Recent content by Jano L.

I am not sure what you mean. My reasoning attempts to answer the question : which forces do the work on the iron body when it is attracted to the magnet. If we have just two charged particles and no magnet, then the original question does not arise. If you are wondering where the increase of...

If the work is done too quickly, yes. But if it is done in a quasi-static manner, the decrease of internal energy can be made arbitrarily small, far less than the work done, because the system(iron body) is not isolated. In general, macroscopic systems in interaction with other systems can do...

Sure, but the Poynting theorem on its own implies nothing about what the expression for EM force acting on a particle is, or what the expression for work of that force is. The former depends on the model of forces - in the Ampere tradition, the magnetic force is always perpendicular to velocity...

It is not zero in general, and can be quite measurable, indirectly, as electric potential that decreases the farther from the wire you go. The direction of this field when near the wire is along the wire, when farther from wire, more radial and hard to determine - it depends on the objects in...

There are two ways to understand the magnetic attraction and resolving the 'magnetic force does no work' problem. The first one is to use the magnetic pole description of matter, where magnets are modeled as collection of equal number of positive and negative magnetic pole that interact via...

In fact, Planck invented and published a model that explains the blackbody radiation as a result of "quantized" oscillators interacting with "classical" radiation. "quantized" in the sense when the oscillators emit radiation, they do so only by losing integer multiples of energy ##h\nu##...

Historians of science have repeatedly pointed out that the problem with the Rayleigh-Jeans spectrum (nicknamed 'UV catastrophe'), although it was known at the time Planck did his work, was not the motivation that lead Planck to his work and results on blackbody radiation. Reportedly what he was...

In such case the Gauss law electric flux through surface = charge inside the surface cannot be used. This is because if charge is distributed on a 2D surface, the surface is a place of electric field discontinuity and neither the electric flux nor the "electric charge inside" have obviously...

Internal energy of the fluid can change as well. This can only happen if the fluid gets compressed or if internal friction in the fluid is significant (large gradients of velocity). When that is so, the assumption of incompressibility that the Bernoulli equation derivation is based on is no...

Only as a special case. In general, pressure is a function of position and usually changes in downstream direction.

Right. Let us first make sure we all understand what the system is: it is that part of the spacecraft that is not converted to burning ejected exhaust gas. The gravitational PE is increasing, but it is unfortunate to say "the gravitational force F_g tries to decrease KE while increasing the...

That's wrong. According to Maxwell's equations, the time derivative of B vanishes if and only if $$ \nabla \times \mathbf{E} = \mathbf{0}. $$

The element of the fluid is conveniently chosen so its two lids are both perpendicular to velocity and have the same area. Only then total force on the element in the direction of flow is ##Adp##.

The first method is appropriate when the process that changes the mass does not result in additional force on the remaining body. For example, when the body ejects mass in two opposite directions with the same rate. It is not valid for a rocket in flight, because mass is thrown away in a...

Newton's laws operate with down-to-earth concepts derived from experience. The 2nd law which uses the concept of mass in "motion" ##m\mathbf v##, is a physical law based on experience only if acceleration, force and mass have meaning independent of the law itself. In present-day parlance this...