## Miscellaneous Science Questions

For the discussion of the sciences. Physics problems, chemistry equations, biology weirdness, it all goes here.

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SWGlassPit
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### Re: Common Questions

A capacitor is a device that can store electric charge (energy) and release it in a short amount of time. Basically it's two metal plates a very small distance apart (the reality is slightly more complicated, but this is the basic idea. Without getting too deep (maybe the electrical engineers can explain it better than I), you can imagine the capacitor as an electrical "spring." Just like a spring will not let you stretch farther than a certain distance when you apply a steady force, a capacitor will only store a certain amount of charge for a given DC voltage. To further the analogy, you can liken the voltage across a capacitor as the force stretching a spring, the charge stored in the capacitor as the distance the spring stretches, and the current (i.e., flow of charge) to and from the capacitor as the speed at which the spring stretches. Using this analogy, it is easy to see the basic properties that make a capacitor useful--it "blocks" DC current, just as a spring will eventually stop stretching when you apply a steady force, and it "favors" AC current, just like a spring will keep moving back and forth if you apply an oscillating force.

Probably the most common use of capacitors is to tune an oscillatory circuit. They can be used along with inductors (which act like a mass hanging on the spring) and resistors (which act like hydraulic shocks in a car suspension) to create circuits that resonate at desired frequencies. This effect is used in radios, audio equalizers, timing circuits that require a steady pulse, and the like.

There is a little more to consider when you want to talk real-world usage, but this should provide a relatively simple, big-picture understanding of how they work and why they are used.
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superglucose
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### Re: Common Questions

Ah, I see now... that makes sense. So a capacitor utilizes AC current mostly... I wish we'd cover more AC current in this physics class. DC makes plenty of sense to me, but AC has me confused to hell and back.

jmorgan3
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### Re: Common Questions

superglucose wrote:Ah, I see now... that makes sense. So a capacitor utilizes AC current mostly... I wish we'd cover more AC current in this physics class. DC makes plenty of sense to me, but AC has me confused to hell and back.

There are applications for capacitors in DC circuits as well. They can put out much more electrical power than a battery for short periods of time, so they are used to power camera flashes.
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superglucose
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### Re: Common Questions

But how does that work? How do the electrons flow?

Sir_Elderberry
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### Re: Common Questions

superglucose wrote:But how does that work? How do the electrons flow?

Hook up a capacitor to a battery, and it eventually accumulates charge equal to capacitance * voltage. Take it out of the circuit now. Then put it in a new circuit, and all that charge goes "WHEEEE!" and rushes back around. You can make it so you get a lot more power out of the capacitor, but it obviously only goes in bursts.
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SWGlassPit
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### Re: Common Questions

To add to that--there are two reasons we use capacitors rather than, say, rechargeable batteries in DC circuits.
1) Capacitors can be charged to any (within reason) voltage a very large number of times. Batteries carry a voltage that is dependent on the chemical materials interacting, and the voltage is going to be more or less fixed.

2) Because capacitors do not rely on a chemical reaction to generate current, they can generate very high currents--releasing their charge at a very high rate--as in camera flashes as said above. The short burst phenomenon is actually quite useful in many situations.
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thoughtfully
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### Re: Common Questions

And we can even toss reality out on its ear and use supercapacitors to replace rechargeable batteries. Just don't short the terminals, 'mkay?

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SWGlassPit
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### Re: Common Questions

Hey, what's this thingy do?
*BANG*

Worse than laying a crowbar over a car battery's terminals.
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jmorgan3
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### Re: Common Questions

Meteorswarm wrote:Well, except that supercapacitors also have really high capacitances, so they'll discharge more slowly than a lower capacitance capacitor with the same energy content.

Wouldn't current be independent of capacitance, at least initially? I think the internal resistance of the capacitor would be the primary factor in initial current (for a given voltage and short). Is the internal resistance of a supercapacitor higher than that of less powerful capacitors?
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jmorgan3
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### Re: Common Questions

Note that the exponent is "-t/RC", so a higher C makes the exponent less negative, increasing i.

EDIT: Re-reading your post, it seems that you're comparing two capacitors of the same energy, rather than voltage. That makes more sense. I'm working on the math for that situation right now.

EDIT2: You're right, and I should learn to read.
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alterant
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### Re: Common Questions

I have a question. It may have been asked before; I can't find it.
Are general relativity and the particle physics idea of the graviton incompatible? Or am I missing something here?

