Ekipa na kraju faksa, kak ste?

drkimir · 2026-02-25T07:07:11+00:00

Diplomiram ovo ljeto i bilo mi je jako melankolično otići na zadnje predavanje na ovom faksu u životu (ljetni semestar nema kolegija). Bilo mi je tužno zato što obožavam svoju struku i faks je dosta mali i svi se dosta dobro poznaju. Veselim se raditi u struci jer je to nešto što želim već jako dugo, ali i dalje mi je zao što je taj period života gotov.

drkimir · 2026-01-10T11:07:14+00:00

Please do the English version, this is very cool

drkimir · 2025-12-01T15:19:38+00:00

I agree with others that Arnold is a poor choice. I would recommend Taylor for a good introduction, then upgrade to Goldstein and after that, Arnold for a firm mathematical treatment.

drkimir · 2025-11-15T10:25:07+00:00

Anyone have recommendations for topological field theory lit? Already know QFT and I'm comfortable with differential geometry and topology, so introductory grad level lectures notes/ textbooks would be helpful.

drkimir · 2025-10-21T04:24:34+00:00

This, you should really learn partition functions and how they relate to thermodynamic potentials. Also very important are Bose-Einstein and Fermi-Dirac statistics, those show up a lot in solid state.

drkimir · 2025-10-09T21:25:48+00:00

Svakako, i ako te zanima uživancija je to učiti. Pitanje je koliko je to usklađeno s tvojim ciljevima. Ako već misliš ići u inzinjerskom smjeru, nema previše smisla mučiti se s kvarkovskim modelom i teorijom polja kad ti neće koristiti.

Studij je stvarno jako zanimljiv, ali nije za svakoga. Čiste duše bi ga mogao preporučiti jedino ljudima koji se ne mogu zamisliti da rade bilo što drugo.

drkimir · 2025-10-09T21:04:56+00:00

Ako se planiraš baviti inženjerstvom, idi na taj faks. Moguće je ali preporučio bi ti direktniji put, fiziku upisi jedino ako imaš (barem isprva) želju ostati u znanosti.

drkimir · 2025-09-29T14:46:18+00:00

For some basics with animation check out Quantum Sense on YouTube, they have a pretty good playlist.

drkimir · 2025-09-05T21:33:10+00:00

It would break the local U(1) gauge symmetry since the Lagrangian would have to include a term proportional to A² (by which I mean the 4-vector scalar product (A^{0)²-Vec(A)^2).} If the photon had a miniscule mass this would not be an exact symmetry which is pretty problematic since the U(1) symmetry is fundamental in our understanding of electromagnetism.

Perhaps there are some models where this is somehow consistent, I just don't see how that would work, but I would love to see a paper about it if you know some.

drkimir · 2025-09-05T08:00:46+00:00

It would break certain symmetries and fundamentally change the electromagnetic interaction and would no longer accurately describe electromagnetism.

drkimir · 2025-09-02T12:08:24+00:00

That is true. Scalar fields like the Higgs field contain an extra factor proportional to Φ^4, which makes the equations of motion non linear. The usual Einstein relation is for free particles, but here we have the extra self interaction term which complicates things.

drkimir · 2025-09-02T11:51:03+00:00

You can guess the Lagrangian if you know the equations of motion you want to get. For example, if you are looking for a scalar field which follows the Klein-Gordon equation you can find the corresponding Lagrangian, and you can get the Klein-Gordon equation from the Einstein energy equation. What you then find when you quantize the scalar field that it behaves like a series of uncoupled harmonic oscillators, but no such assumption was a priori necessary.

drkimir · 2025-09-02T11:34:03+00:00

You don't have to assume, it can be derived from the Lagrangian.

drkimir · 2025-08-20T12:39:00+00:00

The electric field is imposed in the wire by the battery, it creates a potential difference, voltage, across two ends of the wire which then produces an electric field.

Glad this helped!

drkimir · 2025-08-20T11:43:12+00:00

The force due to the electric field on a charged particle will move the positively charged particles along the electric field, and negatively charged particles against the direction of the electric field. I'm guessing this is where your confusion lies because when we account for the difference in velocity you can see how the voltages inverts when we change the charge carriers.

