Expansion of Helmholtz energy

To get an expansion of Helmholtz energy of a) an ideal gas b) a Van der waals gas we must integrate $\left ( \frac{\delta A }{\delta V} \right )_{T}=-P$ I saw the solution is : Can you explain why is that? ....

Christopher Christopher: 4 hours ago


Uncertainty in parenthesis

In a physics text book I read the following: $$e/m=1.758820150(44) ×10^{11} \mathrm{C/kg} $$ In this expression, $(44)$ indicates the likely uncertainty in the last two digits, $50$. How should I understand this uncertainty? Does it mean $\pm 44$ on the last two digits? ....

Alice Alice: 4 hours ago


How does one apply Gauss's Law when dealing with two infinite planes?

If you have two parallel planes with one density of 1 C/m^2 and the other with -1 C/m^2 density and you need to find the electric field at a point in the middle, how do you use Gauss's Law? I drew a box enclosing one plane to find the electric ....

Nova Nova: 5 hours ago


How is the phase of a BEC changing when some particles are being removed?

I have some trouble understanding what happens to the phase of a BEC when some particles are removed. The motivation of the question is the experiment of observing interference between two BECs. In my current picture, a BEC is a state $\Phi(r)e^{i\varphi}$, where $\Phi(r)$ is the N-fold tensorproduct of the ....

Olivia Olivia: 5 hours ago


A simplistic question about virtual particles

Please understand, I am not a scientist. My understanding of "nothing" is that virtual particles are constantly springing into existence on borrowed time and borrowed energy. Almost instantly annihilating each other giving back that borrowed energy. But could an event happen such as 2 antimatter particles appearing near enough to ....

Charlotte Charlotte: 5 hours ago


Commutator of Pauli Matrices in two qubit system

For two qubits 1 and 2 with Pauli Matrices $\sigma_{x_1}, \sigma_{x_2}, \sigma_{z_1}, \sigma_{z_2}$, prove that: $$ = 0$$ I am very confused about what the question means. I thought the Pauli Matrices would be the same regardless of which qubit they belong to, which would mean that $\sigma_{x_1} \sigma_{x_2} = ....

Ayla Ayla: 5 hours ago


Is liquid "air" (nitrogen, oxygen, aragon mixture) flammable?

I was wondering if liquid "air" (nitrogen, oxygen, and argon mixture) was flammable. Imagine you have a container. This container contains liquid air. The air is in liquid form, and is in direct contact with two copper rods, which when a current is ran through them, they spark. I was ....

Ryder Ryder: 6 hours ago


Spherical charge in two different dielectric materials

I am trying to freshen up my memory about electrical fields and I came across this exercise from school. A sphere with a constantly distributed charge is located in between two different dielectrics (see picture at the bottom) and the task is to calculate the electrical field. I have a ....

Ruby Ruby: 6 hours ago


Does GR explain cosmological time dilation?

There is plenty of evidence for a cosmological time dilation effect. For instance a supernova that takes 20 days to decay will appear to take 40 days to decay when observed at redshift $z=1$ (see Ned Wright's cosmology FAQ (, Ned Wright on Tired Light ( I understand how time ....

Piper Piper: 6 hours ago


Naive question - Is Magnetic generator (like this) not a 'perpetual motion machine'?

Perhaps a question that is too naive to be on Physics.SE but I let the experts here decide (and downvote at will), but there seem to a certain company that claims to have created what it calls a "magnetic generator" where, if the machine is powered externally long enough (which ....

Bella Bella: 7 hours ago