Ask a physicist

[Redingold]

How do superconductors work?

Silicon atoms have 4 electrons avaliable to form covalent bonds (valence electrons) and so can make a lattice with 4 bonded neighbours. If you take a sheet of pure silicon and stick an atom with a different number of valenvce electrons in their instead of one of the silicons you can produce charges. For example if you stick an atom with 5 valence electrons if will bond in the lattice with 1 electron left over producing an overall negative charge (an N doped semiconductor). If you stick an aton with only 3 valence electrons however you'll be one electron short and produce an electon "hole" which is taken as a positive charge with the same mass as an electron, this is a P doped semiconductor. Both the elctrons and holes can move through the lattice.

If you bring a P semiconductor into contact with an N semiconductor to for a junction (a join between the two material) the holes and electrons that can will cancell near the junction, this is called the "depleted region" which is neutral. If you put a negative potential on the P side all that will happen is the elctrons and holes will combine to increase the depleated region and no current will flow through the junction. If you put a negative on the N side however it will force the electrons across the junction making the junction conduct. This is the behaviour of a diode. All semiconductor devises are made out of P and N doped semiconductors being brought together in junctions.

Careful choosing of how many donor atoms (the added atoms) and how thick a silicon wafer to use allows control over the depleted region size and so the operating potential of the device.

I said superconductors, not semiconductors. Superconductors are the ones with no resistance at very low temperatures.

 

[Chamale]

I have some complicated physics questions that I'm wondering about. I read a book by Einstein about a month ago, and I want to make sure that I understand what I was reading on relativity. So I'll write some of what I think is correct, and please correct me where I'm wrong.

Matter that is relatively stationary to an observer moves through time at the speed of light. If that matter is moving in space relative to the observer, it is travelling more slowly through time, but its vector through spacetime is equal to the speed of light. Right? Furthermore, when matter is annihilated, it releases energy equal to the annihilated mass times the speed of light squared - e=mc^2, famously. Kinetic energy is .5mc^2, so is it accurate to say that matter annihilation releases the kinetic energy of the matter's four-dimensional vector? If this is the case, why isn't Einstein's famous equation e=.5mc^2?

My next questions are more direct, based on a science-fiction space travel scenario.
Say a spaceship leaves Earth at a speed of 99% light speed, going towards a star 100 light-years away. (Ignore the obvious problems relating to acceleration and safety). To observers on Earth, how much time has passed when the spaceship reaches this star? How much time has passed to the crew? How many years have passed for each group when the ship returns to Earth?

If the crew of the spaceship use a device to measure the speed of light, they will find that c=299,792,458 m/s in all directions. Suppose a laser on Earth beams a binary data signal to the spaceship - will the receiver interpret it normally, or will it need to account for time dilation? When would be the last time a laser on Earth could send data to the crew that they'd receive before reaching the star - 1 year after departure? 99 years after?

I hope these questions make sense. I think the answers will be interesting, so I hope they're not really tedious to type out.

 

[Wintermoot]

how often do people make Gordon Freeman/Half-Life jokes?

 

[TiredGiant]

Fuckin' magnets how do they work?



sorry had to do it....

 

[Zeema]

10 + 15 = ?

25

can i haz Cabbage Medal?

okay what do you think of Religion in regards to your work?

 

[cookyy2k]

I said superconductors, not semiconductors. Superconductors are the ones with no resistance at very low temperatures.

Sorry I completely misread.

Electrons belong to a group of particles called fermions due to the fact they have spin 1/2. All fermions obay pauly exclusion, this is that no 2 electrons in a system can exist in the same quantum state. It is the reason that their are shells instead of all electrons getting as close to the +ve nucleus as possible. Their are 4 quantum numbers when it comes to electrons in an orbit; n(shell number), l(subshell), m(orbit), s(spin). So for n=1 (the innermost shell) you can only have 2 electrons: n=1, l=0, m=0, s=1/2 and n=1, l=0, m=0, s=-1/2. Resistance is caused by electrons scattering off the lattice in a material and causing a rise of temperature in the material.

