Wikipedia:Reference desk/Archives/Science/2014 June 6

= June 6 =

project of physics
'matter for this circuit and working of touch alarm according to this circuit — Preceding unsigned comment added by 14.98.20.92 (talk) 01:27, 6 June 2014 (UTC)


 * Is this homework? At any rate I cannot understand it. IBE (talk) 04:03, 6 June 2014 (UTC)


 * We can't see the circuit diagram. Possibly it's on your PC, but hasn't been uploaded to Wikipedia.  Therefore, only you can see it.  Use the "Upload file" link on the left side margin.  If you have a 2nd computer with internet access, you can check it there after you upload it, to make sure it's really here. StuRat (talk) 17:54, 6 June 2014 (UTC)

How do you conduct medical research?
How do scientists discover cures for diseases and things like that? 203.45.159.248 (talk) 07:18, 6 June 2014 (UTC)
 * The article Medical research gets into some detail about the process. ←Baseball Bugs What's up, Doc? carrots→ 07:28, 6 June 2014 (UTC)

Radiation intensity
The intensity of a beam of radiation travelling in a certain direction is related to its energy density by the formula $$I=uc$$, and it produces a radiation pressure of $$P=\frac{u}{c}$$. Now, I know that if the radiation isn't travelling in a certain direction, ie it's thermalized and travelling outwards in all directions equally, then the radiation pressure will become $$P=\frac{u}{3c}$$. Will the formula for the intensity also pick up a factor of 1/3? 24.37.154.82 (talk) 15:18, 6 June 2014 (UTC)


 * Intensity is the energy flux emitted per unit solid angle, so this will be uc/(4 pi). The power emitted per unit area is then u c/4. To see this consider a small ball of radius r placed inside the encolsure filled with black body radiation. From each direction it will intercept radition with an effective cross section of pi r^2, so if you integrate over all solid angles, it will absorb a power of u c pi r^2 = u c (surface area)/4. Per unit surface area it absorbs a power of u c/4 which in equilibrium it also has to emit. Count Iblis (talk) 17:37, 6 June 2014 (UTC)

Electrostatic discharge
What's the mechanism behind things like sparks, lightning, St. Elmo's fire, etc.? I've heard two different explanations, both of which seem reasonable:

1) Stray electrons are accelerated by the potential difference created between eg the thundercloud and the ground. These electrons attain enough energy to ionize air molecules, creating more stray electrons which ionize more air molecules, and so on, creating an avalanche of electrons.

2) The potential difference is great enough to ionize the air (without needing say stray electrons), creating free charges which can then be accelerated by the potential difference to create a current.

Which one is right? 65.92.6.219 (talk) 21:47, 6 June 2014 (UTC)


 * "2" looks closer to right to me. (If "1" was correct, every spark would ignite the entire atmosphere.)  However, I don't think you need to ionize the air to get a charge to jump from one object to another.  I think that will happen in a vacuum, too.  I believe the ionization is the result of the electrons jumping from one location to another, not the cause.  I believe that ionized air does have less electrical resistance than normal air, so you do sometimes get double and triple lightning strikes, following a path close to the same one, each time.   StuRat (talk) 02:57, 7 June 2014 (UTC)


 * Electric discharge in gases is a complicated subject (there are various different sorts, and the behaviour depends on the geometry of any electrodes as well as the properties of the gas), but basically it's an electron avalanche (i.e. answer 1). In any large enough body of gas, background radiation from cosmic rays ensures there are always a few ionized molecules and electrons around to initiate the process (so you don't need a macroscopic field so enormous as to be able to rip molecules apart from a standing start). It's not clear why one would suppose that this implies that a spark would ignite the entire atmosphere. You can also get discharge in a vacuum, but you need electrodes for that (see e.g. multipaction). --catslash (talk) 01:20, 11 June 2014 (UTC)


 * Well, a chain reaction continues until it runs out of reactants. In the case of a snow avalanche, this means it runs out of loose snow.  What limits this effect in an electron avalanche ?  Is it just because the potential difference is spread out too thinly after a bit ?  If so, I wouldn't really call that an avalanche, as it's not a true chain reaction, where each event is caused by the previous events, alone.  They also require the driver of the potential difference to continue. StuRat (talk) 16:41, 11 June 2014 (UTC)


 * I'm not too sure what you're getting at here. The avalanche tends to propagate, ionizing air and producing a conductive path. The redistribution of charge changes the potential gradient, determining where the avalanche effect is likely to grow, and where it will die out. Even a nuclear chain reaction dies out below the critical mass.  It is not only the availability of reactants, but also the conditions that lead to an escalating or diminishing chain reaction. In the case of an arc forming in air in a potential gradient, if the gradient is insufficient the same chain reaction occurs, only it dies out after each starting event because the charged particles do not gain enough energy to produce more than one ionization event each on average. —Quondum 02:28, 12 June 2014 (UTC)


 * StuRat, that's the wrong picture. Instead of "running out of loose snow", the avalanche approaches a situation where all the loose snow is in the valley – at the low end of its potential. The electron avalanche approaches a similar situation: where the potential gradient has diminished beyond the minimum required to move electrons across the distance. (Imagine that the loose snow forms enough of a pile in the valley to reduce the slope.)
 * The spark doesn't ignite the entire atmosphere (nor the entire volume of air between cloud and ground) because the first electrons ionize the air they travel through, reducing resistance there, and thus they create a literal path of minimum resistance through which most of the follow-ups go. - ¡Ouch! (hurt me / more pain) 06:18, 13 June 2014 (UTC)