# Tag Info

## Hot answers tagged thermodynamics

34

The direction of the gravitational force would not change under time reversal. Your object would feel a force downward, just as it does usually. It might be easier to imagine you had a movie of an object under the influence of gravity. Drop the ball from rest some distance above the floor. You'll see it move downward and speed up. You'd interpret this as a ...

15

There is no mistake. The laws of physics themselves are reversible in time, but the solutions not necessarily so. Thus, the "behavior" of the universe itself does not show symmetry under time reversal, primarily due to the second law of thermodynamics. The second law is about the behavior of the solutions, is not a fundamental law in itself. In your specific ...

13

The low-entropy initial state of the universe is an open problem without a satisfactory answer. Your question is the first time I've heard the suggestion that the initial state should have been a crystal; you remind me that the quark-gluon plasma, which was the state of the universe while it was too hot for nucleons to be stable, has been shown to be a ...

12

One of the problems you will encounter is causality. Imagine you have a ball resting on the ground. Without already knowing how it behaved in the past you cannot uniquely define the next frame of your game. You cannot tell if the ball should: move upwards vertically. move upwards in any direction. roll on the ground towards any direction. do nothing. ...

11

Now I am left wondering why does the heat become lost as if travels slightly. It is not lost. It is spread more out. If you stand so close to the heat source that you are hit by, say 1/10 of it's radiation (1/10 of all photons sent out hit you), then when standing further away you are maybe only hit by 1/100. The heat radiation sent from the source ...

8

the two paradigmatic cases that illustrate these two possibilities is a gas, for the first, and a crystal for the second. Paradigms and examples are well and good, but be careful not to assume they are the only possibilities. In particular, black holes have entropy -- a lot of entropy. In fact they saturate the Beckenstein Bound. The entropy of a black ...

6

I suspect this will be closed as "opinion based". I don't believe there is a canonical answer. Usually microscopic scale relates to phenomena that occur on a level much smaller than the system under consideration (atoms in a crystal when you are thinking about the crystal, for example). There is an analogy with micro- and macro-economics. Micro-economics ...

5

This is such a complicated question! The worst part is that as heat leaves the chimney, it draws air from the room with it - air that needs to be replaced from outside. This actually makes fires quite good as ventilation systems. Whether a fire heats a building depends in very large part on the degree to which cool air can flow past parts heated by the ...

4

What I will state is speculative and based on the statistical mechanics derivation of entropy, and just the way I view it and do not consider that there exists a problem. After all thermodynamic theory emerges from the underlying statistical level of atomic and molecular interactions. where p_i is the probabability of microstate i. Setting aside quantum ...

4

The black hole initially lost the gravitational energy that was needed to create the pair. The pair-creation model is a bad description of Hawking radiation, which for macroscopic black holes is really photons. The second particle that gets created above the event horizon doesn't have nearly enough energy to escape. It does, however, produce photons above ...

4

Imagine a flowerpot sitting on a ledge. A breeze blows the pot off of the ledge, and it falls to the ground. When it hits the ground it shatters into a bunch of pieces, it kicks up dust, it makes a sound, it vibrates the ground, and the shards come to rest. The time-reversal of this is that some pieces of flowerpot are sitting on the ground. Suddenly a ...

4

To make such a game and have it make sense, you would have to use a much simpler physical system than anything you would reasonably encounter on Earth's atmosphere. For heat-dissipating systems, the laws of thermodynamics give time a direction: the direction of increasing entropy. Thus an object that falls to earth dissipates its energy, mostly as heat. You ...

4

No, you cannot prove that mathematical law. All you can say is that your experiments are consistent with your premise. I recommend you take the time to read some introductory books on physics, statistics, and the scientific method (which is to say, don't just take my word that my initial statement is valid).

3

Joule's law, and thermodynamics in general, is a model of the classical world. Here, classical should be interpreted as non-quantum-mechanical. Thermodynamics is the study of large collections of particles and their collective behavior. No microscopic model is assumed, and one tries to extract as many (non-trivial) features as possible based on purely ...

3

The heat radiates away from the source equally in every direction (until it hits something). Imagine the sun. It is roughly spherical and radiates energy (some as heat) in all directions. The energy is radiated in a sphere. As the energy travels away from the source (the sun) the sphere expands but the amount of energy remains the same. When the sphere is ...