Now in the case of superconductivity the electrons pair up into cooper's pairs which have spin 0 and so pauly exclusion no longer applies. Due to quantum mechanical effects the cooper pairs require a minimum amount of energy to excite them, if this energy is greater than the thermal energy (kT where k is boltzman's constant and T is temperature) the electron pairs will not be scattered by the lattice, thus having 0 resistance.

This means a current travveling in a superconducting coil does not dissipate. Another, commonly demonstrated property is the levitation of magnets, this is due to in lorentz's law a magnet producing a current in a coil will be repelled by the magnetic field the current in the coil produces. In the case of a superconductor the magnetic repulsion equals the force the magnet produces on the superconductor, so if you approach a superconducting surface with a north pole the surface acts like a north pole and same with the south pole so it always keeps the magnet repelled away from it.

I have some complicated physics questions that I'm wondering about. I read a book by Einstein about a month ago, and I want to make sure that I understand what I was reading on relativity. So I'll write some of what I think is correct, and please correct me where I'm wrong.

Matter that is relatively stationary to an observer moves through time at the speed of light.

Yes because the co-ordinate for time is ct which is a distance, so by definition of the above without having to add relitivistic corrections in things move through time at c.

If that matter is moving in space relative to the observer, it is travelling more slowly through time, but its vector through spacetime is equal to the speed of light. Right?

Again yes, it's 4 rector will be the same magnitude but it will have been shifted away from the ct axis to provide velocity in space not time.

Furthermore, when matter is annihilated, it releases energy equal to the annihilated mass times the speed of light squared - e=mc^2, famously. Kinetic energy is .5mc^2, so is it accurate to say that matter annihilation releases the kinetic energy of the matter's four-dimensional vector? If this is the case, why isn't Einstein's famous equation e=.5mc^2?

A common mistake, Kinetic energy can nolonger be stated as simply e=.5mv[sup]2[/sup], this only works in classical mechanics. Also E=mc[sup]2[/sup] doesn't tell you the whole thing, this would say photons have zero energy since m=0 but we know this isn't the case. The real expression is E[sup]2[/sup]=sqrt{(mc[sup]2[/sup])[sup]2[/sup]+(pc)[sup]2[/sup]} this reduces to the famous E=mc[sup]2[/sup] when the particle has no momentum.

My next questions are more direct, based on a science-fiction space travel scenario.
Say a spaceship leaves Earth at a speed of 99% light speed, going towards a star 100 light-years away. (Ignore the obvious problems relating to acceleration and safety). To observers on Earth, how much time has passed when the spaceship reaches this star? How much time has passed to the crew? How many years have passed for each group when the ship returns to Earth?

To observers on Earth the ship takes 101 years to get to the star but the crew's clock only advances 14.24 years. When the ship returns (assuming same speed and no turn around deceleration) On Earth 202 years have passes on the crew's clock 28.48 years have passed.

However their is a problem in this, hence its name the "twin paradox";

To the crew the ship takes 101 years to get there and they'd see Earth's clock advance only 14.24 years and same with the return leg. This is a problem with special relativity that is later solved by general relitivity.

If the crew of the spaceship use a device to measure the speed of light, they will find that c=299,792,458 m/s in all directions. Suppose a laser on Earth beams a binary data signal to the spaceship - will the receiver interpret it normally, or will it need to account for time dilation? When would be the last time a laser on Earth could send data to the crew that they'd receive before reaching the star - 1 year after departure? 99 years after?

I hope these questions make sense. I think the answers will be interesting, so I hope they're not really tedious to type out.

They would have to adjust the information stream as if the people on earth sent 1bit/sup the pulse, the crew would get it at .14bit/s, this is due to not only time dilation but the fact that when a bit arrives the ship has moved away a non-trivial amount before the next one arrives stretching out the bit duration and rate. The last possible send time would be after 1 year (Earth time) since the data would take 100 years to get to the star but it only takes the crew 101 to get there by Earth's clocks.

