10581

Physics, Astronomy & Cosmology / Re: Form where do neutrons come from?

« on: 04/07/2012 14:12:12 »

Fusion in stars today does not directly fuse hydrogen nuclei into deuterium or helium, as the two hydrogen nuclei have positive charges, and repel each other strongly. This would require extremely high temperatures and pressures that don't exist in the Sun.
Instead, it is catalysed by Carbon, Nitrogen and Oxygen, where the neutrons in these heavier nuclei provide some of the strong nuclear force, which helps overcome the electrostatic repulsion.
This CNO cycle manufactures neutrons which eventually get emitted as an alpha particle (helium nucleus).
See http://en.wikipedia.org/wiki/CNO_cycle

10582

Physics, Astronomy & Cosmology / Seismic study of Pulsars?

« on: 04/07/2012 14:03:33 »

What is the speed of sound in "Neutronium" (the stuff of pulsars)?
Would it be possible to study the internal structure of pulsars by observing the impact of seismographic echoes from "starquakes" on the rotational speed of a pulsar?

10583

General Science / Re: Does a multi-stranded wire have lower resistance?

« on: 01/07/2012 13:11:51 »

The main reason why you would want to make wires multi-stranded for use at DC (or low frequencies) is to make them flexible.
- One thick conductor is harder to bend around corners, and so is harder to install.
- In case of repeated flexing (eg in wires to an elevator), a single conductor will suffer stress, and fail. Many thin conductors do not get as stressed, and even if one strand breaks, the other surrounding strands can conduct current around the break.

10584

General Science / Re: If my partner is hit by a taser, will I be shocked too?

« on: 29/06/2012 12:39:43 »

I think the biggest risk is if one of the probes misses your partner, and hits you. Then you will feel half of the voltage.

10585

Chemistry / Re: What alters electrolyte concentration in drinks?

« on: 26/06/2012 19:02:31 »

Can you provide some more information, please?

• What are you studying? The drink itself, or the effect of the container on the drink?
• Are we talking about plastic bottles or glass bottles?
• Are you comparing the same drink in a glass bottle, plastic bottle and aluminium can? (I haven't seen many drinks sold in all 3 container types..)
• How are you measuring electrolytes? (eg conductivity or chemical analysis or ??)
• What type of electrolytes can you distinguish? Different salts, acids, carbonic acid ("fizz"), etc?
• Are you comparing a sugary carbonated drink or a caffeine energy drink (which may not have many ions in solution) or an acidic cola drink (the acid could show up as an electrolyte), or a sports energy drink (which has intentional electrolytes added to restore body salts lost to sweat during intense exercise)?
• Do you record the pH (acidity) of the drink?
• Do the cans have a plastic "liner", sometimes used with more acidic drinks?
• Do you test it straight after opening, or do you let it stand to let it go "flat" before testing it?

My expectation is that:

• Glass bottles are chemically inert, and should not leach ions into the drink
• Plastic bottles may leach organic compounds into the drink, but probably not ions
• Aluminium cans could dissolve some aluminium into the drink, but aluminium is protected by a thin, transparent (invisible) layer of aluminium oxide, which reduces this.
• Some types of containers would stand a higher pressure of carbon dioxide than others, which may show up as an electrolyte if you tested it straight after opening.
• Sports rehydration drinks would tend to come in resealable plastic bottles because cans are not resealable, and glass bottles are heavier (and could smash if you fell over: safety issue).

The best way to do this comparison is to look at the same kind of drink from the same manufacturer, sold in different containers. Failing this, buy containers of different types and fill them with drink from the same source, and let them stand for a few days (covered) to absorb any ions from the container, then test them.

10586

Chemistry / Re: Latest negative perceptions - nano

« on: 26/06/2012 18:31:17 »

Asbestos fibres tend to split longitudinally to create nanoscale-wide spears that cannot be broken down by the body's defences, and can cause mesothelioma, especially to people with workplace exposure.
This "nano" is a definite "no-no".
Depending on the application, carbon nanotubes may come as long tubes ("long" compared to the size of a cell), and so there is some geometrical similarity to asbestos, but the chemical behaviour in the body is radically different, and environmental breakdown processes will be quite different. Carbon nanotubes should be separately assessed for health hazards, and not grouped with "asbestos". It may be considered "safe" as long as it is in a form that cannot be ingested.

Every nanomaterial should be considered on its own merits - perhaps to the extent of determining at what scale the behaviour deviates significantly from the Material Safety Data Sheet (MSDS) for the "macroscale" material. 

10587

Physics, Astronomy & Cosmology / Re: Electron Disintegration Field?

