Differences? They are both an electron and a proton, since the neutron decays to a proton and an electron, what's the difference between a neutron and proton + electron? so is it just a higher binding energy between the two?

if i understood this correctly, the determination of voltage for a specific voltaic (gallvanic) cell is determined only by the chemical correlation between the two metals. is this true? for an example , if i use iron instead of zinc in the example from wikipedia, i will get a different voltage?

It's a well known fact that an observer that accelerates at a constant rate from ?c at past infinity to +c at future infinity sees a horizon in flat Minkowski spacetime. This is easy to see from a spacetime diagram once you realize that the union of past light cones on such a trajectory has a boundary that divides the spacetime into two regions - one inaccessible by the accelerating observer.

This leads to the classic result of Unruh radiation when one looks at the quantum field theory for such an observer. The horizon plays a crucial role here.

How does one go about determining whether a horizon is seen by a general class of worldines? In particular, is there any reason to believe that a horizon would exist for an observer that is stationary for all time except for a finite period of acceleration and deceleration?

Is there any other class of worldlines other than an indefinitely accelerating observer for which a horizon is known to exist in flat spacetime?

Something that has bothered me for a while regards the interpretation of chemical potential for different statistics. While I understand its meaning in metals (and its relation with the Fermi surface), I cannot quite relate this definition with the thermodynamic chemical potential, defined as the change in energy of the system when one particle is added to it (or according to Wolfram Demonstrations, " It can be interpreted, for example, as the ability of the system to perform phase transitions or chemical reactions, or its tendency to diffuse").

1) Are those concepts (thermodynamic vs Fermi-Dirac chemical potential) related or they should be thought as different things?

2) Am I missing something trivial or this cited demonstration is misleading? It is mentioned there that the x coordinate corresponds to (E-u). Shouldn't the function blow up for x = 0 independent of temperature, then? I do not understand the shift for negative values as temperature increases.

Quick question for the nuclear engineers/physicists out there

Where does I-134 come from? I cant find it in any of the charts of standard decay products of Uranium fission, but there is tons of the stuff in Fukushima reactor 2 building right now (2900 MBq/ml of water! Nasty!)

half life is 52 minutes, so either it is being made by fission (which would be bad news) or something from weeks ago is still decaying into it in large quantities.

What practical application can we expect from particle physics a century or two from now? What use can we make of quark-gluon plasmas or strange quarks? How can we harness W- and Z-bosons or the Higgs boson? Nuclear physics has given us nuclear plants and the promise of fusion power in the near future. What about particle physics? If we extend our timeframe, what promise does string theory give us? Can we make use of black holes?

Or to put the question another way - what is the result of a proton-positron collision, or an up quark-charm antiquark collision, etc.? As far as I know, annihilation happens only between particles of opposite charge and same mass, but perhaps I am wrong?

And if the types of annihilation mentioned above cannot happend, what are the reasons?

Thank you.

This question came about from a side discussion that arose on this: Does GR provide a maximum electric field limit?

Can we change our choice of coordinate system completely independent of physical motion, and still refer to coordinate dependent components as observations? For example if we have an inertial coordinate system in which we measure the electric field to be E, and now I want to boost the coordinate system to a different inertial coordinate system and state what the electric field E' is in this new coordinate system, do I need to physically accelerate to justify this change in coordinate system (and therefore need to worry about effect such as Unruh, etc. during "changing" the coordinate system)?

I would like to know about the larger picture, current state and future prospects of the sequence of papers that were written by Sheldon Katz and Cumrun Vafa on F-theory. (Freddy Cachazo was also a co-author in many of these papers)

I guess the same is also known as "geometric engineering". (Kindly explain if that is not the same)

There have been recent works on F-theory by Cumrum Vafa, Jonathan Heckman and others.

I would like to know of how this recent work fits in with the earlier work by Katz and Vafa and where do people see this pursuit to be going and what does the community think of its future prospects.

Are these Katz-Vafa works a prospective field for beginning grad students?

I don't really know anything about physics even though I pretended studying it for years.

How is this explained?: http://andrewsullivan.theatlantic.com/the_daily_dish/2011/01/if-you-think-youre-cold-.html

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What is there to be said about it?

My apologies if you find the question too vague or silly.

1. A mass of m = 6 kg has been accelerated by three separate forces acting in different directions in 2-dimensional space. Let each force be given by its acceleration vector in polar coordinates (R, q):

a₁ = 10 m/s² at an angle of 45^o

a₂ = 15 m/s² at an angle of 130^o

a₃ = 4 m/s² at an angle of 320^o

1. Find the magnitude of the resultant acceleration and the direction at which it is pointing.
2. What is the resultant magnitude of the force on the mass m?

A solid sphere of radius 40.0 cm has a total positive charge of 44.4

A jogger travels a route that has two parts. The first is a displacement of 2.90 km due south, and the second involves a displacement that points due east. The resultant displacement + has a magnitude of 3.90 km. (a) What is the magnitude of , and (b) what is the direction of + as a positive angle relative to due south? Suppose that - had a magnitude of 3.90 km. (c) What then would be the magnitude of , and (d) what is the direction of - relative to due south?

A submarine can use sonar (sound traveling through water) to determine its distance from other objects. The time between the emission of a sound pulse (a "ping") and the detection of its echo can be used to determine such distances. Alternatively, by measuring the time between successive echo receptions of a regularly timed set of pings, the submarine's speed may be determined by comparing the time between pings. Assume you are the sonar operator in a submarine traveling at a constant velocity underwater. Your boat is in the eastern Mediterranean Sea, where the speed of sound is known to be 1522 m/s. If you sent out pings every 4.1 s, and your apparatus receives echoes reflected from an undersea cliff every 4.09 s, how fast is your submarine approaching the cliff?

Why is it important to have energy conversion efficiency in our lives and our world?