> From a totally naive point of view it seems like whether matter is a solid, liquid or gas largely has to do with how those atoms behave as a group.

That's correct, phases of matter are properties of large groups of atoms.

Your example of a uranium atom suspended in other matter might not be the best to make this point, because mixtures of different substances can make things more complicated. For example, pure sugar at room temperature is a solid. Add it to a cup of water, and it's a liquid solution. Add much, much more sugar and you get phase separation with some solid and some sirup-y liquid.

It's extremely rare to have qualitative changes in physical properties due to a change in isotopes. Normally, phase transformations occur at slightly different temperatures, and that's about it.

The only example I can think of is SrTiO3, which becomes ferroelectric at cryogenic temperatures when common oxygen 16 is replaced by the rarer oxygen 18.

The superconducting coils that generate the magnetic fields for MRI and NMR spectroscopy systems have zero DC resistance.

They usually operate in persistent mode, meaning that there is no power supply attached to them. As long as you keep them superconducting, you can have hundreds of amps circulating in the coils without any change in the current over several years.

Sure, but usually the liquefication plant fills it into dewars, and the magnet system needs to be refilled manually.

> if you need to turn thr magnet off you need to dump the helium

I've never worked with an MRI magnet, but in general, superconducting magnet systems can be ramped down to virtually zero without evaporating all the helium. You just need a power supply for the magnet.

I don't know if the magnet power supply comes with the MRI system though. I think in NMR spectroscopy, magnet systems are often sold without power supply. The technician who installs the system charges it with their power supply, and takes it back with them. As long as the users don't somehow empty out the helium or damage the magnet, it will hold its field indefinitely.

A changing magnetic field induces an electric field. This is why moving a magnet through a coil of wire induces causes an electric current to flow in e.g. a bicycle dynamo.

Similarly, a changing electric field induces a magnetic field.

An electromagnetic wave propagates because a change in electric field causes the magnetic field to change, which causes the electric field to change, etc. in a self-sustaining wave travelling at the speed of light.

> Is the fact that angular momentum must be quantized a postulate of QM or is it derived from something more fundamental?

Quantisation of angular momentum follows from the mathematical definitions of wave functions and operators in QM. Specifically, it comes from the structure of the group of rotations in 3D, SO(3). In the end, this is Lie group and Lie algebra representation theory.

I wouldn't call this an analogy, it is actually the same thing. Planck's relations tell us that the energy of a particle is the frequency of its wave function time Planck's constant, and momentum is the wavevector times Planck's constant.

You generally don't want to fill a nuclear reactor with a highly explosive liquid.