... whether there was a benefit in having our homes wired up with two sockets (or maybe a 5 pin mains plug) giving standard AC voltage and a low-current DC voltage as well (12V?). So many devices only need low voltage, wouldn't we all benefit in having a power system in our houses in this way?
I have said this before. As soon as you connect devices to each other with wires, their power supplies have to be galvanically isolated from each other. Otherwise, you can get current loops through the signal cables (network cable, audio link) and the power supply. At the very least that will cause interference, in bad cases it will fry the equipment. Example: audio devices that have a 12 V power supply typically create a virtual ground at 6 V such that they can generate both positive and negative voltages on the loudspeakers. Plug those speakers into a computer which has the signal ground at 0 V relative to the power supply and it will burn. Worse even, a computer with a AC switching power supply that is not in a grounded wall socket will have its signal ground at half the main voltage (110 V in Europe, 55 V in USA). As long as everything you connect to the computer isn't grounded you won't notice, but I've personally had a few unpleasant punches when holding a connector that was grounded through the cable radio line. And I fried the sound card along the way.
So you would need a DC/DC convertor in every device, or have very strict rules about how you define the ground level (in that case you would probably need a three-pin power connector with +12V, -12V and GND) and it would still suck for noise-sensitive applications. It will only work for well-defined environments with people with an EE background.
In the end, the only people who'll benefit from that situation will be the bloody users.
I surely hope so. Over here in Netherlands the available online maps are horrible compared to both MS's and Google's. If they would just extend their databases to the rest of the world...
Re:Paitent Records, Kinkos
on
Film to X-rays?
·
· Score: 2, Informative
Option one is to put a piece of white paper behind when you scan it. The black area will stay black, the transparent area will be white.
However, the grey areas will become much darker than intended since the light has to pass twice through the film. If the difference between white and black is a factor 500 in light intensity, then you will need a scanner that can handle a dynamic range of 500*500=25,000. There are no scanners that can do that under practical circumstances. That is why scanners for negatives have a separate light source that illuminates the negatives from behind.
what rights does Google have to protect its own extensive efforts in creating this database?
I'm not sure about the US, but in Europe (Netherlands), such efforts are protected by the Database Act. The information in, for example, a phone book is not copyrightable, but nevertheless the whole organised collection of names and associated addresses and phone numbers is protected.
You'd want to make sure that you could recover from a broken tether, though
With a 200 m tether (100 m from the center of mass) you only need 22 m/s for 0.5g. If the tether breaks, you have to catch up with the other half, which has a relative velocity of 44 m/s. Negligible compared to the center-of-mass velocity (>10 km/s).
The biggest problem foreseen with any rotation scheme is of course the coriolis forces whereby your head experiences an amount of gravity fractionally different than your feet.
Coriolis forces are a problem, but you can make them quite small by increasing the tether length (to 1000 m or so). However, I think you misunderstood the concept of Coriolis forces. It means that any object that moves (except if parallel to the rotation axis) will experience a perpendicular force. It is the same force that causes all hurricanes to rotate the same direction on the northern hemisphere. Only in a spacecraft that makes a revolution every 10 seconds it will also affect motions on a much smaller scale. Imagine for example that your hands end up a few cm off every time you try to pick up something.
Considering that the cable would have to be stronger than the huge construction, I doubt that you'll see that one anytime soon. Nice idea, but it's actually easier to strengthen a larger structure than a smaller one- more materials can mean you can use slightly less strong stuff and accomplish the same thing.
For buildings and things like that you are completely right, since they have to withstand compressive forces (gravity) and the lever effect of forces that try to bend the structure. However, with the space module with counterweight one only needs to have tensile strength. If the space module has a mass of 10 ton and you wish to create 1 g, you only need a steel cable that can handle 10 tons of weight (at 200 kgf/mm2 that is just 50 mm^2 cross-section, plus a safety margin, so say 1 square cm). A 200 m cable would weigh about 160 kg, i.e. much less than the space module.
You need a spinning ring to provide artificial gravity
It can be done much simpler: split the spacecraft in two and connect them with a long steel cable. That way the two halves can rotate around their center of mass and create artificial gravity without the trouble of getting a huge construction into space. Also it is easier to make a large diameter for which a low angular velocity will be sufficient to create 1 g, thus reducing disturbing Coriolis forces.
And for non-visual characters like 'newline'.... what other idea, exactly, did you have? \n is pretty straightforward, once you know how it works.
Yes, it is of course completely silly to want a special character for newline, since you already have one: it is generated by the enter key. The idea of \n is that you can see that there is a linebreak without clobbering the layout of your code. If you want to have some symbol X for newline, then you will have to escape X if you want to display an X instead of a newline. It makes no sense.
