General Physics Thread

Does anyone think they can help me with something physics related?

Silly question:Does acceleration have an upper cap?Velocity has the speed of light.Asking cause I saw something about an experiment accelerating particles to 10^16 gs,which confused me.

Nope. That said, relativistic mass increase applies after some speed. Also, a lot of such accellerations last fractions of a second.

edited 25th Feb '16 2:25:41 AM by SeptimusHeap

Efficient methane C-H bond activation achieved for the first time: "Using a new hybrid breed of computational and experimental chemistry, an international team of chemists was able to solve a puzzle that has been dubbed a 'Holy Grail reaction' and devise a method for catalyzing reactions with methane."

Do all power sources have limits, and when I mean "limits", I mean, do they have time limits?

Well, until they run out of power...

But are there power sources that are limitless, or that isn't possible yet?

... Well, if you could somehow invent a device that can convert whatever cosmic rays that hit it into usable energy, then I'd hazard a guess that the device could theoretically last until the end of the universe.

But are cosmic rays environmentally friendly?

No, in the sense that they're pretty harmful for any known lifeform to be exposed directly to. However, only a fraction of the total "output" gets through Earth's atmosphere, so you'd have to put the device in outer space. I hear it's possible to transfer collected energy "wirelessly" via some sort of microwave-based method, though.

edited 9th Apr '16 11:21:32 AM by MarqFJA

@ Hallow:

Since nobody's invented a real-life Perpetual Motion machine, the answer is yes, they do.

There's a finite amount of energy in the universe, and no more can be created. Even if you created a "zero waste machine" (which is already impossible, but much less so than perpetual motion) any attempts to draw energy out of it would slow it down.

It is possible, however, to make a power source that's extremely sustainable. Solar Power, for example-the sun isn't going anywhere for a mind bogglingly long time (though much less than forever)

edited 9th Apr '16 11:35:59 AM by Protagonist506

The witty version: the universe is a gamble, and entropy is the House.

Reminds me of a reformulation of the laws of thermodynamics I read once :

1. You cannot win, only break even.
2. You can only break even at absolute zero.
3. You cannot reach absolute zero.

(Yes, I know it does not really match the three actual laws)

edited 17th Apr '16 5:24:47 AM by Aetol

One of the things that bugs me is that many of these witty reformulations overlook the fact that the strictest definitions of the laws of thermodynamics apply to closed systems.

While yes, it is indeed true entropy must increase or remain static that doesn't mean that you can't get entropy to decrease on a local scale by transferring the excess entropy (usually in the form of waste heat) to somewhere its not going to have any effect. If this weren't the case we wouldn't have refrigerators, freezers or air conditioners.

You mean isolated systems, right? The equation you're referencing

∆S(system) + ∆S(surroundings)  ≥ 0

is already applied to closed systems. Or am I misunderstanding something here?

edited 17th Apr '16 6:24:51 PM by Elfhunter

I think it may just be a terminology thing. When my lecturers used "closed" system they meant with no external input or output. Open meant external input of energy and the ability to dump waste heat into some sort of external resevoir.

So what did you call systems where there was flow of both energy and mass (which is what we defined as open) or do you consider that as open too?

edited 18th Apr '16 6:01:41 AM by Elfhunter

You could go up a level in pedantry and point out that energy and mass are more or less the same thing anyway.

Let me rephrase that to "significant mass" <_<

I can't remember what my lecturers called them, but I called them a pain in the ass. But I think they were just considered a specialised subset of an open system.

According to The Other Wiki :

• open systems can exchange both matter and energy with their surroundings
• closed systems can exchange energy but no matter
• isolated systems can exchange neither energy nor matter

Yeah, pretty much what I was taught.

True that, yo.

Anyway, I was wondering if someone knows a good source which describes why a given interfacial flow becomes unstable. I've only been able to find papers that state the conditions for instability for various configurations, but were sparse on explanations.

The cosmic speed limit always felt like the universe's middle finger to human exploration. Oh, you want to explore the majesty of the galaxy? Yeah, good luck surviving long enough to get to the nearest star. Scumbag physics strikes again!

I don't know if the cosmic speed limit is an inevitability. In other words, could a Newtonian universe work? How much of physics as we know them depend in some way on relativity?