# I have lots of questions on physics

1. Jun 11, 2007

### NJV

Since I've given up studying at the university, I've had no public lessons on physics, yet my questions about physics have been accumulating ever since, as I still sometimes try to study individually. With the Internet, most notably Wikipedia, as my prime source of information, I sometimes hardly manage to learn anything new at all - especially when I search information about quantum mechanics, the formulae of which have often boggled my mind. Having graduated from secondary education only as of December 2007, I have no considerable physical knowledge - please forgive me if some of my questions will be simplistic.

I've asked for several private teachers to answer my questions, but they either did not have the patience or could not. The list is pretty long, but if some aficionado could help me with some of my questions, I'd greatly appreciate it.

———

Astrophysics

Why is the surface gravity of a Schwarzschild black hole 1/4M? Shouldn’t this be

g = Gm/r2
= Gm/(2Gm/c2)2
= c4/4GM?

———

What is the total thermal energy of the sun?

———

What is the average temperature and velocity of the sun’s particles?

———

Why is the corona of the sun a million kelvins?

———

What is the distance of the event horizon from the black hole?

———

What is the radius of the ergosphere?

———

What is the semi-major axis of the photon sphere(s)?

———

How can one know the total energy received by a moon from the magnetosphere of a planet?

———

The scale factor is defined as

a(t) = L/?

where “L is the physical distance, ? is the distance in comoving units.” What does this mean?

———

What is the rate at which the sun converts kinetic energy (heat) into heat?

———

How can the energy of dark energy be proportional to volume?

———

Why is the surface temperature of the sun so low?

———

What is the total heat of the universe?

———

What is the chance of occurrence of the Big Bang (per unit of space and time)?

———

When do black holes explode?

Electrodynamics

What causes resistance?

———

Paramagnetic materials are attracted by fields because of the presence of unpaired electrons, while for diamagnetic materials it’s just the opposite. Why is this?

———

Why are Maxwell’s equations so significant?

———

How can the electrical current density of a material be known based only upon the its properties?

———

What is the current density of copper?

———

How does a ferroelectric tunnel junction lead to giant electroresistance, and what it its use?

———

What is the Josephson effect?

———

What is cationic lightning?

———

In a ferromagnetic, electrons with the same spin sometimes form pairs. How are these pairs called?

———

What explains the Ochsenfeld-Meissner effect?

Forces

What is the mathematical relation between the speed and strength of a force?

Mechanics

What is the virial theorem?

———

What is the derivation of the formula of gravitational binding energy (E = ? GM2/r)?

———

What is the mathematical relation between the density of particles of a mass, and the heat that will be produced when its movement is inverted?

———

On what, chemically, depends the stress a substance can bear?

———

For a standing wave, y(z,t) = A(z,t) sin (kz - ?t + ?). How is this applied?

———

The speed of sound can be calculated using formula

c = ?(K/?)

Thus, for water, c = ?(2,2 · 109 Pa) ? 50 km/s.

———

If two spheres graze, how can the exchange of momentum be known?

———

What is the work required to compress a mass?

———

What is Langrangian mechanics?

———

Why is the gravitational binding energy equal to ? GM2/r?

Hydrodynamics

If the gravitational acceleration and surface pressure and surface density of a planet’s atmosphere are given, how can the mass of the entire atmosphere be known? Obviously, the problem here is that the atmosphere tapers gradually.

Relativity

How is the stress-energy tensor used?

———

What is a spin tensor?

———

What is a Poincaré group?

———

What is Terrel rotation?

———

Does charge experience a Lorentz effect like mass does?

———

Why is the speed of light 299.792.458 meters per second?

Is this because of the relation ?0µ0 = 1/c2?

———

What would be the maximum length contraction matter could endure before disintegrating?

———

What is the radius for Kerr black holes, Reissner-Nordström black holes and Kerr-Newman black holes?

———

The energy provided by a propellant is:

Enet = m0c2(? - ½?ß2)

The derivative of this to the speed is:

Enet’(v) = p(? - ½ß2?) - Ek/2 [(2ß?/c - ½ ß2(1-ß2)–½]/?2

Then what is the maximum energy a propellant can provide (what are the zeroes of the derivative)?

Thermodynamics

If you pour water on a hot plate, it will form drops as it evaporates. Why is this? Does the plate’s temperature reduce the adhesion of water to the plate, or does it cause it to float briefly?