Viruses are mostly social engineering attacks (worms are the ones that attack engineering flaws).
A virus is a piece of code that can infect other, legitimate, executables (i.e., applications). The virus code is executed only if an infected application is run. It may or may not stay active in computer memory after that. Cleaning the computer of a virus is complicated since you have to revert the changes to all infected executables.
A worm is a stand-alone executable that can propagate itself. Most emailed malware falls into this category, as well as malware that does not require user interaction. These are easier to get rid of: just remove the executable from the computer.
What happens when a few WiFi nodes are positioned in the line of the narrow AP beam?
The important quantity is the ratio W^2/Lamda, where W is the size of the object (obstruction, receiving antenna, sending antenna), and Lamda the wavelength (about 0.12 m at 2.4 GHz). This is an indication for the distance over which the beam profile is smoothened out due to diffraction. For W=1 m, the shadow will be negligible after 8 meters. (For a narrow laser beam, W=3mm, Lamda=600 nm, the distance is 15 meter).
The question is, though, what the effective cross-section is for a high-gain antenna. It might be much bigger than its physical size.
It's not that light sometimes acts like a wave and sometimes like a particle; light always acts in a consistent fashion, which has some features like waves, some like particles, and some unlike anything else.
True. More specifically, light and elementary particles such as electrons propagate as waves, but interact as particles. Still, light behaves more like waves and massive particles more like particles. It is rather hard to create a coherent particle wave consisting of multiple massive particles; any interaction between the particles tends to destroy coherence. On the other hand, it is very hard to create a radiation field with a well-defined number of photons (for example in a laser cavity); any interaction with the surroundings tends to create a coherent wave that is a superposition of infinitely many states with a defined number of photons (number eigenstates).
What's probably more useful, however, is that general relativity is fairly well understood from a mathematical standpoint. Id est, it is relatively simple (no pun intended) to calculate the time/space compression impact on two bodies whose relative speeds are known.
I think you're confused with special relativity, the theory that describes the effect of speed differences on measurements and the equivalence between mass and energy. It combines quite well with quantum mechanics. General relativity - about space curvature due to gravitation - is NOT trivial mathematically. There still isn't a consistent description that combines gravitation (general relativity) and quantum mechanics.
By comparison the same word was borrowed into Swedish, the spelling changed to "kö". The pronounciation is much closer to the original.
Actually, the pronunciation of kö is irregular, because the Swedish pronunciation rules would predict the pronunciation [tjö] (k followed soft vowel). Bad example.:-)
Write down the equations for the magnetic field around a linear conductor and come back then with your smart remarks. A magnet only looks like a dipole if you are far away compared to the size of the magnet, and that is typically not the case with 100 km long power lines.
There is no effort made really to shield MRIs, as it is not very practical.
An MRI magnet is a dipole, so the field drops as 1/r^3 instead of 1/r^2 as with the power lines. Moreover, it doesn't vary in time so it does not create inductive currents.
You don't wear a lead apron when you get an MRI, and your body part is inches away from a rather strong magnetic field.
Actually, the magnetic field goes through the body, that's the whole idea.:-) Lead wouldn't help. An iron coffin might shield out part of the magnetic field, but big chunks of iron have other unpleasant side effects in strong magnetic fields. Over here they somebody once walked into the mass spectrometer lab (which also involves a big helium-cooled magnet) carrying a wrench in his back pocket. It made a big dent in the apparatus and it took a couple of strong men to remove the wrench out of the magnetic field.
2 external supplies (main/redundant) should be all you need with a lightweight re-route internally to get the power onto the rails.
in addition to what other posters mention, there are technical reasons that most equipment must have individual power supplies. Because ground is shared through the power supply among all connected devices AND through the signal cables, you will get lots of ground loops that act as antennas for radio signals and 50/60 Hz. Moreover, it won't take long before someone tries connecting his computer speakers to the 12 V from the main PS, thus burning both power supply and speakers because the ground for the audio input is connected to the ground of the computer, but to +6 V of the speaker power supply.
I wonder why people need 200, 300, 400 W power supplies in their computers. Do you know what that means for your electricity bill to have 300 W running 24/7? Hint: it's about 1 dollar per watt-year...
In reality, the average government transportation vehicle uses less than half its capacity over its lifespan, and thus spends more than half its time burning fuel and polluting the air while driving its routes with most of its seats empty.
Typical car: 25 mpg = 10.5 km/liter. This would translate into 2.8 MJ/km (Assuming 30 MJ/liter).
