# Tag Info

48

“Conserved” doesn’t mean “never changes”. It means “this stuff is real, and the only way you have less or more is if some is taken away or added”. You can then follow that additions or subtractions. Since your cold bottle has less energy, the conservation law says that energy has not disappeared, it’s just gone somewhere. You can find it. You can figure ...

23

I'm not sure why people are saying this only applies to closed systems. This law actually applies to all systems. The first law is conservation of energy. It says the change in energy is equal to the energy that enters/leaves it in the form of work or heat. i.e. $$\Delta U=W+Q$$ where $U$ is the internal energy, $W$ is the work done on the system, and $Q$ is ...

23

A piece of rubber can act as a refrigerator of sorts, as follows: If you take a piece of rubber and stretch it out, the molecular chains are pulled out in tension and the kinks in them get straightened out. they are now stiffer, and whatever frequency at which they were originally vibrating will go up slightly- and this means the rubber is now warmer by a ...

6

Like many things, it depends. And it depends on what kinds of things an engineer/scientist is studying. Here's a short list of examples from aerospace/propulsion applications: Diesel injectors, liquid rocket motors -- the pressure is so high that many of the flows are trans-critical or super-critical. This means they deviate pretty significantly from ideal ...

4

Conserved here doesn't mean that it is conserved only for your system (Not unless it's an isolated system). It is conserved for the whole universe. The total energy is constant. In performing any work or task , all you are doing is taking some energy from the surrounding and giving it to the system or vice versa. These 2 effects balance out or cancel each ...

4

You are correct. Ultimately all heat must be transferred from one molecule to another via collisions. That's conduction. But it makes practical sense to distinguish between conduction and convection because their rates are very different. Place two hot eggs into two cold buckets. Stir the one, leave the other one unstirred. Which egg cools down faster? The ...

4

It's written a little incorrectly, but it does look like Hawking's formula for the entropy of a black hole. The $C$ should be $c$, because it represents the speed of light; and the $K$ should be $k$ or $k_{\mathrm{B}}$, because it represents Boltzmann's constant. That is, it should be written as S = \frac{c^3 k_{\mathrm{B}} A}{4 \hbar G}....

3

This perpetual motion machine fails for the same reason that all others involving the buoyant force fail. If the green molecules really can exert a buoyant force on the blue ones, that means they exert pressure, since a buoyant force is merely a difference in pressure. And a blue ball in the sieve only feels pressure in the downward direction. This one-sided ...

3

In response to light a solar cell generates an electrochemical potential which provides the thermodynamic driving force for extraction of work. This is the fundamental operation of a solar cell and is independent of the nature of the incident light — be it from the sun, a LED or a laser — because information regarding the initial photon distribution is ...

3

There are two possible answers, depending on which end you're approaching thermodynamics from. If you're approaching thermodynamics from the macroscopic perspective, the temperature of a system is a fundamental, natural quantity that you can easily determine (by comparing it to other objects of known temperature), and the entropy is a state function that you ...

2

Thermoelectric effects, the Seebeck potential. But in metals it is very small, because temperature does not have much effect on the kinetic energy of the electron gas. It is much larger in doped semiconductors, where the electrons can be treated as a classical Drude gas.

2

Convection and conduction are essentially two different macroscopic approaches to deal with what is fundamentally the same phenomenon on a smaller scale. Convection is basically just Advection and conduction packaged together for simplified analysis. You mention that it's not "mutually exclusive, collectively exhaustive" (MECE); but if I understand the ...

2

You are mixing up two different 'kinds' of velocity. The one referenced in the Bernoulli principle is a stream velocity, where a fluid moves more or less uniformly in one direction. The velocity referenced in kinetic theory of gases is the thermal velocity, which is the average velocity of the random thermal movement of the particles. Take a gas without ...

2

I recommend to abandon this argument based on $P(E)dE$, which I assume means probability that energy is between $E,E+dE$. Boltzmann's distribution law is not primarily about probability of system having some energy $E$ (but of course, this can be derived from it). What Boltzmann's law says is that given discrete possible states $i$ of the system, ...

2

The question is" "if the above equation is valid for all gases, so that at a given value of reduced volume and temperature, all gasses have the same reduced pressure, how can that value of the pressure not be given by the above equation ?" First, it is is not true that all gasses have the same reduced pressure at the same reduced temperature and pressure. ...

2

The typical depiction of the Carnot Cycle is with the use of a closed cylinder containing an ideal gas fitted with a piston with a shaft that extends outside the cylinder to interact with the surroundings. In the following I will refer to the fluid as an ideal gas. First, we approach a hot source (okay, temperature increases and fluid wants to expand) ...

2

When it is said conservation of energy it's referred to the conservation of the energy on a closed system. In your example, you're considering the bottle of water as that system, but the energy lost by the water and bottle due to cooling, phase change, and expansion is given as heat to the refrigerator. Furthermore, the refrigerator is in a constant energy ...

1

The total energy of the gas is the sum of its internal energy at the molecular level and its external energy at the macroscopic level. The external kinetic energy is due to the velocity of the center of mass of the collection of molecules with respect to an external frame of reference. The internal kinetic energy does not depend on an external frame of ...

1

If you read the details of the Joule experiment you will find that the weight fell at essentially constant velocity. That would mean no change in kinetic energy of the mass from the beginning to the end of the fall. The increase in kinetic energy to get the weight started equaled the decrease in kinetic energy to bring it to a stop. The only change in ...

1

It is difficult to say what would be the effect of air because there's more than just the effect of heat transfer from sun to earth (heath transfer, which as you postulate might be larger/smaller when there would be a different matter in-between the two points). Namely, the other effect is that the presence of matter/atmosphere changes the equilibrium ...

1

Do thermoelectric generators use up heat to generate electricity? Yes, some of the heat that enters a thermoelectric generator does not exit as heat and is converted to electricity. In n-type semiconductors charge carriers are "electrons" and and p-type, charge carriers are "holes". These charge carriers diffuse away from the hot side. see figure "...

1

I am re-requesting that someone try to answer my question after now seeing that MIT has done exactly what I posited, extracting energy from diurnal temperature gradient, 3 years after my post: http://news.mit.edu/2018/system-draws-power-daily-temperature-swings-0215 My question might not have been perfect, but it was not about the specific matter of water. ...

1

There are already some nice answers regarding intuitive interpretations of the Legendre transform. What I want to contribute here, is a more physical reason/motivation for why they appear. I.e., instead of focusing on their physical interpretation, I will focus on the physical requirements that uniquely define Legendre transforms. This explains why you would ...

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