Which idea of universe 'creation' i.e how it started, do you favor?
And isn't the big bang theory just on how it expanded 'n stuff after creation, not creation itself?

...

I haven't done advanced physics.

Christian are you?
Well, basically we don't know what caused it.

Nah, I was actually calling theists quite silly for just pulling this out at every blank. And yeah we don't know, yet.

how often do people make Gordon Freeman/Half-Life jokes?

Not heard one yet.

 

[cookyy2k]

Fuckin' magnets how do they work?



sorry had to do it....

Their are 2 equally good ways I can answer this depending which heory you use to approach it so here goes;

1) Quantum mechanics - Every electron possesses a property called spin, now this in effect makes the electron into a tiny magnet due to the motion of a charge. In a magnetic material their exists many small regions called magnetic domains, in the case where electrons have randomly oriented spins the domains are one electron in size, as you increase the ordering by magnetising the material the spins begin to allign. When this happens the domains begin to merge making larger and larger magnetic regions until eventually the entire material is one domain. At this point the material is as magnetised as possible, the saturation magnetisation. The domain size changes to keep the magnetostatic energy within the material at a minimum.

2)Relativity - If you picture a coil of wire wound around a iron rod with a current flowing trough it this is an electromagnet. If you send an electron along the length of the rod at almost c outside the coil it will experience a deflecting force equal to F=q(E+(vxB)) where q is the charge on the electron, E is the electric field, v is the velocity of the electron and B is the magnetic field.

If you use the rest frame of the coil and look at the force on the electron it would be F=q(0+(vxB)) since outside a stationary coil their is no electric field, so F=q(vxB), the force is entirely due to the magnetic field.

If you use the electron's rest frame the force is now; F=q(E+(0xB)) since the velocity of the electron is 0 in it's own rest frame by definition, however due to length contraction the coil now appears as a point charge, which does have an exterior electronic field. So the force on the elctron is now F=qE therefore the force is entirely due to the electric field.

It can be shown by using real numbers and slogging through the maths that the forces are exactly equal in dircetion and magnitude, meaning that both electronic and magnetic fields are the same thing, just viewed from differing frames.

snip

okay what do you think of Religion in regards to your work?

Well, for a start luckyly religion does not have the power it once did, I don't forsee going jail or having my work banned if I come up with a formation theory for spiral galaxies like the astronomers of old and the whole Earth goes around the sun thing.

And to be honest I haven't once thought how any of my research would agree or conflict with religion but I'm betting their would be those extreemists who declare me a heratic for it. I just think of the God answer as a cop out, no need to ever try and discover anything if you just say oh it's because God says so or whatever. Why should spiral galaxies have these amazing spirals that come from the centre and in which all the stars exist with big gaps of nothing inbetween? Well my answer for this certainly wont be God did it I can tell you that.

 

[NeuroticDogDad]

What is being a postgrad like?

I'm applying to do a research Masters and my professor is questioning my understanding of the effort involved and whether or not I have the motivation to go through with it.

So if you could break down what being a science postgrad is like I'd appreciate it.

 

[Spade Lead]

I've heard that if you move at relativistic speeds, time will slow down for you allowing you to travel forward in time. Is this true? And if so, how does it work?

Also, is it possible to travel backward in time?

Well, basically, no. What happens is, as you travel closer to the speed of light, relative (the universe's) time continues on, but your time is different. A sixteen hour trip for you at the speed of light appears to take two seconds to those of us still on Earth. (That is an example from the movie Contact, and vetted by my Astronomy teacher.) The closer to the speed of light you travel, the slower the universe appears to move around you.

Also, Time travel is impossible according to the laws of physics, as we know them.

 

[Nimcha]

Would you please tell people that an 'anti-matter bomb' is completely ridiculous? People don't seem to believe me when I say it.

Also, have you been to CERN?

 

[Spade Lead]

Good, it's a wonderfully deep rabbit hole and seriously scientists are always needed. The summer after I graduated I got 3 job and 5 PhD offers, quite a descision to make which to go for but atleast it's not unemployment.