Typical electrical train for 500 passengers: 2.5 MW peak power during accelleration, 250 kW to keep it rolling at 140 km/h on flat terrain. Let's say 500 kW averaged and 100 km/h if there aren't too many stops. This translates into 36 kJ/km per passenger. If we calculate 50% occupancy and 40% efficiency of the power plant, it is still only 180 kJ/km/passenger, or 15 times more economic than a car. For this factor 15, the emission limits per unit of burnt fuel can be a bit higher, don't you think so?
And I don't know how technically feasible it would be to have trains traveling 150-250 mph on standard-gauge tracks that were designed in the 1800s.
The gauge isn't the problem. France and Germany have high-speed trains that do 200 km/h (Germany) and 300 km/h (France) on standard-gauge tracks. However, the tolerances for flatness and straightness are much stricter. Also, level road crossings are not an option at these speeds: you need overpasses everywhere. So even though you don't have to rebuild train stations, the high-speed stretches need to be specifically designed as such.
The private automobile is usually a far better price/performance tradeoff than even trains or busses
This is the result of a self-reenforcing system in the USA. Buildings are far apart, with a low density, which means that it is hard to create efficient mass transit. Therefore, people drive cars all the time, which means that all roads and streets have to be ridiculously wide (according to my European standards). This adds even more to the low population density in residential areas.
Compare this to Europe, where in many cities, cars are regarded as a necessary evil that has to be discouraged from congesting the narrow streets that often were constructed before cars were invented. Also, cars create pollution and noise. Therefore, both cars and gasoline are heavily taxed (EUR 1.20/liter = $6/gallon), which leads to public transport being cheaper, although often slower, than cars in many cases. (That public transport is cheaper is often denied by car owners who forget that besides gasoline, they have to pay for the devaluation of the car, maintenance, insurance, and yearly taxes). Because cities are compact, cycling and walking are often a reasonable alternative, especially in the more northern countries (Netherlands, Germany, Scandinavia).
Field strength drops off with the square of distance,
Accidentally, you are be right here, but it is because of Coulomb's 1/r^2 law that applies to single point charges. For line-like charges, such as power lines, the field falls off as 1/r. With power lines, you actually have two line charges with opposite signs, that partially cancel out each other's fields. In that case, the field will fall off as 1/r^2 if you are further away from the lines than the distance between the lines.
Summarizing:
point charge: field goes as 1/r^2
two opposite point charges (dipole): 1/r^3
line charge: 1/r
two opposite line charges (line dipole): 1/r^2
I'll also mention that all this only applies to the electrical field. This electrical field is easy to shield out (grounded aluminum foil will probably do), even though I am rather skeptical of the health benefits. However, the huge currents in the power lines also create a magnetic field (falls of as 1/r^2) and that is almost impossible to shield unless you are willing to cover your house with mu-metal sheet (think $1000 per square meter) or several cm of mild iron. Again, I think this is more relevant for sensitive electrical equipment (microphones, guitar pick-ups) than for your health.
The information is encoded on the key, it's much more cost-effective to do it this way than to try to have "networked" door locks.
When I recently locked myself out of a hotel room, the hotel staff gave me a new card. The old card didn't work anymore after that (the hotel staff confirmed that only one card can work at a time). Either inserting the new card disables any old cards, or the doors are actually networked. In the latter case, it has to be some wireless protocol, since I couldn't find any cables leading to the door (I don't find it very likely that the hinges serve a double role as electrical conductors).
Slashdot Mirror
I have said this before. As soon as you connect devices to each other with wires, their power supplies have to be galvanically isolated from each other. Otherwise, you can get current loops through the signal cables (network cable, audio link) and the power supply. At the very least that will cause interference, in bad cases it will fry the equipment. Example: audio devices that have a 12 V power supply typically create a virtual ground at 6 V such that they can generate both positive and negative voltages on the loudspeakers. Plug those speakers into a computer which has the signal ground at 0 V relative to the power supply and it will burn. Worse even, a computer with a AC switching power supply that is not in a grounded wall socket will have its signal ground at half the main voltage (110 V in Europe, 55 V in USA). As long as everything you connect to the computer isn't grounded you won't notice, but I've personally had a few unpleasant punches when holding a connector that was grounded through the cable radio line. And I fried the sound card along the way.
So you would need a DC/DC convertor in every device, or have very strict rules about how you define the ground level (in that case you would probably need a three-pin power connector with +12V, -12V and GND) and it would still suck for noise-sensitive applications. It will only work for well-defined environments with people with an EE background.
I surely hope so. Over here in Netherlands the available online maps are horrible compared to both MS's and Google's. If they would just extend their databases to the rest of the world...