« on: 25/06/2012 10:24:46 »

Cosmic rays also ionise the air - these are atomic nuclei traveling at almost the speed of light. If one of these went through a charged thundercloud it could indeed trigger lightning.

You could in principle build an accelerator which could fire atomic nuclei through the air at high energy, and use this to guide a man-made lightning bolt. As always, there are some challenges:

• Since a bullet is usually not sitting on the ground, you need to hit it from another direction with another simultaneous ion beam, backed by a high voltage source of opposite polarity, in order for the lightning to go somewhere
• The higher-power cosmic rays have far more power than the LHC can produce
• The LHC is not exactly portable (nor is the power station to drive it)
• You need a "window" that will separate the high vacuum inside the accelerator from the air outside, without absorbing too much energy from the particles.
• Once a subatomic particle hits the air, it produces a shower of subatomic particles which continues in the same general direction, but spreads out, so it is more like a shotgun than a rifle.
  

Possible, but challenging.

10588

Physics, Astronomy & Cosmology / Re: Is electricity really attracted to the earth?

« on: 24/06/2012 11:53:07 »

Sorry - I left out the most obvious factor: What is the voltage between the different points? Lightning will jump between any two points where the voltage exceeds about 1 Million volts per meter.

Storms create voltage differences between clouds and the ground, and also between different parts of the cloud.
I have seen estimates that 90% of lightning strikes are between clouds, so I guess you would have to say that the answer is "NO: Lightning is not necessarily attracted to the earth".

10589

Physics, Astronomy & Cosmology / Re: Is electricity really attracted to the earth?

« on: 24/06/2012 11:45:29 »

For frequencies that are common in lightning current surges (up to a few MHz), the characteristics are quite well understood. High electric currents in this frequency range can easily burn out electronics (and people).

The main factors are:

• Electrical resistance: Copper has lower resistance than dry wood, so far more current will flow through the copper than through the wood.
• Physical distance: Resistance is higher for a longer path, so lightning will tend to take the shortest path (all other things being equal)
• Inductance: A coil in a copper conductor will make it harder for the high frequencies to flow through it
• Capacitance: Two conductors in close proximity can carry small amounts of current without a spark between them
• Skin Effect: As a result of its magnetic field, most of the current will flow through the outside of the conductor. The depth of the skin depends on the material that the wire is made of.
• Wire Thickness: Due to the skin effect, it is better to make lightning conductors as a flat braid of thin conductors, rather than a single round conductor with the same amount of metal.
• Wire Shape: Lightning rods have a sharp point facing the air, as this promotes the very high electric field gradients that create a corona discharge and attract lightning. (Or the converse: high voltage electric power lines have rounded fittings, to discourage corona discharge)
• Resonant effects: Wires of certain lengths & inductance/capacitance will respond more strongly to impulses at certain frequencies. This is called resonance.

For behaviors in the order of nanoseconds, such as in a lightning leader, electrical breakdown in insulators (including air) or electromagnetic pulse (EMP) from nuclear explosions, the effects are less well understood, and harder to control. Moderately strong electric fields in this frequency range can easily burn out electronics, but don't tend to affect people.

10590

Physics, Astronomy & Cosmology / Re: Could a person see X-rays?

« on: 24/06/2012 11:13:25 »

Perhaps the reports of "X-Ray Vision" during atomic bomb tests were due to the very high intensity light flash that accompanies a nuclear explosion.
Someone with their hands over their eyes in preparation for the flash would have an afterimage of the bones in their hand remaining on their retina for a few seconds after the explosion. But this image would be due to visible light, not X-Rays.

Since X-Rays are somewhat absorbed by bone, the only visual indication that could be expected from X-Rays would be a slight dimming of the glow when you faced away from the X-Ray source, compared to when you are facing it directly. This is an effect of shielding the X-rays, not focusing them.

10591

Physics, Astronomy & Cosmology / Re: Could a person see X-rays?

« on: 23/06/2012 09:28:39 »

X-Rays and UV photons have more energy than visible-light photons, and so they could trigger light-sensitive cells (as well as damaging them).

• Before the health hazards of X-Rays were discovered, some early experimenters reported a blue glow when their head was close to the X-Ray tube. So X-Rays can give a sensation of light, but because X-Rays go straight through flesh, your eyes can't focus it into an image.
• Similarly, in the early days of cataract surgery, they used a glass lens to replace the natural lens in the eye. The glass lens let through much more UV than the protein lens, and people reported seeing in UV light. This is probably also doing damage to the retina.
  

Infra-Red waves have less energy than visible light, and so would have trouble triggering light-sensitive nerves. We can detect strong infra-red sources by the heat on our skin, and some snakes can detect even weak IR sources (like a mouse).

