# Tag Info

16

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 ...

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

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 ...

4

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 ...

4

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 ...

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 ...

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.

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

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 ...

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. ...

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.

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 ...

1

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

$E = pc$ 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 $p$

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 ...

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

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

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 ...

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

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 ...

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

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 ...

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.

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