Recent content by da_willem

U're probably right. I'm just looking for an easy way from the expression we have for the force on a single dipole towards the Kelvin force on a magnetized material consisting of many dipoles.

Thanks for your feedback! Given the fact that all of my references date from before the seventies the 'importance' might be not that high. The effects studied are not entirely new but can be better understood with the results of my simulations, also some of the fundamental principles behind...

I worked for a few months on simulating the magnetohydrodynamics of insulating spheres and found some novel relations and insights I have never seen in print during an extensive literature search. My supervisor advises to write one or two publications about this and I would be glad to, but for...

Thanks for your response, so as long as there are no free currents at the position of the dipole the expressions are the same. The first of the expressions stated includes an interaction associated with an inhomogeneous magnetization whereas the second does not. Wouldn't it imply that there can...

The force equation with H (M gradH) (Kelvin force) is the standard expression used in ferrohydrodynamics (see e.g. Rosensweig) when dealing with dilute colloids for example. I'm wondering if the use of H instead of B results from the fact that only the external applied magnetic field should...

The force on a single dipole (dipole moment m) can be obtained by taking the gradient of its energy, i.e. \vec{F} = \nabla (\vec{m} \cdot \vec{B}) One also often encounters for the energy of a magnetized material -\mu_0 \vec{M}\cdot \vec{H} And often the force on a material with...

I guess for example the field of an electromagnet is both solenoidal and irrotational, I had the Helmholtz decomposition wrongly in mind. But still, is the term \vec{m} \times (\nabla \times \vec{B}) often/usually negligible or does one generally have to use the first term \nabla...

There has been some dispute in the past about the validity of the electric current model of a magnetic dipole producing a force \nabla (\vec{m}\cdot\vec{B}) versus the magnetic pole model producing (\vec{m}\cdot \nabla)\vec{B} (see e.g. Boyer `87). I think for elementary particles this dispute...

Most of the light emitted by the sun is what now for us is 'visible light'. Our eyes evolved to become sensitive for a range of light in which the sun emits the most. Ultra-violet is emitted far less by the sun than visible light, so it would not be that useful to be able to see this.

Divide both sides with \sqrt{2}+1 work the product in the denominator to yield one, and you'll see.

Indeed you can have the poles of two magnets oppositely directed repel each other as in these toys. The stabalizing factor here is a large angular momentum, which you have to apply by making the toy spin very fast. I got one myself, but I'm not so skilled in using it however... One can...

I can't help to, again, point to the article of Ohanian (What is spin, Am j Ph '84) who continues calculations of Belinfante. His conclusion is that spin is associated to an intrinsic rotating energy flow around a particle. So, yes it is intrinsic, but it is associated to a rotation. Maybe not a...

That's what I always thought! (see e.g. https://www.physicsforums.com/showthread.php?t=170928) Thanks for your calculations, they confirm what I always thought! Great work!

Often if you see in which direction the force acts you can find the direction of rotation. If not, and you only have the torque, you can use the corkscrew rule to find the direction of rotation. I.e. point the thumb of your right hand in the direction of the torque and then your fingers curl in...

I managed to get the r,theta component of the stress tensor as it is written in the book now. But I still can't see why there would be squares on the r's in the first formula I posted; anyone?