———

If at one point in a space the temperature rises, what will be the heat rate per volume?

———

If the thermal diffusivity and heat are known, what is the thermal diffusion?

———

Upon discharge of greenhouse gases, the atmosphere is heated immediately. But the heat takes time to permeate the oceans and the soil. As the warming caused by greenhouse gases will pervade the planet, the atmosphere will gradually become cooler. Then how will the global warming of the atmosphere alone evolve if the radiative forcing is known?

———

What explains of the Nernst heat theorem?

———

According to kinetic theory,

P = ??v2

Thus,

F/A = ?mv2/V
? ma/A = ?mv2/V
? (v/t) V = ?v2 A
? V/A = ?s
? r = s

But the work performed by pressure is PV!

———

For the energy of heat,

Q = cm?T

where c, the specific heat capacity satisfies the formula

c = T/N (?S/?T).

Thus, it is temperature-dependent. How can this formula be used if the increase in temperature is very high?

———

H = U + PV
= Q + W + PV

How can W differ from both PV and Q?

Nuclear physics

How can the energy produced in nuclear fusion / fission be calculated?

Quantum mechanics - QM

According to the De Broglie equation, Ek = mc2(?-1)/h. How can this be correct?

———

How is the position vector used?

———

If we combine the formulae I(h,v) = (2hv3/c2) (1/(ehv/kT - 1)) and E = hv/1/ehv/kT - 1), we get that the spectral radiance satisfies the formula:

I = ½E?2 = ½hc? = ½ mc2?2

———

What is perturbation theory?

———

According to this site (http://library.thinkquest.org/C007571/english/advance/english.htm), the lifetime of a virtual photon is 1/8?f, which would follow from the Heisenberg principle: the lifetime of a virtual particle-antiparticle pair would be equal to the uncertainty of a particle with that energy. Why is this so?

———

For two carbon atoms with double bonds, what is the electrostatic repulsion if they approach each other in phonon-heat transfer?

———

What causes tachyon condensation?

———

What causes the Higgs mechanism?

———

What is the gauge field tensor?

———

How are the Hawking radiation equations derived? Why is

t = 5120?G2M3/?c4

where t is the evaporation time? More importantly, how can

P = ?c6/15360?G2M2?

If we invert t, we get:

1/t =?c4/5120?G2M3

Now, t is the time in which the total energy of the black hole, Mc2, is evaporated, so that:

P =?c6/5120?G2M2

Why is this three times greater? How can three times the black hole’s mass be converted? Is two thirds of this energy produced by the vacuum fluctuations that cause it?

———

Photons travel according to the spin of a black hole in the inner photon sphere, but not according its spin in the outer photon sphere. Why?

———

How are quantum states calculated?

———

How is the bra-ket notation used?

———

What causes emission of gravitons?

———

How are virtual photons be responsible for electromagnetism?

———

What is Hartree’s energy?

———

Can the lifetime of an elementary particle be calculated based on its properties?

———

According to the equation E = mc2, all energy is contained within mass. Yet, charge also has energy. Charge is not a subset of mass, however - that is, it does not increase mass; conversely, it appears to decrease it, as the difference in mass between protons and neutrons shows.

———

What causes spin-spin interactions?

———

What is the bra-ket notation for?

———

According to Wikipedia, “a dyon is a hypothetical particle with both electric and magnetic charges.” What is the difference between electric and magnetic charges? Doesn’t a particle with electric charge automatically have magnetic charge as well? Isn’t that why it’s called “electromagnetism?”

———

What are polarons?

———

What are Goldstone bosons?

———

What are polaritons?

———

Is there a formula of the strength of the strong and weak forces as there is for electromagnetism and gravity?

———

How are zero-currents of Z particles involved in the weak force?

———

Why is the Planck temperature the upper limit to temperature? A particle with the Planck energy wouldn’t become a black hole - a particle with ?(?) times that energy would!

———

According to Wikipedia (http://en.wikipedia.org/wiki/Orders_of_magnitude_(energy)) the Planck energy is the energy at which the four fundamental forces become equal in strength. What does this mean? In function of what variables?

———

What explains the Bell theorem? Why will “Bob and Alice” get the same results 100% of the time when they are measured at an angle of 180° and different results 100% of the time when they are measured at an angle of 90°?

———

What explains the behavior of Einstein-Bose condensates and Fermionic condensates?

———

What causes superfluidity?

———

What is superselection?

———

What is the diameter of a photon?