Carnildo
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### Re: Common Questions

alterant wrote:I have a question. It may have been asked before; I can't find it.
Are general relativity and the particle physics idea of the graviton incompatible? Or am I missing something here?

This is referred to as quantum gravity, and is probably the biggest unsolved problem in physics right now.

doogly
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### Re: Common Questions

Carnildo wrote:
alterant wrote:I have a question. It may have been asked before; I can't find it.
Are general relativity and the particle physics idea of the graviton incompatible? Or am I missing something here?

This is referred to as quantum gravity, and is probably the biggest unsolved problem in physics right now.

No, particle physics doesn't have a quantum gravity input. I think what alterant is thinking of is the classical field theory approach to gravity. You can treat the graviton as a massless spin two field, and that is actually totally compatible; Weinberg is a fan of this. Neither the geometric nor particle point of view are amenable to quantizing.
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eternauta3k
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### Re: Common Questions

Meteorswarm wrote:Well, except that supercapacitors also have really high capacitances, so they'll discharge more slowly than a lower capacitance capacitor with the same energy content

And it's likely to vaporize whatever's shorting it, so you might lose a finger but probably not your whole arm.

It's not like it's worse than a tank full of highly unstable liquid fuel.

Car explosions are rare. Unless you mean planes, where accidents usually end with fire or chemical burns. But powering a plane with capacitors seems unlikely.
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alterant
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### Re: Common Questions

I guess the problem I'm trying to comprehend is: if gravity is caused by spacetime distortion due to energy; i.e., is geometric in origin, then whence a particle mediating gravity? Is the graviton a traveling disturbance in geometry or something...?

doogly
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### Re: Common Questions

alterant wrote:I guess the problem I'm trying to comprehend is: if gravity is caused by spacetime distortion due to energy; i.e., is geometric in origin, then whence a particle mediating gravity? Is the graviton a traveling disturbance in geometry or something...?

Particles aren't really little particles, they are best thought of as localized bumps in a field. The field concept is what is more important. So, the particle picture does seem to make less sense, but maybe a spin two massless field can sort of work? Instead of viewing the distance as being redefined due to the metric, it is like there is a field pulling at you. I don't know if it is totally sufficient as a way to solve problems though - I know the geometric approach is almost universally preferred. Maybe they are formally equivalent but calculationally useless?
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Sir_Elderberry
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### Re: Common Questions

It's worth noting that nobody's ever seen a graviton to my knowledge.
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### Re: Common Questions

Sir_Elderberry wrote:It's worth noting that nobody's ever seen a graviton to my knowledge.

And it's doubtful that anyone ever will.

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Ralith The Third
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### Re: Common Questions

Tchebu wrote:
Please explain how space itself can expand if expanding means taking up more space.

Well... the first thing you should conclude... is that "taking up more space" is not what is meant by "expanding" when the word comes out of the mouth of a physicist.

Basically it means that an object on which no force acts, actually moves away from you, rather than being immobile (unlike what newton's first law may say). And the further away it is, the faster it will move away.

The analogy commonly used is the surface of a baloon that's being blown up. Now just... you know... imagine that happening in more dimentions...

With ultra-powerful force! Wheee for virtually (infinity symbol) kin energy! Yay!
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Sir_Elderberry
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### Re: Common Questions

Except you don't get any energy out of it, I'm pretty sure. IANACosmologist, but my conservation of energy sense objects very strongly.
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### Re: Common Questions

Sir_Elderberry wrote:Except you don't get any energy out of it, I'm pretty sure. IANACosmologist, but my conservation of energy sense objects very strongly.

Keep in mind that trying to apply Conservation of Energy in cosmological contexts is a good way to contract a severe case of brain-twistage.

The usual example cited is the loss of energy in red-shifted CBR photons. I find this unsatisfying, since it would seem to be the same answer for any gravitationally red-shifted photon (or indeed, any oscillator), and one needn't get into cosmology for that. A tall tower and a couple of atomic clocks will suffice.

I suppose I've submitted my own query. I doubt that it falls under "Common Questions", however.

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Sir_Elderberry
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### Re: Common Questions

Ok, so I saw a UPS (uninterruptible power supply) the other day. For those who don't know, it's a battery/surge protector combination that give syou time to get your computer shut down right if the power goes out. Nothing wrong with that.

However, one of the numbers given as a demonstration of its performance was something known as the "volt-amp". The volt-amp. The (J/C)*(C/S). In other words...the watt.

