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1814
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location San Mateo, CA
age 59
visits member for 3 years, 7 months
seen 12 hours ago

Nov
14
comment How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?
@user3237992: the difference is that microwave ovens and WiFi operate at essentially the same frequency, so changes in absorption due to frequency are off topic here. I believe they penetrate the same, but we care about WiFi penetration and we don't recognize that the oven microwaves penetrate just the same.
Nov
14
comment How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?
@K7PEH: It looks like your comments are directed to OP, not to me. You can comment on the question for that.
Nov
13
comment How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?
@K7PEH: I don't expect the attenuation is any different. I said that a few times. The oven wall provides enough attenuation that we don't care about the signal (just as power) any more, then attenuation through walls knocks it even lower.
Nov
13
comment How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?
@K7PEH: That is (just about) the same frequency most WiFi operates at, so it is fair to expect the same attenuation.
Nov
13
comment How come Wifi signals can go through walls, and bodies, by kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?
How do you know they don't go through a few centimeters of an object. My point is that they will be attenuated by the same factor by any object. We don't normally have any way to detect them and don't care that in fact they do pass through. They are at a level (even before the attenuation) judged to be safe. Yes, there are meters that can detect the oven signals, but we don't carry them around normally.
Nov
13
comment Where does the force to stop a constant velocity object come from?
If the ball truly has a constant velocity it has to pass through the wall, because the wall cannot accelerate it and cannot apply any force. When it is slowing down it is accelerating. Constant velocity is a reasonable approximation when there is no large force acting on the object. For the ball, we ignore air friction. When the ball hits the wall, it no longer has constant velocity.
Nov
12
comment Solar vs lunar gravity: inverse square law
@garyp: I do. The question is asking for a simple calculation, but wanders in many ways off topic.
Nov
11
comment Where is the fine-structure constant in this list?
There is a note that $\alpha(m_Z)=\dfrac 1{4 \pi}\dfrac {g \cdot g'}{g^2+g'^2} \approx \dfrac 1{128}$ and that the unfamiliar value is because of the energy level.
Nov
8
comment How would one best dissipate or absorb shockwaves and pressure caused by an explosion within a confined space
Are you hoping to limit the damage from one explosion or many? In other words, does the material need to survive intact, or can it be damaged? The second provides more options.
Nov
8
comment How would one best dissipate or absorb shockwaves and pressure caused by an explosion within a confined space
I think it is hard to absorb the energy in phase transitions, but my first thought is acoustic impedance mismatches and foams are very good at that.
Nov
3
comment How does lowering your thermostat save energy?
@Trengot: usually you would compute the efficiency of the furnace as heat added to the house divided by energy input. You would ignore (in this calculation) the losses from the house. You are correct that because the loss rate is (a little) higher, the furnace will run (a little) longer to heat the house by $2F$. The furnace outputs its heat at a constant temperature, so its efficiency should be just about constant. Yes, the cooler house will have a cooler input temperature to the furnace. I don't know how that impacts efficiency.
Nov
2
comment How does lowering your thermostat save energy?
Yes, you mentioned that and said it didn't matter much, which I agree with. I thought this added to the already good explanation by showing how the energy savings shows up.
Nov
2
comment How does lowering your thermostat save energy?
It takes the same amount of energy (and furnace running time) to heat the house 2 degrees, whether $66-68$ or $72-74$. It will take more time for the house to cool from $68-66$ than from $74-72$, so the furnace will come on less often. That is where the energy savings comes from.
Oct
27
comment Why is equivalent resistance in parallel circuit always less than each individual resistor?
Once you believe the 2 resistor answer, it extends to any finite number. For each resistor, imagine combining all the rest into a single resistor by the law you cite. Now the combination of the one resistor and the combination resistor is less than the one. Do this once for each of the resistors, and you have that the total combination is less than any one.
Oct
9
comment Voltage and resistors
No, electrons have a potential energy based on the voltage level they are at. That is what changes on the passage through the resistor. It is like a rock rolling down a hill. When it is high on the hill it has a lot of potential energy. After it stops rolling at the bottom, it has the same kinetic energy (zero) as it had before it started rolling, but less potential energy. Electrons in a potential field work the same way.
Oct
8
comment Voltage and resistors
As I said, the same number enter as leave. They just leave with less energy than they entered at. This energy difference is the heat dissipated by the resistor.
Oct
6
comment What is the electric field near an infinite sheet with a point charge near by?
Is the sheet conductive or insulating. For conductive, you need to consider an image charge. For insulating, you can just use superposition.
Oct
5
comment If mass is added to a toy car does it affect its speed making it faster
No, generally it will fall slower. It has less drag, but with a lower mass the drag causes more deceleration. If they are spheres, the drag goes up as $r^2$ (the cross sectional area) while the mass goes up as $r^3$ (the volume)
Oct
5
comment If mass is added to a toy car does it affect its speed making it faster
OP explicitly wants to consider drag.
Oct
5
comment How many are the points which are $n$th nearest to a certain point in a hexagonal lattice
For questions like this, you should count the numbers for small distances, then look in oeis.org. There are three at a distance of one, six at distance $\sqrt 3$, three at distance $2$, six at the next. A search for $3,6,3,6$ gives 21 pages, not too many to look over.