# Tagged Questions

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### Derivation of formula of potential energy by a conservative force [duplicate]

the formula for potential energy by a conservative force is given by: $$F = -\nabla U(r),$$ which in one dimension may be simplified to: $$F = -\frac{dU}{dx} .$$ My question is how is it ...
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### How come the universe is made of matter and not antimatter?

Antimatter is like matter on opposite day: it has the same properties as the stuff that makes up planets, stars and galaxies, but one vital piece is different—its charge. The universe supposedly ...
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### Identification of particles and anti-particles

The identification of an electron as a particle and the positron as an antiparticle is a matter of convention. We see lots of electrons around us so they become the normal particle and the rare and ...
Now, I found three different definitions of Planck's law: $$P_1(\nu,T) = \frac{8 \pi}{c}\frac{h \nu^{3}}{c^2} \frac{1}{e^{h\nu/kT}-1}$$  P_2(\nu,T) = 2\frac{h \nu^{3}}{c^2} ...