However, Microwaves and radio waves have much less energy than IR, and so would have trouble triggering nerves at all. Photons in this part of the spectrum have less energy than the thermal energy of atoms of your body, so they would be overwhelmed by "thermal noise".

10592

Physics, Astronomy & Cosmology / Re: Do gyroscopes work in zero gravity?

« on: 23/06/2012 02:13:07 »

Gyroscope Capabilities
Gyroscopes can be used to change the direction your spaceship is pointing, but it won't change the direction your spaceship is traveling. With current technology, to change the direction of travel involves firing a rocket, or taking an orbital slingshot around a massive object like a planet.

Sub-Light Travel: Rockets & Fuels
There are some current projects that are attempting to produce energy from controlled nuclear fusion, for example the Joint European Torus (JET) and the US National Ignition Facility (using lasers). If/when controlled nuclear fusion is achieved in a compact size, this would make a great space rocket, as the fuel is fairly light and compact; it is efficient because it burns at a high temperature. At present both these experimental projects take up a large warehouse and large conventional power stations to make them work. Managing the high temperatures of fusion is one of the big problems of fusion.

Ignoring the temperature problems, scooping gas off the sun would yield a mixture that is about 90% hydrogen and 9% helium. You can fuse the hydrogen, but it takes very high temperatures (more temperature problems). Deuterium is "easier" to burn,  but it is more common in comets or in the atmosphere of a gas giant like Jupiter. Tritium is even easier to burn, but it is radioactive, and you have to make it as you go.

If you can't sleep, why not read some science fiction:- one story I read suggested flying to a comet, and strapping on a fusion rocket. The comet acts as a fuel source and as a shield against gas and dust in space. (It won't be much protection against a meteorite, but these are rare in space.)

For rockets, anti-matter is the most powerful potential fuel we know, but so far scientists have only managed to bottle a few anti-atoms together, which is not enough for a practical fuel tank.

Faster-than-light travel
There was  a report that neutrinos had been observed traveling slightly faster than light, but that has now been traced to a loose connector on some equipment.
 
Travel at the speed of light is not a theoretical challenge: in principle you could  locate the position of every atom in a person's body, encode that onto a laser beam, and send that to a receiver which would reconstruct every atom in its original position. Far beyond current technology, but not forbidden.

Travel beyond the speed of light is also theoretically possible: There are some hypothetical particles called tachyons that can only travel faster than light, but they have never been observed.

The challenge that is beyond current physics is to make the transition between traveling slower than light and going faster than light. If you try to do it by rockets, it would take all the mass in the universe and you still wouldn't be going faster than light.

We don't have a solution to this today, so many science fiction writers gloss over this aspect of interstellar travel, or speculate about travel through another dimension, which is theoretically possible (even if we have no idea how...).

10593

Physics, Astronomy & Cosmology / Re: Electron Disintegration Field?

« on: 22/06/2012 22:29:51 »

I can confirm that diamagnetic, paramagnetic and ferromagnetic bullets are all effectively shielded by SiO₂-filled sandbags.
The military has been researching this technology for quite a few years, and I have seen photos of some deployments.

10594

Physics, Astronomy & Cosmology / Re: Electron Disintegration Field?

« on: 22/06/2012 22:19:37 »

There is such a mechanism as you describe: It's called lightning, and contains an intense burst of current at 30,000 Amps or more.

Before lightning can occur, you must first establish an electric field of around 1 Million volts per metre (or yard), which would be most hazardous for the people nearest the electric field generator. You may be able to reduce this voltage with a laser, but lasers carry their own risks.

As well as sufficient voltage, you must also build up enough electric charge to drive the high currents. The best high-voltage laboratories have problems simulating the intensity of direct lightning strikes.

Lightning could burn out electronics in an anti-aircraft shell, and perhaps detonate the charge in an explosive shell, but because lightning only lasts a very short time (like 10-30 microseconds) it mostly does cosmetic damage to solid metal or insulating objects like bullets.

If an attacker was aware that their bullets would be used in a lightning-prone area, it would be fairly easy to ensure that the current path goes around the sensitive areas, rather than through them. Airplanes are designed this way, and contain a lot of electronics; I have seen an aircraft struck by lightning on takeoff, and the plane continued no no apparent ill effects.

10595

Physics, Astronomy & Cosmology / Re: Electron Disintegration Field?

« on: 22/06/2012 14:13:23 »

Electrons moving at almost the speed of light create a near-impenetrable electric field that will change the course of any charged object (like the electrons in a bullet).

Surrounding yourself by a source of such relativistic electrons would provide a good shield against bullets - the hardest part is manipulating the location and path of these high-speed charged particles.

Silicon dioxide is a good, cheap source of electrons, which are conveniently localised within the covalent bonds of the SiO₂ molecule for easy transport. SiO₂ can be easily obtained from open-cut mines on beaches or deserts.

