574 reputation
312
bio website henning.makholm.net
location Copenhagen, Denmark
age 41
visits member for 2 years, 2 months
seen Aug 17 at 14:21

Jul
10
comment Special relativity and electromagnets
@PhilFrost: I think a more complex analysis is necessary to explain Faraday's law -- a naive application seem to lead to an effect of the wrong sign. However, one also has to take into account that changes in the electic field propagate with finite speed, and that the electrostatic repulsion from a moving charge in the direction of its movement is less than that of a stationary charge. (In the transverse direction it is the same). This is not part of Coulomb's law but has to be derived using relativity -- it's the only way for things to fit together in a consistent way mathematically.
Jul
10
comment Special relativity and electromagnets
@PhilFrost: From the cat's frame, some electrons are removed from the wire during the transition -- because the cat sees electrons begin leaving the front end of the wire before new electrons begin moving into its back end. (Of course it is an impossibly idealized assumption that the current starts instantaneously from the wire's rest frame, but the net outcome is the same for less sharp transitions).
Jul
10
answered Special relativity and electromagnets
Jul
10
comment Special relativity and electromagnets
But the row of electrons is contracted in the lab frame. You can see that in your screencaps from the video too -- Derek sees 10 electrons per image width, whereas in the rest frame of the electrons there are only about 8½ electrons per image width.
Jun
21
awarded  Yearling
May
26
comment Why do turbine engines work?
@BrysonS.: If the pressure drop across the turbine is smaller, that just aggravates the original problem I had.
May
25
comment Why do turbine engines work?
@Nathaniel: Yes, that is exactly what I was asking.
May
25
comment Why do turbine engines work?
No, the question was not "why doesn't the hot expanding gas from the combustion chamber flow back out through the compressor", but why the pressure generated by the combustion doesn't propagate backwards through the airstream to slow down the compressor by the same amount that it speeds up the turbine. Where the individual molecules of gas go is not really the point, but I seem to have worded the question confusingly.
May
25
answered Why do turbine engines work?
May
25
revised Why do turbine engines work?
fix typo; the simplifies system might not have fewer degrees of freedom
May
25
asked Why do turbine engines work?
Apr
21
awarded  Critic
Apr
8
comment Why does milli- mean 1/1000
Also, "femto-" is derived from Danish femten "fifteen", so it wouldn't make sense to bump that down from $10^{-15}$ to $10^{-18}$.
Mar
30
comment Why cant Electrostatic field lines form closed loops?
@Venik: Yes, that's the "Hmm..." comment immediately afterwards. In my train of thought I had considered the potential to be something whose gradient is the field, but in the usual physical convention the field is minus the gradient. That makes no difference for the high-level argument sketched here.
Mar
29
answered Why cant Electrostatic field lines form closed loops?
Mar
15
comment Why does a candle blow out when we blow on it? Our breath is 16% oxygen and only 4% CO2
@shortstheory: Flames have nothing at all against water vapor. The reason liquid water puts out fire is that vaporizing it takes away heat that would otherwise have sustained the combustion, and to a lesser extent that the volume of produced steam tends to displace oxygenated air. Neither of these effects are available when the water is already vaporized.
Dec
6
comment If the universe is full of dark matter, why is it only 2.73 K cold?
@JanHirschner: Since it is hard to hit the black hole exactly, most infalling matter will end up orbiting the hole for some time until it loses enough energy from inelastic collisions to finally fall in. This lost energy eventually becomes heat and is radiated away in addition to the Hawking radiation from the hole itself. (This glowing matter is how we actually observe most objects considered to be black holes). (And even a directly infalling object would be heated a bit by tidal stresses before it reaches the event horizon, I suppose).
Oct
24
awarded  Popular Question
Oct
4
awarded  Enlightened
Oct
3
awarded  Nice Answer