# Tag Info

## Hot answers tagged models

20

### Why do people categorically dismiss some simple quantum models?

I can tell you why I don't believe in it. I think my reasons are different from most physicists' reasons, however. Regular quantum mechanics implies the existence of quantum computation. If you believe in the difficulty of factoring (and a number of other classical problems), then a deterministic underpinning for quantum mechanics would seem to imply one of ...

9

### Modeling non-quantum objects (in finance, sociology etc) using fermionic fields?

Actually a paper recently came out, and highlighted in Popular Science, discussing using fermionic field concepts to model crowd avoidance at Netflix. You can imagine that the same concept could be used to consider in any situation where there are large numbers of people competing for limited preferred items. Update Now that we have a few minutes, ...

7

### Why do people categorically dismiss some simple quantum models?

This could have been a comment, but as it actually anwers the question asked in the title, I'll post it as such: As far as I can tell there's no rational reason to dismiss these models out of hand - it's just that quantum mechanics (QM) has set the bar awfully high: So far, there's no experimental evidence that QM is wrong, and no one has come up with a ...

5

### Bohr's model of an atom doesn't seem to have overcome the drawback of Rutherford's model

Classically emission is continuous and the electron would need to occupy a "in between" energy level for a while, and that is forbidden in Bohr's scheme, so the emission can't be allowed to happen. This doesn't really explain why it can't happen, but that's phenomenology for you: you line keep lining up facts until your kludge (1) gets the right answer and ...

5

### Macroscopic laws which haven't been derived from microscopic laws

This is an example from hydrodynamics. When the effects of viscosity can be ignored (inviscid flow), a uniform incident flow can exert on immersed bodies only lift forces perpendicular to the asymptotic flow velocity. However, there exist an infinite number of solutions of the flow equations of motion satisfying the asymptotic conditions at infinity and the ...

3

### Bohr's model of an atom doesn't seem to have overcome the drawback of Rutherford's model

Unfortunately, nobody reads Bohr nowadays so Bohr's arguments are not understood and transmitted. Modern quantum mechanics is more complete and superior as a physical theory to the old quantum mechanics, so the omission is perhaps understandable, but it is not forgivable. Bohr's ideas explain the stability of H-atoms in a reasonable way, which is also ...

3

### Why do people categorically dismiss some simple quantum models?

There are two questions here: Why criticize your models? And are there better ideas? I will try to answer the second question in a separate answer. Here I only give some comments of a general nature to adress the first question. I personally agree with you, and I think most people who care about this stuff do too, that it is disconcerting to have a theory ...

2

### Macroscopic laws which haven't been derived from microscopic laws

As Ron noted, there are many, many examples within condensed matter; they often share a very similar story where the microscopic laws are known well (exactly, for the case of simulations), but the macroscopic laws are derived by symmetry concerns. Take for example, liquid crystals. We could simulate a collection of hard rods or ellipsoids - this is our ...

2

### A problem of approximation [duplicate]

Even a physical quantity which changes by discrete amounts can often be well approximated by a continuous function of time. The derivative is a property of a mathematical function. Any differentiable function must necessarily be continuous, and a continuous function will change by arbitrarily small values for an arbitrarily small change in inputs. The ...

2

### Why do people categorically dismiss some simple quantum models?

Foundational discussions are indeed somewhat like discussions about religious convictions, as one cannot prove or disprove assumptions and approaches at the foundational level. Moreover, it is in the nature of discussions on the internet that one is likely to get responses mainly from those who either disagree strongly (the case here) or who can add ...

2

### Modeling incoming solar radiation

If you are interested just in the direct irradiance you can neglect the emmision and scattering terms in the Radiative Transfer Equation RTE wich can in this case be simplified to allow only for absorbtion and is known under the name Beer-Bougert-Lambert's law of absorption $$\cos\theta_{0}\,\frac{\partial}{\partial p}S^{i} = -\frac{\kappa^i}{g}\,S^{i}$$ ...

2

### Physics of the electric hot plate

I think there is some interesting physics to be had here. The rate of change of temperature depends on the rate of heat flow in from the electric heating element and the rate of heat flow out as heat is lost to the air. If we write the heat capacity of the hotplate as $C$ ($C$ is the traditional symbol for heat capacity) then:  \frac{dT}{dt} = C \left( ...