3

In fact it's quite the opposite : cold places in a room are due to contact with the cold outside : door, window, bad isolation ... So putting heater here make you also heat the outside, and waste some money. The reason to do so is that the point of a heater is to make cold places hot : as you said you want an uniform temperature.

3

This is an excellent question, but not readily answered, as discussed in Floris's Answer. Here is a stab at how to get an estimate of efficiency. It is the method which I believe is reasonably accurate: the actual values will need to be refined by experimental measurement: I am not too confident of the actual numbers that fall out owing to the ...

3

The heat simply disperses and the further away from the heat source you get the more space the heat has to disperse into

2

The simple answer is the inverse square law which states that the intensity (power per unit area) of a heat source drops off with $1/r^2$ where $r$ is the distance (assuming a point source). Looking at your camp fire: imagine people standing side by side around the fire. Each gets a share of the heat. When they make the circle wider, more people can get ...

2

The answer is, it depends on the type of fireplace you have. An ordinary brick chimney consisting of a fireplace directly venting up and to the outside has a very low efficiency. The draft generated by the fire pulls warm air from the house, and most of the heat travels directly up. There are various types of fireplaces which specifically attempt to ...

2

Actually, there is a reversible process that will allow you to end up with a larger temperature, and this does not violate the second law. To see this, you can couple your system to Carnot engine, which will extract and "store" work until both the reservoir and the case are at the same temperature. Now use the stored energy to heat the case. Actually your ...

2

That's a very hard question to answer with the appropriate level of detail! Very broadly speaking in an ideal metal all atoms are forming a perfectly regular crystal lattice. Conduction band electrons can move freely around these atoms, which makes it easy to pass a current trough the metal. In a (theoretical) metal with perfect crystal lattice the ...

2

In reverse time, it would seem gravity is still attractive. But the quick impulse force that stopped the motion of something falling would instead send it upward at some inital velocity, which gets slowed by gravity. Otherwise, the object would accelerate from rest away from earth. My hunch would be that all conservative force fields remain the same ...

2

Proporties that are independent of time will not change when time is reversed, forwards or backwards, or changed in any other way. Anything dependent of time will run backwards. In general we may say that any change of any proporty (causing a difference $\delta f(t)$ over a time difference $\delta t$ between "before" $t_{1}$ and "now" $t_{2}$, that is ...

2

In Reverse Time, objects would experience a inversion of velocities and accelerations. Instead of a falling ball moving toward the ground and gaining velocity, it would be moving away from the floor and losing velocity. When it returned to the spot it was dropped, its velocity should equal 0. To achieve this, change the sign on it's velocity along the ...

2

Physics is not mathematics. Physical laws are not axioms. A physical theory is not the derivation of all possible hypotheses from a set of axioms. Please repeat this like a mantra a hundred times a day for the next three weeks. Instead a physical theory is patterned as a set of naive ontological assumptions about the approximate usefulness of a set of ...

2

In short, ice-cream is a solid, but it is filled with air bubbles. All this air that was incorporated in the ice-cream as it froze (through various industrial processes) means that the dairy product doesn't freeze as a solid, but rather a foamy delicious treat. There are many websites explaining what it actually is, and how it is made, such as this one. ...

2

The type of blankets is not defined, different can mean many things. If one makes two classes, more insulating ones and less insulating ones the answer of Photonicboom applies. The more insulating one next to the skin will give a comfort value sooner than the less insulating one because the body will have to heat up a smaller amount of matter (air and ...

2

In a still air environment, the previous answers are correct and sufficient. In an environment with unheated air moving over you, however, the answer greatly depends on what blankets you have, exactly. Humans lose a lot of body heat to evaporation of sweat, so the amount of air passing across you makes a big difference in how warm you stay. Let's play a ...

2

Roughly speaking solid matter is on a lattice form, A three-dimensional lattice filled with two molecules A and B, here shown as black and white spheres. The molecules fit like LEGO , the forces tying them together are mainly the spill over electric field forces , attractive and repulsive forming the patterns of the lattice. In a single crystal one ...

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