# Tag Info

1

The obvious Google search finds various articles on the subject, including this one that has a graph of excitation lifetime against temperature: The lifetimes vary from about 600$\mu$s to about 3ms, so a 5 kHz signal (200$\mu$s) would indeed appear steady.

0

The conditions needed to produce lightning have been known for some time. However, exactly how lightning forms has never been verified so there is room for debate. Leading theories focus around separation of electric charge and generation of an electric field within a thunderstorm. Recent studies also indicate that ice, hail, and semi-frozen water drops ...

-1

Basically.. it's not basic at all!! The whole phenomenon is quite complicated and there are bits and parts that we still don't fully understand! For a quite nice introduction I suggest you to read Feynman's second volume, chapter 9: "Electricity in the Atmosphere", you can find it online too!.

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Checking for electron degeneracy is a matter of comparing the Fermi kinetic energy with $kT$. If $E_F/kT \gg 1$, then you may assume the electrons are degenerate. The central density of the Sun is around $\rho=1.6\times 10^5$ kg/m$^3$ and the number of atomic mass units per electron is around $\mu_e =1.5$. The number density of electrons is therefore ...

0

There is no need of a potential for the Schrödinger equation to have a solution, namely $$i\hbar \frac{\partial}{\partial t} |\psi\rangle = H |\psi\rangle$$ does possess solutions even when the Hamiltonian contains no potential. Questions of normalisability may arise, but that is another point (and can anyway be solved by expanding in Fourier terms). ...

1

$E = pc$ is only true for massless particles. For massive particles you have the mass-shell relation: $E^2 = m^2c^4+p^2c^2$ After you use $E=T+mc^2$ and you can find $p$

0

Yes, a free particle has a wavefunction. If it has sharp momentum $p$, it is given by the plane wave $$\psi_p(x) = \mathrm{e}^{\mathrm{i}px}$$ which might look a bit strange, because it is non-normalizable/not square-integrable, but that should not be all too troubling - no momentum uncertainty at all is rather unphysical, after all. The general ...

0

The decay is proportional to the number of atoms left. Where growth (or decay) is proportional to the size of the population, you have exponential growth (or decay). In fact the time constant is basically defined in terms of this exponential.

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Single particle energy eigenstates for a system of particles in a box are given by $$E_n=\frac{\hbar^2 \pi^2}{2mL^2}\,n^2 + E_0.$$ The Fermi energy for a single particle is, by definition, the value of its energy that exhausts all the possible states given by $N$ indistinguishable particles; in the case at hand, for fermions (electrons), this is given by ...

1

You might find the wiki article on this topic helpful. Summarizing: When you have a 1-D box, the energy states of an electron can be given by $$E_n = E_0 + \frac{\hbar^2 \pi ^2}{2 m L^2} n^2$$ Now the things to note are this: Two electrons (with opposite spin) can occupy the same level The Fermi level is the energy of the last electron After each pair ...

5

Yes excited states have a non-zero lifetime. Electronically excited states of atoms have lifetimes of a few nanoseconds, though the lifetime of other excited states can be as long as 10 million years. The decay probability can be calculated using Fermi's golden rule. The lifetime is then an average lifetime derived from the decay probability. The lifetime ...

0

The particle and antiparticle pair don't emerge from nothing, but rather the field (e.g. lepton field for electrons and positrons) that permeates the vacuum over all space. So pair creation an annihilation isn't tied to the vacuum but to the quantum field, and it happens everywhere - not just in an experimental vacuum. I.e it happens in the nucleus of an ...

1

The characteristic time of interaction - energy of interaction relation between two systems is usually written as $\delta E\cdot\delta t\sim\hbar/2$ (do NOT mix with the uncertainty principle). So the characteristic time would be about $\delta t\sim\hbar/(2\delta E)$, where for $\delta E$ we can take the difference of energies between two states.

0

As you have noted, there can be multiple contributions to the thermal conductivity. Basically, any aspect of the material that can move 'independently' can transfer energy from one place to another. Since the electronic subsystem can often be taken as independent of the ionic (lattice) subsystem, those are the two main terms that are written out. However, ...

0

Resonant frequency of an object: A frequency at which the object will best capture and retain energy from a driving force. Driving energy into an object or system with a force at the natural frequency of the object will maximize energy transfer.

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The resonant, frequency is the frequency at which an object tends to vibrate. Every rigid object in existence has a natural structural resonance frequency, a frequency at which it, metaphorically speaking, wants to shake more than any other. If you vibrate an object at its resonant frequency, it will gradually shake more and more wildly. This frequency ...

0

Sometimes electrons do "crash into the nucleus" - it's called electron capture and is a mode of decay for some unstable isotopes.

2

For those wondering why zinc sulfide is important, I will note that "a zinc-sulphide screen in vacuum" is specifically called out in the original Geiger and Marsden papers on alpha particle scattering. It was already well accepted as the coating for the early cathode ray tubes, and zinc sulfide would become one of the main phosphors for CRTs for television. ...

2

If you had a mole of electrons and a mole of protons and put them together, they would make hydrogen. The transition from ions to ground-state atoms would release 13.6 eV/atom or about 1300 kJ/mol. This mole of hydrogen would have a mass of one gram. For comparison, combustion of 1 kg of gasoline releases about 44 MJ of heat; your completely-ionized ...

1

I assume that what you're getting at is something like "what would it look like if we created something the size and mass of a basketball, made of only neutrons?" If this is what you're getting at, you should consider what is meant by what something "looks like". This generally means how does visible wavelength light interact with it? A regular basketball ...