Bullets, whether conducting, insulators, magnetic or superconductors, on close approach to the SiO₂ are affected by intense electric fields, diverting and slowing their flight. By Newton's law, the SiO₂ feels an equal and opposite force, so it needs to be anchored in some way. Fortunately, more SiO₂ is able to fulfill this function.

To keep the silicon dioxide grains where you want them, ready for use on any incoming bullets, you can pack them in economical hessian sacks, and stack them near the vital target that you wish to protect.

Unfortunately, the relativistic electric field generator tends to be rather heavy, so it's best used on fixed installations, rather than for portable applications. But I think this could produce one of the cheapest and most effective bulletproof shields: the sandbag.

10596

Physics, Astronomy & Cosmology / Re: Copernicus & GR

« on: 22/06/2012 13:47:19 »

A "coordinate system" is how you measure the position of things, and how far apart they are.

• Before Galileo, philosophers and astronomers thought of the earth as the center of the universe, so you would measure distance from the Earth. And this is probably close enough if you are mainly interested in the orbit of the moon, and when there might be an eclipse.
• Copernicus suggested that the orbits of planets might be easier to calculate if you measured their distance from the sun.
• This was certainly true for Kepler and Newton in their analysis of the elliptical orbits of the planets, where the sun sits at one of the foci of the ellipse. This is fine if you are mainly interested in the orbits of the planets.
• It was with the mapping of the galaxy in the 1900s that astronomers identified motions within our galaxy.
• With Hubble's work, the motion between galaxies was measured.
• Later, the Cosmic Microwave Background Radiation (CMBR) was discovered, and you can measure relative to the CMBR.

Each of these psychologically removed the earth one step further from the center of the universe.

So, what's the best coordinate system (or "frame of reference") to make your measurements?
It mainly depends on what problem are you trying to solve!

• Throwing a ball: It's good enough to consider the playing field as the coordinate system. The maths is very easy.  (The Sun, Moon and planets won't affect the result measurably.)
• Launching a satellite into low earth orbit: It's convenient to consider the center of the earth as your frame of reference.
• Sending a mission to Mars: You have to consider the Sun and planets, so the Sun is probably the simplest frame of reference
• Voyager studying the Heliosphere: It is probably easiest to use the center of our galaxy as the frame of reference (the planets and other galaxies are too far away to impact the results much).
• The LHC studying collisions of protons at near light-speed: You can still take the earth as your "stationary" frame of reference, but some behaviors like particle half lives have to take into account that the particles have their own frame of reference.

The theory of relativity explicitly states that the choice is fairly arbitrary.

10597

Question of the Week / Re: QotW - 12.06.17 - Why do women live longer than men?

« on: 22/06/2012 12:15:15 »

About 10% of the human genes are carried on the X Chromosome.
Women have 2 copies (1 from each parent), while men have only 1 copy (from their mother).
All chromosomes carry mutations, but a woman will have a second copy of the X Chromosome, which can mask defects on the the other one.
In the female, each cell randomly inactivates one of the X chromosomes, but the presence of two copies in the body is enough to reduce the chances of women suffering from "male diseases" like haemophilia. It probably also protects against a range of less-obvious conditions that could slightly shorten lifespan in males.

10598

The Environment / Re: What does "the oil will settle back into the ground" mean?

« on: 21/06/2012 12:18:51 »

Perhaps:
At present the oil pipes are full of oil, and that will continue as long as they keep pumping. As soon as they stop pumping, the oil will start to sink down the pipes, so it will take longer to start flowing.
They often inject water into older wells to improve the flow. If they stop injecting, it may take longer to get the oil flowing later.
Equipment only works smoothly while it is being maintained. If you leave it unmanned for some time, it will need to be repaired before it can be used again.

10599

The Environment / Re: Why is there no water in the desert?

« on: 21/06/2012 12:06:32 »

Some of the most surprising deserts are near the North and South poles.
Because it is very cold there, there is not much moisture in the air, and so you get very little precipitation (which we call rain in warmer climates or snowfall in colder climates).

This technically makes them deserts, even though there may be large amounts of frozen water there, representing the slow accumulation of snow over many years.

10600

Physiology & Medicine / Re: Why are some people bald?

« on: 21/06/2012 11:57:04 »

Hair loss can occur due to hair pulling, disease or medical treatment. Gradual hair loss occurs with age.

But the most common hair loss is male-pattern baldness, and is determined by genes which are triggered by male hormones. This constricts the hair follicle, so a normal, pigmented hair cannot grow.
The genes are active in a specific region of the scalp, leading to a distinctive pattern of hair loss.