Physics

### Why do wheels spin?

I'm pretty sure this is probably a stupid question but it is annoying me a lot. In my physics textbook it says, cars move because the wheels exert a backward force on the ground and the ground exerts an equal and opposite forward force on the car. If the wheel ....

Sophia: 12 hours ago

Physics

### Factors on which barrier potential in a p n junction depends

I have a few queries regarding the barrier potential in a p-n junction diode (http://en.wikipedia.org/wiki/P%E2%80%93n_junction). Is the barrier potential dependent on temperature? Why/why not? Does the barrier potential depend on doping concentration? Why/Why not? Why is the barrier potential different for different semiconductors? Basically, what determines the barrier potential for ....

Naomi: 12 hours ago

Physics

### Group theory and quantum optics

This is a question about application of group theory to physics. The starting point is the group $SU(n)$. I have a representation $R$ of $SU(n)$ that takes values on the unitary group on an infinite dimensional separable Hilbert space $H$. This representation can be written as the direct sum of ....

Hazel: 12 hours ago

Physics

### Stress-energy tensor of Jordan-Fierz-Brans-Dicke action

I have a question about the JFBD action, in particular the part that involves a scalar field. It is given by $$S_\phi = \int d^4x \sqrt{-g} \left( \frac{1}{2} g^{\mu\nu}(\nabla_{ \mu}\phi)(\nabla_{\nu}\phi)-V(\phi)-\frac{\xi}{2}R\phi^2 \right).$$ The energy-stress tensor is defined as $T_{\mu\nu}^{\phi}:=\frac{-2}{\sqrt{-g}}\frac{\delta S_{\phi}}{\delta g^{\mu\nu}}$. I should get the following: $$T_{\mu\nu}^{\phi}=(\partial_{\mu}\phi)(\partial_{\nu}\phi)-g_{\mu\nu}\left -\xi G_{\mu\nu}\phi^2-\xi\phi^2,$$ where ....

Daniel: 12 hours ago

Physics

### Accurate mathematical description of angular acceleration?

My physics textbook says that angular acceleration is the derivative of angular velocity, but I think that this isn't accurate. For example let's imagine that a block is tied to a rope that passes through a hole in a table. Then let's make the block revolve in a circle on ....

Ruby: 13 hours ago

Physics

### Why is neper frequency called a frequency?

In the context of complex frequency of RLC circuits, the real part is called neper frequency, according to units it's understandable that it has 1/s as the unit which is same as frequency but what is repeating at this frequency as I nowhere see any repetition? Why is it a ....

Liam: 13 hours ago

Physics

### "Hollow Earth" like Theories for pedagogical porpuses

I recently encountered the Hollow Earth theory (http://en.wikipedia.org/wiki/Hollow_Earth), and I realize that This kind of theories can be used to give a really interesting discussion of a physical law. For example, If we take the hypothesis that the earth is hollow and if we ignore the border effects of the ....

Mateo: 14 hours ago

Physics

### Potential is with respect to what

When we say a body has "high" electric potential and "low" electric potential, to what potential is it with respect to? ....

Colton: 14 hours ago

Physics

### What happen when two galaxies collide?

As the title suggests, I know collision of galaxies happens. Questions are Why do they collide (obviously because of gravity, but in what conditions)? Aftermath of the collision? If there is a super massive blackhole in the center of the galaxies, what happen to them? ....

Kayden: 14 hours ago

Physics

### How do we presently understand time?

How do we presently understand time? Can a second be related directly to physical phenomena? What is a second really? Why are we sure that time is linear? ....

Lincoln: 15 hours ago