.

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 ...

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 ...

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, ...

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 ...

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

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 ...

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 ...

0

There is a point of temperature called Planck temperature where are understanding starts to break down. Advances in quantum gravity will help us understand this incredibly high temperature and its effects on molecules.

0

When you heat up an ionic crystal, the nuclei oscillate more quickly (increase in kinetic energy) and this causes the atoms to become more spaced apart, i.e. the material expands (increase in potential energy). You can see this effect come out of the virial stress.

0

You're basically describing a car radiator. I can't see any reason why you'd need a more sophisticated heat exchanger.

0

The reason why a gas heats up when it is compressed into a smaller space, is because the ambient heat that the gas possessed in its original volume, has now been confined to a smaller volume—same amount of heat but now more concentrated—the temperature goes up. When the vessel storing the newly compressed gas cools off to the ambient temperature of its ...

0

For an ideal gas, we neglect all intermolecular interactions (except for the trivial case of elastic collisions). $\Delta U$ therefore depends purely on the kinetic energies of the particles. The 'decreasing of intermolecular distances' has no role to play here. However, these things are taken care of to some extent by the van der Waals equation Melting is ...

0

If you compress an ideal gas adiabatically, the average kinetic energy of the particles will increase because collisions with the moving piston will increase the kinetic energy of the colliding particle. For an ideal gas, the intermolecular distances have no bearing on the internal energy. More specifically, suppose the piston is moving inwards at velocity ...

2

It would certainly require a material that allows electron release from energies lower than those of the visible spectrum. The energy of a wave is given by E=hf where h is the planck constant (6.63 x 10^-34) and f is the frequency. The wavelengths of IR light range from 0.001 m to 750 x 10^-9 m. (Hyperphysics.com, infrared) Using this knowledge you can get ...

2

Q = mc(t1-t2), Now, m = (density)(volume), Specific heat of water, c(in joule/gramCelsius) = 4.186, Hence, you can find the energy it would require for this conversion. . And the work you do can be a bit more pertaining to your efficiency.

0

You asked: "Does one imply another?" No. Neither implies the other. However, I think there are benefits to first being clear what the ideas are, particularly since I think each idea actually already assumes an arrow of time. In the first case, you start with an arrow of time that only earlier times affect later times, and then end up strengthening that to ...

2

You are sloppy with units, but the result is correct. To go from 25C to 3C is 22 cal/g. When you multiply by 300 g you have cal and your conversion to kJ is correct. Converting to W-hr is silly, but that is the unit of energy, not W/hr. You have 8.3 W-hr you want to remove. That chills the water assuming no new heat is added, so insulate the water. ...

1

This might be better on the engineering SE site but here is some physics to consider: You are right the the heat you need to remove is the mass of water times the temperature difference times the specific heat capacity. Peltier devices and other heat pumps typically have a parameter called a COP - coefficient of performance. This compares their efficiency ...

0

There is a simple way of looking at this. Would a container of gas have a change in temperature if the container was given a different velocity? For your second question, the vibrating membrane acts like a spring pendulum which transfers energy into the surroundings. The membrane does not have a change in temperature until it absorbs the energy back from ...

1

Water forms close to perfect spheres in zero gravity due to it's surface tension. There's a variety of videos of water in the space station. Ice, assuming you start with one of those balls of water, you have to ask first, would it freeze outside in (say, the temperature of the station is dropped below 0 C), or would it freeze inside-out, say you stick a ...

0

Sound will behave just like on earth provided it has a medium to travel through (Astronauts on the ISS can communicate normally; watch some videos)

0

In first place, the temperature is a quantity that measure thermal equilibrium by the zeroth law of thermodynamics. We have the contact with this quantity by with a thermal equilibrium can do. For example, the Celsius units is constructed by define $0°C$ as the volume of mercury in contact with freezing water and $100 °C$ as the volume of mercury in contact ...

0

Since both are at the same temperature, both have the same degree of hotness ie. Temperature, hence similarly cold or hot. The difference is that water and ice both have different enthalpies, Water when converted to ice requires only phase change enthaply(assuming water to be at 273K), the enthalpy of freezing is then, Q(f) = ml

0

First law of thermodynamics is the extension of Law of Conservation of Energy for non-isolated system. There are two forms of first law of thermodynamics(both are actually same): Followed by physicists best suited for dealing with heat-engines: $$\partial E = \partial q - \partial w$$. Here $\partial w$ is the work done by the system. Followed by chemists ...

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 ...

0

Can anyone help with this derivation? There are $N!$ ways to arrange $N$ different objects, but you don't have $N$ different objects, you have $n_0$ indistinguishable objects in the ground state, $n_1$ indistinguishable objects in the first state, and so on. So that means, from your $N!$ different permutations (assuming all the objects are ...

-1

Amps travel in a straight line and so must travel inside the wire. Volts travel around the amps and usually outside the wire. So amps will generate heat - because of the atoms and valence electrons create degrees of resistance - while volts , generally, will not. But if you use thick enough wire you will not notice the heat increase.

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 ...

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 ...

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 ...

1

So the first law for an system where we don't have mass flows in or out ( a closed system ), is $\Delta Q + \Delta W = \Delta U$ Where $Q$ is your net heat added, and $W$ is your net work added, and $U$ is your net internal energy change: internal energy being like the sum of all the different kinetic energies of the molecules. This is why when we add ...

0

In your setup with a moving wall the state of the system after heating is not simply the same as the state of the system at the start other than the fact that the wall has moved. In particular the particles in the gas are moving faster, i.e. the gas has a higher internal energy (I am assuming that the other than the heat you added, no other heat transfer ...

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 ...

0

It can be shown that the work done on an object follows the following equation: $$W = \int F\cdot ds = \int P\cdot dv$$ where $F$ is the force that acts upon the object, $ds$ is the distance it acts over, $P$ is the pressure acting on the object, $dv$ is the change in volume of the object, and the dot is the dot product. In thermodynamics, often when we ...

0

What your textbook says is that during the course of your process, the system is in thermodynamic equilibrium with its surroundings if it's reversible. Such a process is just a theoretical concept as such a process would take an infinite amount of time. Real processes(I mean irreversible processes) take place so quickly that it is impossible for it to be in ...

0

The equipartition theorem says that if the system is in contact with thermal reservoir of temperature $T$ and has Hamiltonian description with Hamiltonian being a sum of terms, one of which is quadratic function of some canonical variable $q_c$ ($aq_c^2$, where $a$ is constant), then the expected average value of $aq_c^2$ is $k_B T/2$. This theorem follows ...

0

Entropy is a state function, correct ! But entropy itself depends upon how you got to that state (final state). In thermodynamics Entropy is considered the quality of heat (hotness/coolness) whereas temperature is considered quantity/degree of heat (hotness/coolness). The reversible processes (and adiabatic i.e. no heat exchange) do not mess with the ...

1

I suggest to compare human produced heat with the incident heat of the sun which is around 1 kW/m$^2$. The usual comparison is "the sun delivers more heat in an hour than humans use in a day". While such a comparison may not remain accurate forever, a difference in scale of 7000x suggests that even if humans doubled thei energy consumption every 17 years ...

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