Stack Exchange Network

Stack Exchange network consists of 175 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.

Visit Stack Exchange
9

The charge of $1C$ was derived from the definition of Ampere. If you look at the SI units, you'll check that, surprisingly, intensity of current is a basic unit, whereas charge is a derived quantity. This is a bit weird, because charge is seen as "more fundamental" than current, current is "charge per unit time". So why is it? Because measuring the charge ...


6

Electrons will flow as long as there is an electric field to move the. When you first connect the wire to the negative terminal of the battery, the electric field generated by all of those electrons stuck on the negative terminal will cause them to move into the wire. They will basically move to distribute the electrons evenly along the wire. This happens ...


4

Drift velocities in metals are low, also in superconductors, typically less than 1 mm/s. But self-induction may cause a spark if you cut the circuit. And that does not depend on superconductivity.


4

Maxwell's equations (ME) don't by themselves theoretically predict spin 1/2 matter, such as, e.g., electrons. The source term $j^{\mu}$ in ME could in principle consist of, say, scalar matter. It doesn't even have to arise from point sources. We need experiments to tell.


4

Electrons are actually always moving, even when there's no potential difference (aka voltage). They just move in random directions. What happens when there is a potential difference, i.e. when the wire is connected to two terminals of the battery, is that although the electrons continue to move in random directions, there's now a slight bias towards one ...


3

I edited the question to be of the right form. The ampere is now defined by fixing the numerical value of the elementary charge (the charge of an electron or proton) in the International System of Units to be exactly equal to $1.602176634\times10^{-19}$ coulombs. So why not a nice round number, like $10^{-19}$ coulombs, or a nice round number such as ...


3

You think you are observing “classical physics” when you observe humans and planets, but what you are actually observing is the classical limit of quantum physics. Quantum physics applies to humans and planets as well as to electrons and atoms, but what we think of as quantum effects are not noticeable for large objects. There are no special rules for small ...


3

Maxwell's equation don't say anything about the structure of charges, or about the smallest unit of charge. We put the structure of the charge (whether point particle or some continuous distribution) while solving the equations. In classical electrodynamics electron is simply taken to be a point charge because it agrees with many excitements. We assume ...


3

Does it take energy to keep the Earth orbiting the Sun? It does not! It just takes gravitational force, which “costs nothing”. Something similar applies to the obsolete Bohr model of the atom that you are thinking of. The electron has orbital energy, but it doesn’t constantly need more energy to keep going around. All it needs is the electrostatic ...


2

Newton's third law doesn't tell anything about mass. It is about forces, and it says in fact that momentum is conserved in every interaction. See Compton scattering, where a photon interacts electromagnetically with an electron and then it scatters.


2

Yes, a very small body can exert a very large force if it is being acted upon by a very large force (and vice-versa). There is no limit in terms of the mass of the objects involved as to whether they follow the third law of Newton or not. For example, I would like to point out an example which is trivial but is similar in an emotional sense to what you have ...


2

Why do electrons revolve around the atom's nucleus? The first approximate planetary model, Bohr model, is discussed in G.Smith's answer. There the electron is caught at the lowest energy bound state, given by the rules of the model. The current model of the atom has no revolutions, because the term revolution is a classical mechanics term, and atoms belong ...


2

It's a mix of the two. The sequence: [incoming photon and atom; interaction; outgoing photon and atom] is called scattering. There can be elastic scattering, where both the photon and the atom retain their energies, and inelastic scattering, where the energies change. In elastic scattering the photon 'bounces' off the atom, so as to change direction but ...


1

1-Current is flow of electrons . Electrons will only move only and only if there is a potential difference between the ends of a wire . This case also applies to battery as the positive end is maintained at a higher potential than that of negative terminal . Also if you connect an end of a 1km long wire to battery and ground the other end . You will notice ...


1

The pursuit of knowledge is the asking of many questions. The question you have asked here is a complex one, about which whole books have been written, but I will try to give you a brief answer. The quantum rules that govern the behavior of objects as small or smaller than individual atoms are quite different from the classical ones that govern the ...


1

In general, when a bound system is said to be excited, it is in a state other than the ground state. So when an atom absorbs a photon, it's energy increases and it gets into an excited state. However, it is not possible to say what your textbooks mean by an excited state without further context.


1

One way of thinking about this kind of problem is that in the case you bring exactly one electron near the conductor, you will begin to see effects from the fact that any real conductor is made of atoms and the classical theory of a conductor is only an approximation. The classical theory of conductor that you are used to is valid only in the regime in ...


1

Newton's third law isn't a law about massive objects. The law is about forces. Newton's third law essentially just tells us that all forces are interactions. It doesn't tell us anything about the dynamics resulting from such interactions. Like if we electromagnetically put tremendous force on a single electron, would it push back with the same force when ...


1

An isolated atom has no way to get rid of the energy other than to emit a photon, so an isolated atom will always decay by emitting a photon. However if there are other atoms around, for example in a gas, the atoms are frequently colliding with each other. If the excited atom collides with another atom the electronic energy can be converted to kinetic ...


1

In materials there are radiative transitions and non-radiative transitions. When photons are absorbed, they might be 'swept away' by non-radiative transitions. These transitions can be Auger-recombination or simply the electrons falling back down to a lower energy level and release phonons into the material instead. Photons could also be reemitted in ...


1

The following definition of electric current comes from the NCEE reference manual for the PE FE exam in Electrical and Computer Engineering: "Electric current $i(t)$ through a surface is defined as the rate of charge transport through that surface or $$i(t)=\frac{dq(t)}{dt}$$ which is a function of time $t$ since $q(t)$ denotes instantaneous charge." ...


Only top voted, non community-wiki answers of a minimum length are eligible