However, the grey areas will become much darker than intended since the light has to pass twice through the film. If the difference between white and black is a factor 500 in light intensity, then you will need a scanner that can handle a dynamic range of 500*500=25,000. There are no scanners that can do that under practical circumstances. That is why scanners for negatives have a separate light source that illuminates the negatives from behind.
I'm not sure about the US, but in Europe (Netherlands), such efforts are protected by the Database Act. The information in, for example, a phone book is not copyrightable, but nevertheless the whole organised collection of names and associated addresses and phone numbers is protected.
With a 200 m tether (100 m from the center of mass) you only need 22 m/s for 0.5g. If the tether breaks, you have to catch up with the other half, which has a relative velocity of 44 m/s. Negligible compared to the center-of-mass velocity (>10 km/s).
Coriolis forces are a problem, but you can make them quite small by increasing the tether length (to 1000 m or so). However, I think you misunderstood the concept of Coriolis forces. It means that any object that moves (except if parallel to the rotation axis) will experience a perpendicular force. It is the same force that causes all hurricanes to rotate the same direction on the northern hemisphere. Only in a spacecraft that makes a revolution every 10 seconds it will also affect motions on a much smaller scale. Imagine for example that your hands end up a few cm off every time you try to pick up something.
For buildings and things like that you are completely right, since they have to withstand compressive forces (gravity) and the lever effect of forces that try to bend the structure. However, with the space module with counterweight one only needs to have tensile strength. If the space module has a mass of 10 ton and you wish to create 1 g, you only need a steel cable that can handle 10 tons of weight (at 200 kgf/mm2 that is just 50 mm^2 cross-section, plus a safety margin, so say 1 square cm). A 200 m cable would weigh about 160 kg, i.e. much less than the space module.
It can be done much simpler: split the spacecraft in two and connect them with a long steel cable. That way the two halves can rotate around their center of mass and create artificial gravity without the trouble of getting a huge construction into space. Also it is easier to make a large diameter for which a low angular velocity will be sufficient to create 1 g, thus reducing disturbing Coriolis forces.
Yes, it is of course completely silly to want a special character for newline, since you already have one: it is generated by the enter key. The idea of \n is that you can see that there is a linebreak without clobbering the layout of your code. If you want to have some symbol X for newline, then you will have to escape X if you want to display an X instead of a newline. It makes no sense.
A virus is a piece of code that can infect other, legitimate, executables (i.e., applications). The virus code is executed only if an infected application is run. It may or may not stay active in computer memory after that. Cleaning the computer of a virus is complicated since you have to revert the changes to all infected executables.
A worm is a stand-alone executable that can propagate itself. Most emailed malware falls into this category, as well as malware that does not require user interaction. These are easier to get rid of: just remove the executable from the computer.
The important quantity is the ratio W^2/Lamda, where W is the size of the object (obstruction, receiving antenna, sending antenna), and Lamda the wavelength (about 0.12 m at 2.4 GHz). This is an indication for the distance over which the beam profile is smoothened out due to diffraction. For W=1 m, the shadow will be negligible after 8 meters. (For a narrow laser beam, W=3mm, Lamda=600 nm, the distance is 15 meter).
The question is, though, what the effective cross-section is for a high-gain antenna. It might be much bigger than its physical size.
True. More specifically, light and elementary particles such as electrons propagate as waves, but interact as particles. Still, light behaves more like waves and massive particles more like particles. It is rather hard to create a coherent particle wave consisting of multiple massive particles; any interaction between the particles tends to destroy coherence. On the other hand, it is very hard to create a radiation field with a well-defined number of photons (for example in a laser cavity); any interaction with the surroundings tends to create a coherent wave that is a superposition of infinitely many states with a defined number of photons (number eigenstates).
I think you're confused with special relativity, the theory that describes the effect of speed differences on measurements and the equivalence between mass and energy. It combines quite well with quantum mechanics. General relativity - about space curvature due to gravitation - is NOT trivial mathematically. There still isn't a consistent description that combines gravitation (general relativity) and quantum mechanics.
Actually, the pronunciation of kö is irregular, because the Swedish pronunciation rules would predict the pronunciation [tjö] (k followed soft vowel). Bad example. :-)
Not negligible, but O(1/r^2) rather than O(1/r) for a single wire. My point is that the field reaches further than with a confined dipole [O(1/r^3)].
Write down the equations for the magnetic field around a linear conductor and come back then with your smart remarks. A magnet only looks like a dipole if you are far away compared to the size of the magnet, and that is typically not the case with 100 km long power lines.