-1

Can electrons reflect light? Yes. Like CuriosuOne said, electrons are shiny. I kid ye not, google on electrons shiny. Metals are shiny because they have free electrons. Check out this question about the colour of metals, where Ali said a metal is are silvery because it "reflects all wavelengths specularly (more or less)". Also see this article by William ...

1

DC current is organized as following: positive potential applied to one end of the wire, negative potential applied to the other. Electrons move from one end to another with some speed. If you have one electron in vacuum and electric field from A to B, then there will be force acting upon that electron due to $F=eE$. Movement should happen along line ...

0

You're correct that it's not the high voltage itself which ionizes the atoms, but rather the free electrons accelerated by this electric field. In any system at finite temperature, there is a non-zero probability for some atoms to be ionized at any given time. Applying a strong electric field causes the free electrons to accelerate. Collisions with bound ...

1

It is important to remember that van der Waals' forces are forces that exist between MOLECULES of the same substance. They are quite different from the forces that make up the molecule. For example, a water molecule is made up of hydrogen and oxygen, which are bonded together by the sharing of electrons. These electrostatic forces that keep a molecule intact ...

1

To understand what is going on, you need to understand something called unitarity. Unitarity basically just says that anything that can happen in forwards in time can also happen backwards in time. So in this case, unitarity means that if the particle can go from $\Psi_0$ to $\Psi_1$, then it can also go from $\Psi_1$ to $\Psi_0$. Now what does that have to ...

1

Even in the classical model, an infinite amount of levels doesn't necessarily mean that it occupies an infinite amount of space. You can divide any finite distance into infinitely many bits (for instance, $1 = \frac{1}{2} + \frac{1}{4} + \frac{1}{8} + \ldots$). EDIT: I'd forgotten about the $r\sim N^2$ relation that the OP mentions below, so yes, although ...

2

The lightning only 'sees'positive and negative charges. If the storm clouds are negatively charged then they drag a positive charge along the surface of the ocean. When the charge reaches a certain capacitance a lighting strike will neutralize the potential. Like some people, the strike follows the path of least resistance; which is usually the highest ...

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I have seen lightning hit the middle of a sea lake. ( very happy I had not gone swimming). The water did not boil enough to be observed at my distance, about 500 meters. No dead fish were washed out. A boat or a head in the sea water will become a focus for the upward streamers that will join the downwards leaders and form a path for the energy of the ...

2

I guess the answer you are looking for is that the electric field propagates at the speed of light. Suddenly add a voltage source to a complete circuit and the electric field will spread at the speed of light $c$. Depending on how far away a specific electron is in the circuit, this electron will soon feel this electric field and then immediately react to ...

2

Electrons do not "decide" which path to take in any meaningful precise sense (they don't take any particular path at all unless an interaction fixing their position takes place every step along the way), hence there is no time span in which that decision is made.

0

Your statement "Maxwell's equations imply that magnetic fields are due to changes in electric fields." is not complete. A corrected statement is that Maxwell's equations imply that magnetic fields are due to changes in electric fields AND due to currents (which can be stationary): $$\nabla\times\mathbf{H} = \mathbf{J}+\partial\mathbf{D}/\partial t$$ As ...

1

This view would not be accepted by physicists today. Charged particles have mechanical mass, momentum, and energy (rest and kinetic) and the fields have energy and momentum. Total energy is conserved. Total momentum is conserved. Are there cases where it can be sensible to imagine field momentum as an additional mechanical momentum? Sure, consider the ...

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The problem is that the two calculations have hardly anything to do with one another - so it's no wonder you don't get the same result. The electron volt, as you say, measures the work you need to move an electron across a potential difference of one volt. On the other hand, if you want to calculate the mass of an electron using $E=mc^2$, what you need is ...

1

It not produced from, it is the exact same force that do difference thing. Magnetic force is like a torque coiling around direction of electricity. And so if electric charge run in circle then the magnetic that coiled around will be merged in one direction, produce a strong magnet Still the force is actually came from attractive and repulsive force of ...

1

I'd just like to add to diracpaul's excellent answer, which to summarize, makes the point that both electricity and magnetism belong to an inseparable whole: you can't treat one as a more fundamental phenomenon producing the other. Probably the most compelling reason for looking at things in this "indivisible" way is special relativity, wherein we accept ...

6

There is neither electricity alone nor magnetism alone produced by it. There exists an inseparable electromagnetic field produced by (moving) electric charges, in mutual interaction with them, and the splitting to its "electric" and "magnetic" parts depends upon the motion of the observer.The concept of electricity alone and magnetism that produces is due to ...

5

More than one photon can be absorbed, but the probability is minute for usual intensities. As a scale for "usual intensities" note that sunlight on earth has an intensity of about $1000\,\mathrm{W/m^2} = 10^{-1}\mathrm{W/cm^2}$. The intuitive reason is, that the linear process (an electron absorbs one photon) is more or less "unlikely" (as the coupling ...

1

I'll attempt to "explain" it as simply as possible but without cheating ("not simpler"). This is indeed not an answer to the "how" does something happen, but rather how physicists describe it. in physics, one calls a scalar field a function $\psi:\mathbb{R}^4 \rightarrow \mathbb{R}$ from $\mathbb{R}^4$ to $\mathbb{R}$, (where $\mathbb{R}^4$ is the ...

2

It is an observed result that a moving electric charge can produce a magnetic force. As to the 'how', that's a bit troublesome. At this level, one has to accept some things as axiomatic. As to the force at a distance, this is also true of ordinary electric potential. So magnetic force at a difference should be no more troublesome than electric force at a ...

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