Is there a good, electrical engineering reason behind not calling it a watt, or am I missing something? Watt is hardly that obscure of a measurement--hey, the kilowatt-hour seems to be the most popular unit of energy out there, and can you imagine how the joule feels about that?
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ATCG
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### Re: Common Questions

When volt-amps show up, you know that power factor has entered the discussion. Real (resistive) power, the kind that the electric company gets to bill you for, is measured in watts. Apparent and reactive power (both of which are reactive in part or in whole) are measured in volt-amps and include power (the reactive part) that you trade back and forth with the electric company 60 times a second (on my side of the pond) and don't get billed for. This makes the electric company unhappy, especially if you are a large industrial user.
Last edited by ATCG on Thu Mar 05, 2009 3:24 am UTC, edited 1 time in total.
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Sir_Elderberry
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### Re: Common Questions

And thus was I enlightened.
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gmalivuk
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### Re: Common Questions

Sometimes the unit given is such because that's easier for users to actually get something useful out of. Rechargeable batteries measured in mAh, for example, which, sure, when you multiply it out, just gives you a quantity of charge. (Or, for that matter, kWh, which is just a number of Joules.)
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doogly
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### Re: Common Questions

Yeah. Nobody wants the Hubble Constant in hertz.
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Sir_Elderberry
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### Re: Common Questions

gmalivuk wrote:Sometimes the unit given is such because that's easier for users to actually get something useful out of. Rechargeable batteries measured in mAh, for example, which, sure, when you multiply it out, just gives you a quantity of charge. (Or, for that matter, kWh, which is just a number of Joules.)

Yes, and I'll permit that, although part of me still wants to grumble about kWh. But my confusion was with the fact that the "watt" was a very common unit, and decently understood. My assumption was that "volt amp" just sounded cooler to some marketing guy, or they were aiming for obfuscation.
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Carnildo
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### Re: Common Questions

doogly wrote:Yeah. Nobody wants the Hubble Constant in hertz.

But square millimeters are such a fun unit to measure fuel economy in!

ATCG
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### Re: Common Questions

Sir_Elderberry wrote:My assumption was that "volt amp" just sounded cooler to some marketing guy, or they were aiming for obfuscation.

You weren't entirely wrong. With respect to UPSs, the very same UPS might be rated as both 500 watts and 750 VA - with a distinct emphasis on the 750 VA in the marketing literature. (And doesn't 750 sound 50% better than 500?) The seeming contradiction is because 500 watts refers to the real power that can be supplied by the UPS while 750 VA refers to the apparent power that can be supplied. This is where the real and apparent powers drawn by the load, along with their ratio - the power factor, enter the picture. The gory UPS-specific details are covered here.
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### Re: Common Questions

Maybe a not so common question, but here it goes:

Say you have two positive charges moving in the same direction with the same velocity. An observer watching them go by will observe a magnetic force attracting the two charges due to it's movement, and an electric force repelling the two charges. An observer moving with the charges (such that they appear stationary to the second observer) will see only the repulsive electrical charge. This seems like a relativistic problem, but it seems to be able to happen at velocities much lower than speeds normally considered relativistic. What's going on?
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doogly
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### Re: Common Questions

hideki101 wrote:Maybe a not so common question, but here it goes:

Say you have two positive charges moving in the same direction with the same velocity. An observer watching them go by will observe a magnetic force attracting the two charges due to it's movement, and an electric force repelling the two charges. An observer moving with the charges (such that they appear stationary to the second observer) will see only the repulsive electrical charge. This seems like a relativistic problem, but it seems to be able to happen at velocities much lower than speeds normally considered relativistic. What's going on?

Purcell Chapter 5! Yesssssssssss it is definitely a relativistic problem.
Are you familiar with only SI units? It may help to know that
$\frac{1}{\mu_0 \epsilon_0}=c^2$
And in other unit systems, this is more apparent. So if the charges and observers aren't moving so fast, something still is - namely, the electromagnetic field. It is behaving relativistically even in classical E&M. You can derive the rules of special relativity from taking E&M more seriously than Newtonian mechanics, it's actually rather neat.
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### Some questions

I have gotten some questions and wrote them down to ask some scientist, and now I need to settle for you guys:

1. How does a photon have momentum?
2. How does the basic transistor work?
3. What is the difference between electrons and photons?
4. How does electrons release energy during collisions, and does the electrons slow down?
5. How does magnitism work?
6. Is it possible to remove a photon's "field" and can it be used for heating?
7. I was in "an electronic field", for lack of better words, and my finger started to glow, however it was just the tip of my finger, would that be static moving from flesh to fingernail?
8. What is the molecular structure of a super-liquid?
9. What is the molecular structure of plasma?

