# All Questions

2k views

### What is the reason behind band gap narrowing in semiconductors

I want to know why some semiconductors band gap decreases after doping with elements. Burstein-Moss band-filling effect can be useful to explain band gap widing in a semiconductor materials but i was ...
52 views

### prediction of a moving object

OK, this may be a hard question to answer and really all I am looking for is an equation as I don't even know what to call this. This is all for a game so bare with me please. In the game two ...
101 views

### How to calculate the heat of a metal?

Lets say I leave 100g of copper outside under a convex lens that is 1m squared in surface area (I've been told that 1m2 of sunlight equals 1Kw). How would I figure out the temperature reached for a ...
175 views

### Velocity in the lift equation

What exactly is the nature of the velocity in the lift equation? I have read that it is the velocity of the aircraft, and I think I have read that it is the relative velocity of the air to the wing/...
237 views

### Does turning sharply on a bicycle conserve more energy than a wide turn?

I use a bike to commute, so I spend a lot of time thinking about how to get the most bang out of my momentum. Aside from the extra distance traveled in a wide turn, does making a sharp turn save you ...
2k views

### How to freeze the Niagara waterfalls?

Here is a picture of the usual vigorous Niagara Falls (in the winter). Here is the picture of Niagara Falls frozen in 1933 (in the very cold winter). Here is the picture of Niagara Falls frozen in ...
1k views

### Could fast vibrations cause us to travel forward in time

Assuming it's possible to vibrate a human at near light speed without harming him, would a few minutes of this from his point of view be much longer from a stationary observer's point of view? In ...
619 views

### How do I calculate the curve of a parabolic mirror to melt the ice from my street gutter?

I want to make a concave parabolic mirror that will have a focal point six inches below where I place it (on the edge of the sidewalk) so that it will melt the ice collecting in the gutter below and ...
216 views

### Really nothing special when falling into a black hole?

It has been said time and again, that an observer who falls into a black hole will not notice anything special. Is this really true? There is of course the problem with the tidal forces, but I ...
606 views

### Can we explain Huygen's principle taking into account Maxwell's predictions?

Descartes gave the corpuscular model (1637) of light. Corpuscular model was further developed by Issac Newton. Model predicted that if the ray light (on refraction) bends towards the normal then the ...
109 views

### What does this summation mean in relativity?

Equation 1.2 of 't Hooft's Introduction to General Relativity gives the Lorentz transformations: $$(x^\mu)' = \sum\limits_{\nu = 1}^4 {L^\mu}_\nu x^\nu$$ Is this the sum of four square matrices ...
126 views

### Geometric quantization AND nuclear physics

Classical mechanics has a natural mathematical setting in symplectic geometry and it may be asked if the same is true for quantum mechanics. Geometric quantization is one formalization of the notion ...
87 views

Could the radiation pressure of a black body (theoretically) perform work on the perfectly reflecting apparatus in the figure below? Assume that the block does not hinder the passage of light through ...
117 views

### Transforming a lagrangian to hamiltonian and vice versa

I am not refering to Legendre transform, but to something more simple. In analytical mechanics, the Lagrangian can be described as $L=T-V$, and the Hamiltonian is if the Lagrangian doesn't explicitly ...
4k views

### What happens when a charged (negative or positive) object touches an insulator?

I know that insulators do not conduct electricity because they do not allow the free movement of electrons. Let's assume the object that's going to touch the insulator is negatively charged. Does some ...
93 views

### Are mesons color polarized?

The binding of quarks in mesons baffles me. It's an Occam's Razor thing. Since a meson is a colorless, the simplest way to bind its two quarks together is to use a $U(1)$ Cartan subalgebra of $SU(3)$....
38 views

### Charges and their configurations

Can we treat any charge configuration as small point charges by using superimposition principle to derive electric fields, forces and other things ? For example suppose we have a symmetrically ...
70 views

### What is the “momentum” referred to in the energy-momentum tensor

What is the "momentum" referred to in the energy momentum tensor from GR? Is it $m\dot{x}$ or is it the canonical momentum $\frac{d}{dt} \left(\frac{\partial L}{\partial \dot{x}}\right)$ Also, I ...
2k views

### How does potential energy work in the context of objects in space?

It's said that potential energy is "energy of position." If an object is sitting on a shelf five feet above the floor, its potential energy can be thought of as equal to the amount of energy that ...
40 views

### Red Shift Effects

I dong know much about physics, but I was curious about the red shift effect. I think I have the basic idea of it down in that as an object 1 accelerates from object 2 the light from object 1 will ...
101 views