And here I was, unsure if my decision to go for an Astro-physics or some other type of Astronomy based Doctorate was a good idea. I can't wait to have that title... Thanks for the spark that will keep my drive going.

 

[Hungry Donner]

I've been building some planets lately (stupidly I started in my basement, I have no idea how I'm going to get them out when I'm done) and I've been wondering about gravity and water displacement.

If you had a terrestrial planet with higher gravity would boats have an easier time floating (because the water would be denser at sea level) or would floating be more difficult (as gravity would be pulling them down with more force).

 

[llafnwod]

Would you please tell people that an 'anti-matter bomb' is completely ridiculous? People don't seem to believe me when I say it.

Also, have you been to CERN?

It's ridiculous right now. Antimatter is not an energy "source", as it is not naturally occurring in any usable quantities. Rather, it is a (potentially) powerful energy store; since it's annihilation with matter is 100% efficient, a matter-antimatter explosion has by definition the highest possible energy yield of any material by mass. As of right now, antimatter takes much, MUCH more energy to create than it releases when annihilated, and cannot be stored in any significant quantities, nor in any insignificant quantities for any significant length of time. By the second law of thermodynamics, it will always take more energy to create antimatter than that amount of antimatter will release via annihilation.

But if, through major technological advances, a fairly efficient means of creating antimatter and an effective (and portable) means of storing it are developed, an antimatter bomb would be an ideal weapon, with well over 100x the energy yield of a similarly massed thermonuclear bomb (and, incidentally, none of the fallout).

By the way, if this is about Angels & Demons, don't worry. Seriously. Dan Brown doesn't know a single goddamn thing about anything, except apparently how to write multiple best-selling novels.

 

[Aglaid]

Not sure if this is a stupid question to ask but what are "dimensions" exactly? And what I mean is like when people says "3D" or talk about a fourth dimension.

CAPTCHA: Purpose forsin, well captcha if you say so, I mean God would have died for nothing if we had not sinned.

 

[cookyy2k]

This may have already been asked (I honestly couldn't be arsed to go through every post just to see if this question's already been put out there), so if it has, feel free to ignore this post. That said:

I remember recently reading something in the paper about a phenomenon called "quantum entanglement." The main idea I seem to remember from this article (without being able to recall the maths or physics to really explain it) is that if you get two particles, scramble them together, then separate them again, the two particles have become inherently connected, despite their separate nature. So, what happens to one particle has an effect on the other. With that in mind, I was wondering if you could clarify:

-if this process is actually anything like what this article explained to me, or if it was a horrible bastardisation of an incredibly complex idea.

Well what you've gleamed from the article and said above is right so I'd guess it's a decent one.

-just how the process works, because even though I understand a little science (though I admit, only a little), the idea of two particles having a direct effect on each other is so strange I can only see "magic" or "witchcraft" being genuine explanations.

The act of entangling two particles is fairly complex. For example electrons are spin 1/2 particles (spin is a quantum property) if you could decay a spin 0 particle into 2 electrons they'd be entangled, since they came from a spin 0 particle they'd always have to sum to spin 0. So one would be spin 1/2 the other spin -1/2. Now if you flipped the first to spin -1/2 the other would have to instantly flip its spin to 1/2. The interesting part is this can violate relativity. the two electrons could be seperated by miles but the information from one to the other to "flip" its spin is instantaneous, thus the information has travelled faster than the speed of light.

-lastly, the article in question then took the idea a little further, basically putting forth the possible theory that because at the start of the Universe, all matter was held together in a single point of singularity (hope I'm not butchering the term), theoretically all particles have been scrambled together, and thusly every particle of the universe could theoretically be entangled with every other particle. Would I therefore be right in thinking that Quantum Entanglement offers some very psychedelic explanations of the make-up of the universe, and that (dare I say it) some of the more New Agey ideas out there may have at least a fraction of scientific plausibility to them.