An MRI magnet is a dipole, so the field drops as 1/r^3 instead of 1/r^2 as with the power lines. Moreover, it doesn't vary in time so it does not create inductive currents. You don't wear a lead apron when you get an MRI, and your body part is inches away from a rather strong magnetic field.
Actually, the magnetic field goes through the body, that's the whole idea. :-) Lead wouldn't help. An iron coffin might shield out part of the magnetic field, but big chunks of iron have other unpleasant side effects in strong magnetic fields. Over here they somebody once walked into the mass spectrometer lab (which also involves a big helium-cooled magnet) carrying a wrench in his back pocket. It made a big dent in the apparatus and it took a couple of strong men to remove the wrench out of the magnetic field.
in addition to what other posters mention, there are technical reasons that most equipment must have individual power supplies. Because ground is shared through the power supply among all connected devices AND through the signal cables, you will get lots of ground loops that act as antennas for radio signals and 50/60 Hz. Moreover, it won't take long before someone tries connecting his computer speakers to the 12 V from the main PS, thus burning both power supply and speakers because the ground for the audio input is connected to the ground of the computer, but to +6 V of the speaker power supply.
I wonder why people need 200, 300, 400 W power supplies in their computers. Do you know what that means for your electricity bill to have 300 W running 24/7? Hint: it's about 1 dollar per watt-year...
Typical car: 25 mpg = 10.5 km/liter. This would translate into 2.8 MJ/km (Assuming 30 MJ/liter).
Typical electrical train for 500 passengers: 2.5 MW peak power during accelleration, 250 kW to keep it rolling at 140 km/h on flat terrain. Let's say 500 kW averaged and 100 km/h if there aren't too many stops. This translates into 36 kJ/km per passenger. If we calculate 50% occupancy and 40% efficiency of the power plant, it is still only 180 kJ/km/passenger, or 15 times more economic than a car. For this factor 15, the emission limits per unit of burnt fuel can be a bit higher, don't you think so?
The gauge isn't the problem. France and Germany have high-speed trains that do 200 km/h (Germany) and 300 km/h (France) on standard-gauge tracks. However, the tolerances for flatness and straightness are much stricter. Also, level road crossings are not an option at these speeds: you need overpasses everywhere. So even though you don't have to rebuild train stations, the high-speed stretches need to be specifically designed as such.
This is the result of a self-reenforcing system in the USA. Buildings are far apart, with a low density, which means that it is hard to create efficient mass transit. Therefore, people drive cars all the time, which means that all roads and streets have to be ridiculously wide (according to my European standards). This adds even more to the low population density in residential areas.
Compare this to Europe, where in many cities, cars are regarded as a necessary evil that has to be discouraged from congesting the narrow streets that often were constructed before cars were invented. Also, cars create pollution and noise. Therefore, both cars and gasoline are heavily taxed (EUR 1.20/liter = $6/gallon), which leads to public transport being cheaper, although often slower, than cars in many cases. (That public transport is cheaper is often denied by car owners who forget that besides gasoline, they have to pay for the devaluation of the car, maintenance, insurance, and yearly taxes). Because cities are compact, cycling and walking are often a reasonable alternative, especially in the more northern countries (Netherlands, Germany, Scandinavia).
Hmm, you seem to have reading difficulties. The pages says that 90% would put you around 65 wpm.
Accidentally, you are be right here, but it is because of Coulomb's 1/r^2 law that applies to single point charges. For line-like charges, such as power lines, the field falls off as 1/r. With power lines, you actually have two line charges with opposite signs, that partially cancel out each other's fields. In that case, the field will fall off as 1/r^2 if you are further away from the lines than the distance between the lines.
Summarizing:
I'll also mention that all this only applies to the electrical field. This electrical field is easy to shield out (grounded aluminum foil will probably do), even though I am rather skeptical of the health benefits. However, the huge currents in the power lines also create a magnetic field (falls of as 1/r^2) and that is almost impossible to shield unless you are willing to cover your house with mu-metal sheet (think $1000 per square meter) or several cm of mild iron. Again, I think this is more relevant for sensitive electrical equipment (microphones, guitar pick-ups) than for your health.
I assume with a battery. It only needs to blink a LED when a card is inserted and power a small electromagnet for a few seconds.
When I recently locked myself out of a hotel room, the hotel staff gave me a new card. The old card didn't work anymore after that (the hotel staff confirmed that only one card can work at a time). Either inserting the new card disables any old cards, or the doors are actually networked. In the latter case, it has to be some wireless protocol, since I couldn't find any cables leading to the door (I don't find it very likely that the hinges serve a double role as electrical conductors).