### Lorentz force helix

I have a few questions about the Lorentz force. Is it possible to create a Lorentz force helix in a cloud chamber and if so how would this be achieved?﻿ How would the pressure inside the helix be ...
1k views

### Understanding the difference between co- and contra-variant vectors

I am looking at the 4-vector treatment of special relativity, but I have had no formal training in Tensor algebra and thus am having difficulty understanding some of the concepts which appear. One ...
44 views

### Experimental information: is momentum conserved?

Could I say that it is conserved during the time, or instead there is a slight decrease?
313 views

### Air conditioners and the law of conservation of energy?

Sorry if this is a stupid question, but I have often been told that air conditioners are more efficient than an electric heater, because, the story goes, if you put say 1 kW of electricity in the ...
1k views

### Conservation of mass energy and kinetic energy in different reference frames

With a little work it's easy to show that kinetic energy by itself is not necessarily preserved when switching between frames of reference. And it is my understanding that energy should be preserved ...
1k views

### Vector and scalar potentials of plane wave

Consider a simplest 3D solution of Maxwell's equations: $$\vec B=\vec e_z \cos\left(\frac{2\pi}{\lambda}(ct-x)\right),$$ $$\vec E=\vec e_y\cos\left(\frac{2\pi}{\lambda}(ct-x)\right),$$ and ...
5k views

### What is the direction of static friction?

Note: My question is duplicate of the following Direction of friction when a car turns Why does friction cause a car to turn? I've gone through many related questions especially the first. As ...
534 views

440 views

### Reflection between two mirrors?

If I put my hand between two perfectly aligned mirrors and then remove it will the images continue to reflect for a few nanoseconds? If yes then will the images be of a combination of the front and ...
802 views

### Finding out the minimum deviation angle [duplicate]

Let a light incident on a prisom at right angle. I want to determine the minimum deviation angle. I know a relation $$\frac{\cos i_1}{\cos r_1} = \frac{\sin i_1}{\sin r_1}$$ and minimum angle of ...
349 views

### Do Cooper pairs act like Cheshire cats?

Could the pairing up of electrons be explained by their spin being in a different position? What would separating the spin from an electron in matter do in theory?
175 views

### How would you explain the cause of earths magnetism to an 8th grade student

What would happen if I freely suspend a bar magnet at north pole, will it become vertical? I am the 8th grade kid.What would happen if earth brakes into two pieces,will there be new north and south ...
22k views

### Is the Earth negatively or positively charged?

The Earth carries a negative electric charge of roughly 500 thousand Coulombs. Does that mean the Earth is negatively charged?
314 views

### Do electromagnetic waves have endpoints?

When learning about electromagnetic waves at school we never talked about any endpoints as we did with standing waves, so I assumed that light has an endless length, but that doesn't make sense. So my ...
160 views

### Transport theorem derivation question

I am trying to rigorously go through some fluid mechanics proofs and theorems. I am currently going through a proof related to the transport theorem and I am having trouble with a step. The steps in ...
200 views

### What software do publishing companies use to create drawings for text books?

What software is used to create free body diagrams, inclines, springs and so on in the introductory physics textbooks ? I know Adobe, Luxology, and many others could, but I am more interested in ...
34 views

### Are “phase-specific” masks ever used in the context of double exposure lithography? Can these sorts of things exist?

I was recently reading an 08' paper on double exposure lithography: and I was wondering if there existed some material that could be used to create a mask for e.g. a 193 nm light source that was only ...
166 views

### What is the interstellar density of mm sized “dust” particles? [duplicate]

If you assume an interstellar spacecraft of 100 m in diameter traveling to the nearest star - that ship would plow through 3 x 10^20 m^3 of interstellar space. At 0.5 c the energy release of such a ...
430 views

### CP violation from the Electroweak SU(2)$_{weak,flavor}$ by $\int \theta F \wedge F$

Question: Why there is NO Charge-Parity (CP) violation from a potential Theta term in the electroweak SU(2)$_{weak,flavor}$ sector by $\theta_{electroweak} \int F \wedge F$? (ps. an explicit ...
38 views

### Placatory motion of more than two bodies

Recently in a documentary I heard that Newton's law of gravitation very well explains motion of two bodies such as sun and earth. And when applied to three bodies the answer is chaotic and not stable. ...
977 views

### Is Conformal Symmetry Local or Global?

I'm just brushing up on a bit of CFT, and I'm trying to understand whether conformal symmetry is local or global in the physics sense. Obviously when the metric is viewed as dynamical then the ...
229 views

### Is it feasible to transfer energy from power stations to communities via photons instead of electrons?

Electrical wires are relatively inefficient in transferring energy--especially when the place of production is quite far from communities. Would it be possible to transfer that energy via photons? I ...