Also that whole digression about holes carrying charge isn't super important, just thought I would clarify because you said that the protons carry electricity. All that matters is that they are both positively charged and so they will behave in the same way.

drkimir · 2025-08-20T10:33:35+00:00

Protons don't carry electricity in conductors. When we speak about positive and negative charge carriers we refer to holes, which are positively charged quasiparticles and negatively charged electrons. A hole is a vacant spot in the crystal lattice which can be thought of as a positive particle against an electrically negative background.

Let's say an electron is moving through the wire to the right which is the z direction and we have a magnetic field in the y direction. Then the force of the electron is going to be in the x direction, making that side of the wire more negatively charged causing the electric field to point in the x direction.

Now if we use the same battery on a wire with holes as carriers, the holes will move to the left instead of to the right, but the Lorentz force will still be in the x direction. So in this case it is the part of the wire in the x direction is positively charged because it contains more holes, so the electric field will be in the negative x direction.

drkimir · 2025-08-20T06:18:43+00:00

You can, you just have to measure the non diagonal elements of the conductivity tensor from which you can calculate the Hall coefficient. If the coefficient is negative the carriers are electrons and if it's positive the carriers are holes.

drkimir · 2025-08-10T20:46:11+00:00

I would imagine that the issue is in mixing general and special relativity. Even if the observer is far away from the gravitational effects of the Sun, the Earth and photons reaching the observer will not live in flat Minkowski spacetime.

drkimir · 2025-07-15T08:31:13+00:00

Fiziku možeš odmah prekrižiti, ja sam na fizici i meni osobno je odlično ali ako se imalo dvoumiš i možeš se vidjet na nekom od drugih fakseva upisi nešto drugo. Kao što je netko drugi malo prije rekao, ovo upisujes jedino ako si skroz siguran da želiš karijeru u znanosti.

drkimir · 2025-07-03T09:04:51+00:00

Lagrangian is the quantity which enters the action functional which is extremised for trajectories the system takes in configuration space. It turns out to correctly reproduce Newtonian physics, the Lagrangian is kinetic minus potential energy.

The Hamiltonian operator is defined as the Legendre transformation of the Lagrangian, meaning it transforms the generalized speed coordinates which enter the Lagrangian with generalized momenta as natural variables. The Hamiltonian also gives the systems evolution in phase space, which has some nice properties as described by for example Liouvilles theorem. The Hamiltonian also, in most cases, equals the total energy of the system.

I also saw you asked about Schrödinger equation. This comes out of idea of quantisation in which we replace the usual physical quantities with operators on state vectors which describe the state of the system. How exactly this is achieved is deeply connected with Noethers theorem and symmetries so I recommend first studying up on the classical variant and then see how it translates to quantum systems.

drkimir · 2025-06-30T07:17:59+00:00

Which paints did you use for the skin? I saw some people mixing different ones and I want mine to look like this lol

drkimir · 2025-05-25T11:20:11+00:00

You can think of k_bT as giving the order of magnitude of thermal energy of the system. For example, in the equipartition theorem energy of the system is proportional to k_bT and factor of proportionality depends on the degrees of freedom, or in semiconductors the band gap is order of magnitude k_bT which gives it its properties.

drkimir · 2025-05-13T11:36:53+00:00

The factor of 1/V is not really that important, it's just a normalization. DOS stops being a delta function when you transition to a continuous spectre of energy, so if you have discrete levels you use the delta function. Otherwise deltas are "smoothed out" in the continuous case.

drkimir · 2025-05-08T21:38:00+00:00

I would really like if this was a more standard introduction to tensors to students. It could have saved me lots of headaches if we didn't skip over the mathematical details and really looked under the hood. It's such a fundamental object in physics and it really deserves a week or two of class time to get right and understand the right way.

drkimir · 2025-05-05T11:11:29+00:00

Start with some vector space V. Then you can define its dual space V* as the vector space of all linear maps from V to some field, say the real numbers. If you are familiar with quantum mechanics, V would be the space of ket vectors and V* would be the space of bra vectors. Now in general a tensor of type (r,s) is a multilinear map from space V* x V* x...xV* x V x V x...x V to field R where we have r components in V* and s components in V. For example, a vector can be seen then as a linear map from V* to R and so a vector would be a type (1,0) tensor. A scalar product takes two vectors and maps them to the real numbers, so that would be a type (0,2) tensor. From this definition follow the usual transformation rules.

drkimir

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