I hope someone can help.
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KyleOwens
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### Re: Some questions

While I'm not quite a scientist I'll do my best to answer the ones I can.

1. Photons have energy, and everything with energy has momentum. While it's tempting to just assume [imath]p=mv[/imath] but this obviously breaks down when m=0. The relation [imath]E^2=m^2 c^4 + p^2 c^2[/imath] for relativistic energy is the one to use when dealing with massless particles. From there it is easy to justify that [imath]p=\frac{E}{c}=\frac{h\nu}{c}[/imath].

2. While there are many types of transistors they all operate on the same basic premise. A transistor has three pins, by applying a voltage across two of them we can control the voltage across the third. This allows us to use them as a sort of dimmer switch. By applying the right voltage to 2 of the pins we can have output on/off or somewhere in between. The exact mechanism that is used depends on the type of transistor in question but it is generally done by manipulating some property of the semiconductor that makes up the transistor.

3. Apples and oranges. Electrons have mass, charge, and can never go as fast as light. Photons are massless, chargeless, and can never go slower than c (although c is dependent on whatever material the photon is in). Electrons are also fermions meaning they have spin 1/2 and must obey the Pauli exclusion principle. This states that no two identical fermions can have the same quantum numbers (all electrons are identical). Photons have spin 1 and are Bosons. Because of this they are allowed to exist in any state they want regardless of whether it is occupied by another photon.

4. This question is a bit vague. When an electron is roaming freely then it behaves pretty much like you would expect. If it hits something its going to slow down, go off in another direction, etc, making sure to not violate any conservation laws along the way. If it's bound to a nucleus, however, and is "struck" by a photon then there is a chance that it will jump to a new energy level. The catch is that within this system the electrons energy is quantized, meaning it can only absorb certain wavelengths (energies) of light. If the incident photon has the proper energy the electron may instantaneously jump to another energy level and then decay back down to the ground state emitting photons along the way.

5. The long answer to this question can be found in any one of many books about E&M (Griffith's Intro to Electrodynamics is a great book although probably a bit above your head). The short answer is that moving charges generate magnetic fields. Basically magnetic fields have to happen when you mix electric fields and relativity.

6. This question doesn't really make sense. Concerning heating, however, whenever a material absorbs a photon it is heated.

7. I don't know what to say to this other than that you may be a witch, I'd stay away from 17th century Massachusetts.

8. As far as I know there is no such thing as a "super-liquid.' Superfluids on the other hand are a phase of matter (like solid, liquid, plasma, etc) found at very low temperatures in some materials that exhibit some very strange properties, most notably a complete lack of viscosity. This means they can flow right up and over the edges of whatever vessel they are contained in. Superconductors can be considered superfluids of electrons existing in a metal.

9. Plasmas dont really have a predefined molecular structure. They are simply gasses that have been heated enough that some (or all) of the electrons have jumped right off their respective nuclei and exist freely in the gas. Because of this they can be manipulated in various ways that are impossible with neutral matter (magnetic bottles, etc).

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### Re: Some questions

1. How does a photon have momentum?
Everything acts as both a wave and a particle. Particles have momentum. To think of it another way, they can transfer momentum to things so they must have some of their own.

2. How does the basic transistor work? I don't know.
3. What is the difference between electrons and photons?
Electrons have mass, charge, and a half-integer spin. Photons have no mass, no charge, and an integer spin.

4. How does electrons release energy during collisions, and does the electrons slow down? You got it. It can also give off electromagnetic radiation.
5. How does magnitism work?
When a charge moves the electric field around the charge changes. The change in the electric field is the magnetic field.
6. Is it possible to remove a photon's "field" and can it be used for heating? I'm not sure what you're asking. But if you shine photons onto the something it will heat up.
7. I was in "an electronic field", for lack of better words, and my finger started to glow, however it was just the tip of my finger, would that be static moving from flesh to fingernail?
You got a shock?
8. What is the molecular structure of a super-liquid? I think it's just the atoms touching each other but not bonded strongly.
9. What is the molecular structure of plasma? No structure, just ions and electrons flying around.