———

In the Wikipedia article of the finite potential barrier, they keep using the term “barrier height” without explaining it. What is it?

———

What is the delta potential barrier?

———

As they are deflected by other charges, photons must essentially have charge. What is the charge of a photon, then?

———

What is the rate of emission of virtual photons from a charge?

———

The number of electrons per shell equals 2n^2. Is there such formula for nucleonic shells?

———

How did the Dirac equation lead to the discovery of the positron?

Last edited by a moderator: Apr 22, 2017
2. Jun 11, 2007

### rewebster

are you looking for single word answers for each question?

3. Jun 11, 2007

### G01

WOW!!!!!!! This is a lot of questions. You really should consider maybe taking a physics class or two at the local community college if your this interested. In fact, if possible you could even try to go back to school for physics. Really using wikipedia and forums(even this one) isn't the best way to learn physics. The best way is from an physicist in a classroom.

I can't, nor do I have time, to answer all of these questions but I'l try to answer a few:

What is the derivation of the formula of gravitational binding energy (E = ? GM2/r)?

I hope you know some calculus, otherwise many of the answers to your questions are going to be confusing.
Newton's Law of Gravitation states that:

$$F_g=G\frac{Mm}{r^2}$$

Now, for conservative forces such as gravity, the potential energy of the force is:

$$U=\int F dr$$

Now, from Newton's Law of Gravitation:

$$U=\int G\frac{Mm}{r^2}dr = -G\frac{Mm}{r}$$

What would be the maximum length contraction matter could endure before disintegrating?

You are confused about length contraction. When something's length is contracted in a certain reference frame, it is actually shorter in the frame. The object is not "squashed" into a smaller space. It is actually taking up less room in that frame. From the object's frame, nothing seems different, it sees itself just as long as it always was.

So matter won't disintegrate because of length contraction.

Why is the speed of light 299.792.458 meters per second?

Light moves at this speed because of the units we pick to measure distance and time. If your asking about where we compute value for the speed of light in some units, then it comes from Maxwell's Wave Equation for electro-magnetic waves. Using the equation, you can find the term describing the speed of the wave:

$$v_{wave}=\frac{1}{\sqrt{\epsilon_o\mu_o}}$$

Find the values for epsilon and mu in SI units and see what the wave speed comes out to be!

What is the mathematical relation between the speed and strength of a force?

Newtons second law is defined(for constant mass) as:

$$F=ma=m\frac{dv}{dt}=\frac{dp}{dt}$$ where p is the momentum.

So the magnitude of a force is proportional to the rate of change of the speed of the object, not the speed itself.

Last edited: Jun 11, 2007
4. Jun 11, 2007

### cesiumfrog

lol. first answer just looks like a conventional choice of units. but so many.. if you'd broken them up into groups that fit on a screen, and posted them into appropriate subforums (not all at once), it's likely you would get more answers.

5. Jun 11, 2007

### Kurdt

Staff Emeritus
Whoa dude! I think thats a weeks work for ten men. I doubt any moderators would enjoy trying to split this up (thats even if they can) as well. But as cesium suggests it may have been wiser to post separately. i'll certainly have a go at some of them, but for others like 'what is lagrangian mechanics?' I'd suggest getting some introductory texts.

6. Jun 12, 2007

### NJV

I'd assumed this thread would be inconveniently long, but I wouldn't pollute the other subforums with my questions; they'd have to be divided over almost each of the physics forums. I was aware that there were far too many questions to be answered, and that not all could be answered, and that many of those that could be answered would take to much time, but I'm satisfied with the answer to just a few of the questions listed above.

G01, thank you for your help. You surely helped me out with two of my questions, although the other two were asked too ambiguously to be understood.

The gravitational binding energy of an object is the energy needed to counter its gravity in taking all its mass away to an infinite distance. The formula was incomplete because the vulgar fraction wasn't displayed: the actual formula was E = (3/5)GM^2/r.

I often find relativity to be counterintuitive. Suppose that an object would be infinitely Lorentz-contracted, yet but its mass would somehow not increase despite its increase of momentum. (Granted, that's impossible, unless the particle is a massless luxon, but luxons apparently don't undergo Lorentz contraction at all, as their wavelength would otherwise be infinitely Lorentz-contracted.) How could the object remain intact despite being infinitely Lorentz-contracted? Obviously, being infinitely small, it would be smaller than its Schwarzschild radius.
Wait, I get it: the Schwarzschild radius gets contracted as well.

