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Ever seen a spring? They are everywhere. What kind of material would you consider incompressible? Steel? If you put a force on it (called "stress"), it will be squeezed by a certain amount (called "strain"). The ratio of the two is called "elastic modulus" (if you like big words). In fact, if you have a rod of that material, and tap one end of it, sound ...

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Atoms themselves cannot be compressed in general. Liquids are not very compressible (they are compressible) because of distances between atoms. Gases have the most distance between atoms. Liquids have very little space in-between atoms, so when you press on them, the their is not much more room to budge. Solids can budge because of there crystal ...

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Neutrons have almost the same mass as protons and the unit of atomic mass is (at the moment: see footnote) $\frac{1}{12}^{th}$ of the mass of a carbon 12 nucleus. The mass of nucleons varies a little depending on which nucleus they belong to, but the difference (the binding energy) from their mass when free nucleons is small. So, to a good approximation, the ...

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Density is a 3-form, since you would write it as $$\omega:=\rho\text dx\wedge\text dy\wedge\text dz.$$ In special relativity it remains (the time component of) a 3-form. More specifically you have a current density $J$ of the form $$J = \rho\text dx\wedge\text dy\wedge\text dz + J_x \text dt\wedge\text dy\wedge\text dz+ J_y \text dx\wedge\text dt\wedge\text ... 0 Your first equation is k_F in the Sommerfeld model, so long as the variable n is the electron density, n = N/V, where V is the volume of the system and N is the total number of electons in the system. I'm not quite sure what you meant to write for n, but it probably works out to the electron density. 1 If the electric charge density of a region of space is negative, that would mean that there are more negative charges than positive charges in that region. When people use the word "density" casually, they usually mean mass density (or sometimes number density). Mass (as far as we know) can only be positive, and the number of particles can only be ... 0 It tells us that the charge is negative. Surface charge density is negative when the surface is covered by -ve charge. Same goes for volume charge density. Like any density, charge density can depend on position, but charge and thus charge density can be negative. Wikipedia 1 In a subsequent video the same person attributes the phenomenon to mercury's high surface tension and non-wetting, non-wicking chemistry. Mercury makes a convex meniscus at the interface between the mercury and a dry surface. The gaps between the salt grains are smaller than the radius of this meniscus, so the mercury can't flow between them to lift them ... 2 If you have an object immersed in air, then you can calculate the forces on it using Archimedes' principle. There are two forces to consider. Firstly you have the weight of the object, which is simply:$$ F_g = mg  where $m$ is the mass of the object and $g$ is the acceleration due to gravity. This force acts downwards. Secondly you have the bouyant ...

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