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### Re: Some questions

Thank you, I waited for two posts because I prefer to have two oppnions on things.
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Dobblesworth
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### Re: Some questions

On 2), the transistor stuff was covered well. It's a pretty advanced topic, as I'm learning and applying it at university-level electrical engineering, well only first year, but still. Some random blips more on it...
Semiconductor material used is often silicon, Germanium is also used in some rather specific low-concentration for higher performance at times.
The bonding of the atoms and electrons in the semiconductor works where one portion form almost negative ions and have extra electrons, while others have 'holes' for them. It's P-type (positive) if there's holes, N-type vice versa. For the transistors, the strip of semiconductor is one section of p-type that gets jacked up through 'doping', and one section of n-type that goes otherwise.
The n-type 'substrate' is entitled the source, p-type substrate is the drain, i.e. in terms of electrons. For the transistor to work, a voltage is applied to the 3rd node, or gate, to shift electrons in the semiconductor close to the gate, to create a channel for flow of electric current. That gate voltage determines its performance, current/voltage output, and how much voltage gain can be produced when you use transistors in amplifiers.

I'm too rusty on particle physics to contribute on the rest.

Rhubarb
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### Re: Some questions

shadowslayer wrote:I have gotten some questions and wrote them down to ask some scientist, and now I need to settle for you guys:

1. How does a photon have momentum?

I'm a leeetly tipsy when I write this so take care! If you understand that photons have energy (they must because EM waves transfer energy between things) and that the flow of energy is momentum (it's true) then you can see that photons have momentum! Photons can never be at rest after all...
2. How does the basic transistor work?

Something to do with semiconductors... when you bring two SCs together, and one has a chemical potential closer to the valence band and one has it closer to the conduction band, by equalization of potentials, there is an energy barrier for electrons traveling towards the conduction band, but an energy drop for electrons traveling towards the valence band. Hence anisotropy of current...
3. What is the difference between electrons and photons?

Electrons generate electric fields, photons ARE the electric field. Aw yeah.
4. How does electrons release energy during collisions, and does the electrons slow down?

If an electron collides with something that it can pass energy to, it may do, in which case it will lose energy by conservation of energy, and hence slow down.
5. How does magnitism work?

Exchange of photons... or something. It's like electric fields, only it only acts on moving charges only (cuz of relativity yo)
6. Is it possible to remove a photon's "field" and can it be used for heating?

The photon IS (an excitation of) the field... The number of photons in the world is not constant (unlike, say, electrons), so you can just remove the whole photon and use it for heating!
7. I was in "an electronic field", for lack of better words, and my finger started to glow, however it was just the tip of my finger, would that be static moving from flesh to fingernail?

If your finger starts to glow in an E field, go see a doctor.
8. What is the molecular structure of a super-liquid?

First, make sure it's just bosons (integer spin), then make sure the whole lot is moving slow enough that the fluid doesn't gain excitation energy due to friction cuz any excitation speeds up the whole lot...(that might be completely wrong!!) then you have a super fluid! That's all you need!! Note that going that slow means it has to be hella cold...
9. What is the molecular structure of plasma?

All the molecules have been torn asunder by the high temperature... you just have nuclei and electrons floating about independently.

Hrmm I hope that helps... drunk physics== baaad idea! Holy fuck it's nearly gone half past 4!!!! What the hell is wrong with me!!!!!

I probably shouldn't post this, it's probably mostly really inaccurate...

cpt
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Joined: Sun Jan 04, 2009 5:02 am UTC
Location: Boston/Cambridge

### Re: Some questions

Perhaps to clarify number 3, you should read about gauge bosons. Sorry I don't have a good source for you, but it won't be hard to find. Basically, a very big difference between photons and electrons is that electrons are basic particles while photons are created/destroyed to be carriers of the force of nature called EM.

evilbeanfiend
Posts: 2650
Joined: Tue Mar 13, 2007 7:05 am UTC
Location: the old world

### Re: Some questions

just to clarify plasmas do not have to be high temperature (in terms of the bulk matter) and they do have to be more than ionised gases, its crucial that the charged particles are close enough that they all exhibit an influence on each other and you get collective behaviour
in ur beanz makin u eveel

Posts: 645
Joined: Mon Dec 15, 2008 2:56 pm UTC

### Re: Common Questions - Now With Extra Relativity!

Where did the spot of space, matter, and / or time that exploded come from when talking about the big bang?

Searching didn't give me anything, so I think this is a new question here.
Maybe.
Seems like someone would raise it, but...
Well, anyway, back to the question.

It confuses me.