I knew Maxwell's wave equation (except the name) under the form (epsilon)0µ0 = 1/c^2, but I thought that this relation flowed from the speed of light, rather than the other way around (as one could think that mass flows from gravity).

Ignore the last question: it's probably quite simplistic. The "speed of a force" referred to its speed propagation. As the speed of gravity is thought by some to be twenty billion times faster than the speed of electromagnetism (light), I assumed that gravitons would have a smaller cross-section than photons, and therefore travel a greater distance before they have an influence, hence the fact that electromagnetism is 6,48 · 10^40 times stronger than gravity. But as this is fringe, speculative, and probably wrong, I doubt there's an answer to the question how the speed of propagation of a force is related to its strength.

Also, thanks for your suggestions. Perhaps I'll study at the Open University of England when I'm old enough (I'm 17 as of the first of June), and perhaps in the meantime I'll study mathematics for a year at the University of Ghent.

Last edited: Jun 12, 2007
7. Jun 12, 2007

### G01

The speed of a force you experience in everyday life would propagate at the speed of light I guess because those forces are electromagnetic in nature at a fundamental level.

Also, Gravity is NOT thought to travel 20 billion times faster than light. Gravity is thought to propagate at the speed of light itself. Where did you here this?

Last edited: Jun 12, 2007
8. Jun 12, 2007

### NJV

Note that I said "thought by some." These claims are highly controversial, though. The speed of gravity was claimed to be six exameters per second by Tom van Flandern. (As you can read http://metaresearch.org/cosmology/speed_of_gravity.asp" [Broken]). It may not be correct, of course, but it's certainly an interesting hypothesis… imagine that the speed limit would actually appear to be at least twenty billion times higher than we'd thought - we could travel across the Milky Way in 2,6 minutes.

Last edited by a moderator: May 2, 2017
9. Jun 15, 2007

### JohnDuck

Wikipedia is a spectacularly bad resource to teach oneself physics.

A lot of these questions are very vague or don't lend themselves to simple or non-technical explanations. You'll have a lot more success in self-studying physics if you pick up some introductory textbooks. Any university library will have a number of texts on classical and modern physics, and you don't always have to be an enrolled student to check books out.

Depending on your mathematical background, though, you'll probably find many of these books inaccessible. I'm sure if you give us some idea of your educational background and what you're interested in studying, someone here can give you more detailed information.

10. Jun 17, 2007

### NJV

That's hard to say, as was home-studied for the bulk of my secondary education. Let's say, though, that I had nearly the highest amount of maths one could get.

Thanks for your helpful suggestions. Just a few days ago I registered in the university libraries and borrowed a few books. The first one showed that picking books almost at random isn't very likely to be enlightening.

Furthermore, I've ordered an introductory textbook on statistical thermodynamics at Amazon.

11. Jun 19, 2007

### Claude Bile

I will attempt to answer some questions with some 1 liner answers - but there are certainly some questions here that do warrant their own thread I believe. (I started from the bottom and worked upwards).

As they are deflected by other charges, photons must essentially have charge. What is the charge of a photon, then?

Photons do not possess charge, they are not deflected by charge.

In the Wikipedia article of the finite potential barrier, they keep using the term “barrier height” without explaining it. What is it?

The upper voltage level (in units of volts) - the term height is sometimes used because height is somewhat analogous to voltage in that they are both ways of expressing potential energy - though using the term height in this manner isn't strictly proper.

What is the diameter of a photon?

A photon does not have defined physical dimensions. It's only defined physical quantities are energy, and momentum.

How is the position vector used?

A position vector is simply a way of describing the spatial dependence of a function in terms of a single vector variable (r), rather than multiple scalar variables (x,y,z).

For a standing wave, y(z,t) = A(z,t) sin (kz - ?t + ?). How is this applied?

This formula is not correct for a standing wave, only a single propagating wave. A standing wave is the sum of two waves, a forward propagating and a counter-propagating component. For a standing wave, y(z,t) = A*sin(kz)*sin(wt) - note that the sin(kz) governs where all the nodes and anti-nodes will be located and is dependent on wavelength - sin(wt) governs the position of the string at any instance in time, and is dependent on frequency.

Why are Maxwell’s equations so significant?

Because they underpin all known electromagnetic phenomena, which roughly accounts for 50% of all known phenomena in our universe.

Claude.

Last edited: Jun 19, 2007