Recent content by Arham

Hi. Consider a solid body placed in vacuum. How much is its pressure? Can we describe this body using thermodynamics if its pressure is zero?

Hello. In Molecular Dynamics simulations, the Newton's equation of motion is used to calculate the time evolution of system. Once, I read in an introductory text that when the thermal de Broglie wavelength $$\Lambda=\frac{h}{\sqrt{2\pi mkT}}$$ is much smaller than the interparticle distance...

Dear mfb, I think I found the solution. \partial{P}/\partial{t} is some kind of current (bound charges are moving). So if we write total current density as J=J_f+\nabla\times{M}+J_p where J_p is polarization current density, we can solve the problem.

\partial{D}/\partial{t}=\epsilon_0\partial{E}/\partial{t}+\partial{P}/\partial{t}. The second term is underivable from Ampere's law in vacuum.

Hi We can derive equation \nabla.D=\rho_f from equation \nabla.E=\rho/\epsilon_0. But what about Ampere's law? I tried to derive \nabla\times{H}=J_f+\partial{D}/\partial{t} from \nabla\times{B}=\mu_0J+\epsilon_0\mu_0\partial{E}/\partial{t} but I could not. This is strange because I thought...

I think it's possible. After all, sunlight is nothing but a kind of electromagnetic wave.

Hi Consider a beam of electrons. It is a non-neutral current of charges. You can assign a current density to it. In magnetostatics, currents are steady, so \partial\rho/\partial t=0, and the continuity equation becomes: \nabla.J=0, where J denotes the current density.

Thanks for your good and clear answer. The only thing I'd like to correct is that in magnetostatics fields are NOT time-dependent.

Hello In magnetostatics theory, there exists a current of charges. So in this situation charges are not stationary, and the Coulomb's law, and all the relations derived from it, are not valid. My question is how can we obtain electric field when dealing with steady currents (within...

Consider just these two systems: 1. The under study system which is in steady state, and 2. the rest of the world.

Coordinates of a system in steady state don't change with time either.

Thanks. I know this. Look! When can we assign a temperature field to a system? When there is local thermal equilibrium in the system. But what is the exact definition of thermal equilibrium?

Thanks. But I think definition of temperature is based on definition of thermal equilibrium. In other words I think at first we must define thermal equilibrium, and then temperature.

Hello In Zemansky's book there is this definition of thermal equilibrium: "Thermal equilibrium exists when there is no spontaneous change in the coordinates of a system in mechanical and chemical equilibrium when it is separated from its surroundings by diathermic walls. In other words...

D.J. Griffiths in his "Introduction to Electrodynamics" writes: "Some authers consider Einstein's second postulate redundant - no more than a special case of the first. They maintain that the very existence of ether would violate the principle of relativity, in the sense that it would define...