# All Questions

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### Why is the change in enthalpy, $\Delta H$, defined as $\Delta H = \Delta U+P\Delta V$?

If we define work as $w=-P\Delta V$ (so that during expansion work is negative), wouldn't it make more sense to say that $\Delta H = \Delta U-P\Delta V$ since $\Delta H = \Delta U+w$? Here, $U$ is the ...
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### Frequency and Wavelength peak for Wien's displaement law of a blackbody

This is a question relating to Wien's displacement law for the Planck function. As we all know frequency and wavelength are related to the speed of light by: $$\nu\lambda=c$$ However, why is it ...
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### how do photons move with respect to EM (I'd like to picture wave magnitudes frame by frame)

(I'm aware treating photon as particle and talking about its position is not exactly, conceptually right but I think it makes sense, at least in the point of view of a beginner. Please just assume ...
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### Obtaining an equation regarding isothermal compressibility

The isothermal compressibility $\kappa_t$ of a substance is defined as $$\kappa_t = -\frac{1}{V} \left ( \frac{\partial V}{\partial P} \right )_T$$ Obtain an expression for the isothermal ...
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### Is it possible for a planetary system to orbit around something that wasn't a sun?

Say for example that at the center of our solar system you replaced the sun, with something that wasn't a star, lets say that it was electrified and still produced the same amount of heat, would that ...
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### What really is mass?

I was always taught that mass was how much matter something is made of, but I recently read that it is how resistant an object is to acceleration; as such, you could add mass to an object by ...
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### Reverse spinning wind generator

I broke a blade on my wind generator. The maker says they have new blades that rotate the opposite direction now. At first that sounds like a no-go, but then after thinking about it, the wires are ...
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### Stationary states

I am a beginner in Quantum mechanics and as I understand,the superposition of stationary states is also a solution of time-independent Schrödinger equation (TISE). The wave functions that are the ...
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### How to “read” Schrodinger's equation?

I am starting to learn Quantum mechanics. I can't wait for my completion of QM, as I am running behind all the concepts taught in the class; but I can't even go on studying chemistry, or I can't even ...
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### Problem in understanding Feynman's explanation of the Dirac-Delta function

This is quoted from Feynman's Lectures' Normalization of the states in $x$: We return now to the discussion of the modifications of our basic equations which are required when we are dealing with ...
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### Primitive unit cell of fcc

When I consider the primitive unit cell of a fcc lattice (red in the image below) the lattice points are only partially part of the primitive unit cell. All in all the primitive unit cell contains ...
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### How do I calculate the speeds in x, y, and theta of a spinning rod?

Suppose the object looks like the diagram below (it is the horizontal rod). On the vertices, are two velocity vectors $v_r$ and $v_l$. The question is: how is the motion of the rod affected by these ...
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### Relative displacement of a point in a deformed object

I have a model and it undergoes large rigid body translation or rotation at different time steps.I want to understand the deformation relative to some point in the model which is kept as reference.How ...