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Jul
25
comment How can the orbit of Jupiter's moons be used to calculate the speed of light?
@Javier: No. We are not looking at the time between eclipses, we are looking at the absolute time of each eclipse. If the distance from Earth to Jupiter did not change the times would be a nice arithmetic progression. The motion of Earth imposes a sine wave on the time with amplitude the light time over the diameter of the Earth's orbit around the sun (assuming Jupiter's orbit is circular). We asses the effect and find the light travel time over the diameter of Earth's orbit.
Jul
23
comment What is the work done?
@JohnM: I believe showing the calculation of the PE change in the barrel is enough to satisfy this. OP has isolated the question to which value is being asked for.
Jul
8
comment Tritium decay is spontaneous even if the binding energy of tritium is higher than the binding energy of 3He. Why?
This is spot on. The point is that the masses of the atoms include the electrons, so the decay is favorable from an energy standpoint.
Jul
6
comment Why do we ignore rotational energy in monatomic gases?
Once it contributes, you have three axes for rotation, so $\frac 32kT$ becomes $3kT$. This is the same for molecules larger than diatomic. For diatomic molecules near room temperature we ignore rotation around the long axis, so there are two instead of three degrees of freedom. The motivation is the same. The MOI is so small the DOF is not excited.
Jul
6
comment Why do we ignore rotational energy in monatomic gases?
Then the MOI is decreased because now you have a bare proton. That means the temperature where the degree of freedom becomes important is higher-I suspect much higher for the reasons in my last comment. Other things may be going on by then-increasing number of electrons contributing the $\frac 32kT$ would be my first guess.
Jul
6
comment Why do we ignore rotational energy in monatomic gases?
Aside from hydrogen, an ion has almost the same moment of inertia as an atom. I think (but didn't do the calculation) that the electron cloud has most of the MOI of an atom in the classical picture. The electrons are $2E3$ times less massive than the protons, but the radius (which gets squared) is $1E5$ times greater or more.
Jul
3
comment Drag in low earth orbit
If you don't include gravity gradient, then there is nothing to prevent the arrow from flying straight. The damping comes from transverse drag. If the arrow is spinning, there will be drag to slow the rotation down. That may be in your step 4.
Jul
3
comment Radiation from home heaters
@CuriousOne: It is true you can get burned by a heater, but the wording convinces me that OP believes all radiation is harmful, not can be harmful.
Jul
3
comment Radiation from home heaters
This assumes all "radiation" is equally dangerous, which is clearly false. Different types of radiation (note that sound is included as well as electromagnetic) have different damage pathways and different danger levels. -1
Jun
26
comment Cylindrical capacitor in an electric circuit
If you think about having $R$ connected to the cable and then connecting the battery/$R0$, the capacitor starts completely discharged. As the capacitor cannot change voltage instantly, the current will flow through $C$. With a time constant of $RC$ the current will shift to the resistor. As $t \to \infty$ the current in $C$ will go to zero and you will have a constant current in $R$.
Jun
26
comment Cylindrical capacitor in an electric circuit
Yes, that is the capacitor. Once you calculate $E(r)$ you can use that to compute the capacitance of the coaxial fiber per unit length.
Jun
26
comment Unable to understand relative motion
You probably wouldn't see the same effect if everything outside the van is the same distance from your route of travel. I made a long drive yesterday with mountains in the distance on both sides of the road. It looked like the mountains were moving with us in the direction of travel with everything nearer to us than the mountains moving rapidly backwards.
Jun
26
comment Cylindrical capacitor in an electric circuit
As I reread it, I think you are correct that the wrapper is connected at both ends. Your circuit is correct as well. They say that at large time no current flows through the capacitor, which I misread as the wrapper. In your circuit, you have current flowing around the outside loop permanently in just the amount stated in the problem. Now C charges according to the voltage divider formed by the two resistors across the battery.
Jun
26
comment Cylindrical capacitor in an electric circuit
I had seen the picture as showing an external wire from $R_0$ to $R$. If the bottom line is the bottom edge of the wrapper and $R$ is connected from the wrapper to the center conductor, you have continuous current in the wrapper.
Jun
21
comment Calculating the resistance of a 3D shape between two points
Glad to see that. As long as your mesh is very small compared to the dimensions of any element of the problem you should get about the same result. Fine meshes imply many nodes, which imply long relaxation times. If you did it in 3D it would require a factor $n^{3/2}$ more nodes. The pros in this area (I am not one) have techniques for choosing the nodes so the mesh can be coarser and you can get good results. They also have software that accelerates the convergence. Even so, models with thousands of nodes are routine.
Jun
20
comment Calculating the resistance of a 3D shape between two points
I think usually a non-cubic mesh is used, maybe the classic sphere packing, which also helps with making the situation isotropic.
Jun
20
comment Calculating the resistance of a 3D shape between two points
No, the resistances are fixed during the relaxation, only the potentials at each node are varied. When the relaxation converges, you compute the current flow for unit input voltage and get the resistance. The node spacing has to be small compared to all the feature sizes of the object for exactly the reason you mention. If your node spacing is small compared to the width of the rod, you will get $\sqrt 2$ more lines of points sharing the current to compensate for the added length and get the same resistance.
Jun
20
comment Are Feynman's Six Easy Lectures still relevant today?
The suggested duplicate is for the three volume set, not the Six Easy Pieces. They are quite different.
Jun
18
comment Determining the final state of the gas in the tank
My divider changed it to a control mass. The mass in the volume away from the vent is constant. The process inside the tank is reversible-you could make the divider a piston and recompress the control mass. The escape out the vent is irreversible, but we are not concerned with that gas.
Jun
10
comment What is the voltage at every pair of points along a ideal wire that is connecting the two terminals of a battery?
Idealized cases are very useful, but they are approximations and you have to watch for limitations. If I have a 12V 3W light bulb, its resistance is $48\Omega$. If I hook it to a battery with wires with a resistance of $0.1\Omega$, it is not a bad approximation to say the wire resistance is zero, which saves a bunch of calculation. If some "high" resistance is not present, the resistance of the wire becomes important.