1

### Eddy current losses in electric steel by harmonics of a magnetic field

I'm no expert, but ... MIT's "Magnetic Circuits and Transformers" discusses eddy current losses in chapter V.2. An approximate formula for a magnetic sine wave at frequency $f$ and peak amplitude $B_{max}$ (in Tesla, mks units throughout) is: $$P_e = k_e f^2 t^2 B_{max}^2 V$$ where $k_e = \pi^2/(6 \rho)$ theoretically (but in practice is often ...

1

### Modeling incoming solar radiation

Ok, I'm still not sure on what level you want to do this, but I will start you off with some basics. The most important factor is probably the solar elevation angle, $\theta$. As described on the wiki-page it can be calculated using this formula: $$\sin\theta=\cos h\cos\delta\cos\Phi+\sin\delta\cos\Phi$$ where $h$ is the hour angle, $\delta$ is the solar ...

1

### Is energy always proportional to frequency?

Yes. For photons in vacuum, the energy per photon is proportional to the photon's classical, electromagnetic frequency, as $E = \hbar \omega = h f$. Here, we see a connection between two classical properties of light: the energy and frequency. What is surprising is that the relation holds for matter, where there is no classical equivalent of the frequency. ...

1

### If a cart hits a wall, does the weight of it affect how it moves, when the center of gravity is constant?

No. When you hit the wall, the bicycle rotates around the front axis. The angular momentum L that you create for an arbitrary number of mass particles is $$L=\Sigma_i(r_i \times m_iv_i) .$$ If you split location r=R+r_i and v=V+v_i with R and V being center of mass location and velocity, respectively, and r_i and v_i deviation from it, then it can be shown ...

1

### Does a scale model with 1/2 the linear length have 1/8 the mass?

OK, now we have a better idea what you're trying to do. If we can assume elastic collisions, then answer should be independent of the mass of the bike (though it will depend on the relative distribution of the mass, i.e. center of gravity and moments of inertia). However you will need to think how to scale the velocity of your model. One way to think of ...

1

### What is the simplest system that has both, discontinous and continous phase transitions?

First, make sure to read up on definitions to clarify what you are looking for - classification of phase transitions isn't 100% science, and has a little bit of fussiness to it. Wikipedia's page isn't terrible. Second, I can't tell you whether it is the simplest or not, but as I understand your question, the Ising model itself satisfies your conditions, as ...

1

### What is the simplest system that has both, discontinous and continous phase transitions?

I think you are wrong with water. Above the critical point there is no transition in water at all. This is also true for any other isostructural transition: as soon as there is no symmetry difference between the phases, in the continuous case you cannot say, if the transition has already taken place. A correct example should be probably, KHP (potassium ...

1

### Why do people categorically dismiss some simple quantum models?

This question tries to reproduce quantum mechanics from classical automata with a probabilistically unknown state. Probability distributions on Automata states Start with a classical CA and a probability distribution on the CA. To keep things general, I allow the CA to have some non-determinstic evolution, but only stochastic probability, no quantum ...

1

### A problem of approximation [duplicate]

Physics is all about making the right approximations, in the hope that we can gain some actual physical insight into our problem and make verifiable predictions. For example, say you wanted to calculate the trajectory of a cannonball that has been fired from a cannon. It would be a Sisyphean task to account for all the possible variables that could affect ...

1

### Bohr's model of an atom doesn't seem to have overcome the drawback of Rutherford's model

Ron Maimon's answer is very good and I agree with him, but I would like to point out that Bohr's model is not completely consistent with classical electromagnetism. On the one hand, the electron does not radiate as dmckee and Ron have explained. But, on the other hand, in Bohr's model the electron is a pure particle in a closed orbit and it is therefore ...

1

### Formulation of general relativity

General relativity is a classical theory. I will restate your dilemma as follows, since this is how Einstein stated it: We have an abstract manifold consisting of points, vectors that link nearby points, and a metric tensor that tells you the distance between nearby points. What makes these points physical? How can we tell point A apart from point B? Since ...

1

### Can anyone estimate what proportion of water remains after I flush a toilet?

Use a commercial blue dye in the tank, and flush until the blue color disappears. You can find the e-folding rate by eye. The amount of water removed per flush depends on the toilet, from memory, it's about .9 in a normal flush, so that 90% of the blue color is gone after one flush, so that you lose all noticible blue after about 4 flushes.

1

### Macroscopic laws which haven't been derived from microscopic laws

Any problem that requires solving of non-trivial Schroedinger equations. For example, protein folding problem. It is known what equations the system should satisfy and those equations can be written down. Yet they cannot be solved with modern computers which would take millions of years tor that.

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