Quantum entanglement may well be responcible for the universe properties, we don't really know. Something had to cause density fluctuations in order for structure like galaxies and stars to form so maybe. What is known if you can't say what one electron in an electron gas (a cloud of electrons and nothing else is called an electron gas) is doing without taking into account the state of all the other electrons in that gas. So this may be true of every particle in the universe, and a reason why quantum mechanics is only a good approximation not a true solution.

I realise this is a little rambly, but it's a subject which I'm interested in, yet I lack the knowledge of scientific jargon to really make much sense of it. If you could offer any kind of explanation in more laymans terms, that would be much appreciated.

Well I hope that helped, it's certainly an interesting field and I've done quite a bit of quantum mechanics thought this is not exhaustive and I'm no expert.

I've been building some planets lately (stupidly I started in my basement, I have no idea how I'm going to get them out when I'm done) and I've been wondering about gravity and water displacement.

If you had a terrestrial planet with higher gravity would boats have an easier time floating (because the water would be denser at sea level) or would floating be more difficult (as gravity would be pulling them down with more force).

In this case it would be equivalent. The weight of the boat is more so it would need to displace a volume of water equal to that incresed weight to float, the water would be denser by the same amount so it would need to displace as much as it does on Earth.

 

[cookyy2k]

What is being a postgrad like?

I'm applying to do a research Masters and my professor is questioning my understanding of the effort involved and whether or not I have the motivation to go through with it.

So if you could break down what being a science postgrad is like I'd appreciate it.

It's hard, expect 60-70 hour weeks not your standard 40 like in a job. Expect frustrating problems and dead ends that last weeks. On the plus side it's an increadible feeling when you get something you've been working on for weeks or coming up with something and knowing you're the only person living or dead to know this, it's a strange feeling. So in summary, very hard work and you have to push yourself every inch of the way but it's amazingly rewarding.

Would you please tell people that an 'anti-matter bomb' is completely ridiculous? People don't seem to believe me when I say it.

Also, have you been to CERN?

It's not completly ridiculous atall, not overly possible under today's technology since we can't make much (8g so far have been made) but it's certainly a possibility that hopefully they'll never be able to make...

I haven't managed to go CERN no, but it's on my list of things to do.

Not sure if this is a stupid question to ask but what are "dimensions" exactly? And what I mean is like when people says "3D" or talk about a fourth dimension.

CAPTCHA: Purpose forsin, well captcha if you say so, I mean God would have died for nothing if we had not sinned.

dimensions are degrees of freedom, for example a particle can move in x, y, z or t aswell as some others. It's had to pictue having extras but it's neccessary to make the maths work. It's awkward writing 4D matacies but luckily computers can sort that for me.

It's ridiculous right now. Antimatter is not an energy "source", as it is not naturally occurring in any usable quantities. Rather, it is a (potentially) powerful energy store; since it's annihilation with matter is 100% efficient, a matter-antimatter explosion has by definition the highest possible energy yield of any material by mass. As of right now, antimatter takes much, MUCH more energy to create than it releases when annihilated, and cannot be stored in any significant quantities, nor in any insignificant quantities for any significant length of time. By the second law of thermodynamics, it will always take more energy to create antimatter than that amount of antimatter will release via annihilation.

But if, through major technological advances, a fairly efficient means of creating antimatter and an effective (and portable) means of storing it are developed, an antimatter bomb would be an ideal weapon, with well over 100x the energy yield of a similarly massed thermonuclear bomb (and, incidentally, none of the fallout).

By the way, if this is about Angels & Demons, don't worry. Seriously. Dan Brown doesn't know a single goddamn thing about anything, except apparently how to write multiple best-selling novels.

I would question the non of the fallout thing since yes while their is no radioactive waste left over annihalation turns the mass of the objects completly into gamma radiation at an extreem rate (E=mc[sup]2[/sup] and all). So the shear amount of radiation will be beyond current comprehension and that is bound to cause havoc with elecments in rocks and trees etc and make them radioactive leading to indirect fallout answell as a highly irradiated area.