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1d
comment Is a given charge density a surface charge density or volume charge density?
If you don't, consider a cylinder that crosses the surface, large in diameter compared to the height. There is no E field through the sides of the cylinder, only through the circular faces. Use Gauss' law to find the difference in E field between the two circles.
1d
comment Is a given charge density a surface charge density or volume charge density?
You have the E field between the conductors given in the problem by taking the derivative of the potential. You know the E field is zero inside the conductors. This gives you a step change as you cross the surface of the conductor, caused by the surface charge. That is where I say you should have an equation for the step change in E field caused by a surface charge.
1d
comment Is a given charge density a surface charge density or volume charge density?
There is no volume charge density in conductors, because it would set up an electric field in the conductor, which it cannot support. If you have an insulator, you might have (and would expect it to be) a volume distribution.
Sep
26
comment Why doesn't this perpetual motion machine work?
This does not answer the question of why the given machine will not work. The power for the spring can be negligible. You are right that if we have an electromagnet things are different, but that was not the question. In questions like this we don't worry about things failing.
Sep
23
comment Cable being pulled at more than two points with a load at one end
As I understand it, A is the end of the rope at the upper right, B is the place where the rope attaches to the mobile pulley and C is the connection to the load. What does the rope at B connect to? Are there two ropes coming up from the load to the top left pulley, so if the load doesn't move neither does B?
Sep
23
comment Special relativity; rocket moving towards a mirror
@dmckee: I don't think this is a duplicate of that one. That one just was looking for a faster/slower than light answer. This one wants an actual calculation.
Sep
23
comment How can I find an error formula for density?
A partial derivative of a multivariable function with respect to one of the variables it depends on is just the ordinary derivative if you assume all the other variables are constants. So for your $p=\frac {4m}{\pi td^2}$ you have $\frac {\partial p}{\partial d}=\frac {-8m}{\pi td^3}$ by the power rule. I suggest you read the Wikipedia page I linked to.
Sep
19
comment Observing lunar lander and footprints on the moon?
The link you give has been updated with LRO photographs. They do show the hardware and even footprints.
Sep
19
comment Why do we only talk about three options when it comes to the Shape of Spacetime?
Do you mean three three dimensional spaces (sphere, plane, hyperboloid)?
Sep
18
comment How to combat the black-body temperature of an object?
The outer surface temp considers the dissipation inside as well. You have two inputs-solar and dissipaton. Now find the surface temp that balances that with radiation. Then find the temperature difference from the inside to the surface by dividing the heat flow (the dissipation) by the conductivity. Add that to the outside temp and you have the inside temp. To really do it, you should have a model with lots of nodes, because the outer surface will not be all at the same temp. Then a conductivity between every close pair of nodes and a big model to solve.
Sep
18
comment Path of wheels of a bicycle
This is the title puzzle in Which Way Did the Bicycle Go? and a good one.
Sep
18
comment How to combat the black-body temperature of an object?
If you dissipate heat inside the body, you can maintain a temperature differential, just like your oven at home. You can model it with a conductivity between the various layers (or nodes in a 3D model). Conductivity is measured in W/K (if you are working in layers) or W/m^2/K (if you consider the area). In any case, a low conductivity can maintain a temperature differential as long as heat is being input into the center (say from solar arrays)
Sep
16
comment What's wrong with this temperature-in-space calculation?
The first one you link to ignores the fact that only half the body is in sunlight and doesn't use your absorbtivity of 0.3. The second is not so different from what you have. The third doesn't give a justification for the number.
Sep
16
comment What happens when I place an ice cube into boiling water
You now have a competition between the heat supplied to the water from the container and the heat going from the water to melt the ice. It sounds like the ice was absorbing enough heat to stop the boiling. If you looked very close to the edge of the container you might have seen water still boiling.
Sep
14
comment Estimating the rotational speed of DC motor
I only watched the first bit. The fact that you can see it changing means it isn't turning very fast. 100 RPM would be a good guess. You could try to find out what the frame rate of the camera was, then download the video and look at it frame by frame to do better. In theory, it could be spinning very fast and the camera frame rate could be almost the same so the images get slowed down (as the difference of frequencies) but that is unlikely.
Sep
14
comment Estimating the rotational speed of DC motor
What is the purpose of the estimate? That determines how good an estimate you need. If you can't see it spin, it must be at least high tens of RPM. Unless it is carefully designed it doesn't spin a few thousand RPM, probably much less. Is that range good enough for your purposes?
Aug
30
comment What distance does one travels in his lifetime?
You might look at what-if.xkcd.com/86 where he compares the distance traveled by Voyager 2 (launched in 1977) to the distance traveled by a 1977 Plymouth Voyager and some other objects in various coordinate systems.
Aug
29
comment It seems that the harmonic (integer multiple) overtones of a sound usually all have the same phase. Is this true, and if so why?
If you pluck a string at the center and do the Fourier analysis, all the harmonics are in phase. That is the expansion of a triangle wave.
Aug
29
comment How would normal matter behave under conditions found in the core of the sun?
@AbanobEbrahim: Oops, you are right. I counted the $9$ twice. It is $10^{31}$ J That comes even more quickly.
Aug
29
comment How would normal matter behave under conditions found in the core of the sun?
This page estimates the heat of vaporization of crust as $10 GJ/m^3$ From Wikipedia we find the heat of vaporization of iron to be about $11 GJ/m^3$ dominated by heating the material to boiling. With the volume of the earth $10^{21} m^3,$ we need $10^{40}$ J to vaporize it. That comes very quickly.