Berkeley CSUA MOTD:Entry 34020
Berkeley CSUA MOTD
2017/10/23 [General] UID:1000 Activity:popular

2004/10/10-11 [Science/Electric] UID:34020 Activity:high
10/8    Dear physics/EE experts. If I put electricity in motors, they
        turn. But if I turn motors, will they generate electricity?
        \_ How do you think electric power plants work?
        How efficient could they do that? The other question is,
        the Toyota Prius has a regenerative brake. How much more
        components did they add to make that happen (how many more
        motors, circuitry, coils, etc), and how efficient is the
        regenerative brake? In another word, suppose you put in
        100X 'energy' into the car to make it go ye fast, then you
        utilize the regenerative brake, what percentage of 100X do
        you get back?
           \- a neat demo i would think you would see in high school
              physics is to put a coil at the end of a pendulum and swing
              it through a magnetic field ... when the coil is open, the
              pendulum behaves normally and continues to swing back and
              forth, but when the coil is closed, the potential energy
              becomes kinetic energeny and then raher than being converted
              back to PE, becomes electrical energy and the velocity of
              the pendulum drops. it is pretty dramatic with an efficient
              coil. ok tnx.
              \_ i have thought of the same thing while banging yermom
              \_ a *really* dramatic demo in a similar vein is to drop one
                 of those super-strong rare earth magnets down a copper tube.
                 the currents induced in the tube create enough field to make
                 the magnet take tens of seconds to drop through a few
                 feet of tube.  cooling the tube with liquid nitrogen makes
                 it even more dramatic.
                 \_ A brilliant read on the matter:
                    \_ thanks!
        \_ I wonder why subway trains don't use regenerative motors.  I'd think
           we don't even need to add batteries to the train.  Just make it pump
           the electricity back to the powerlines such that another train
           accelerating somewhere along the same powerlines can use that
           \_ BART does, at least in theory.
        \_ it really depends on the motor design. If it has no permanent
            magnets and relies on current to generate the magnetic fields it
            uses to provide motive force, turning a 'dead' motor to generate
            current won't do anything at all.  Also the brushes in the motor
            would need different timing settings to run as a generator than
            they do as a motor.
2017/10/23 [General] UID:1000 Activity:popular

You may also be interested in these entries...
2005/3/28-30 [Science/Electric] UID:36926 Activity:low
3/28    So I'm really confused about the DC to DC, car-to-laptop power adapter.
        How come most of them are "inverter" types, where you convert from
        DC to AC, and then AC to DC? Isn't that inefficient?
        \_ Using a resistor or transistor to do DC->DC also gives power loss.
           -not an EE.
        \_ It's easier to step up/down the voltage if the current is in the
2004/12/27-28 [Science/Electric] UID:35451 Activity:moderate
12/27   Anyone have experience with voltage regulators? Ie, it will
        correct under and over voltage back to 120v? I am trying to
        find one... Thanks.
        \_ Yes, they will do that. Is there anything more specific
           you wanted to knwo?
           \_ What particular ones have you used? How well does it work?
2004/9/11-12 [Science/Electric, Science/GlobalWarming] UID:33475 Activity:insanely high
9/11    Is it practical to have, say, a large portion of CA or AZ running
        on solar power? My coworker runs his entire household via solar
        and it got me to wondering what would happen if everyone did this.
        Obviously industrial plants need more power, but could we do away
        with a lot of our polluting plants if everyone went solar? If so,
        then why don't we? What are the technological obstacles, if any?
2003/7/14-16 [Science/Battery, Science/Electric] UID:29030 Activity:moderate
7/14    Do DC to AC power inverters cause any strain / damage to car batteries?
        Would the car battery wear out faster from continued use of the
        inverter?  Specifically, I'm looking at the Xantrex XPower 75 watt DC
        to AC inverter, available on at  If it
        does not damage the car battery or the device, is this a good price?
        \_ no, no, yes
Cache (1989 bytes)
Hybrid Insight most efficient vehicle Gas-electric Honda beats Toyota hybrid Prius for best mileage; October 8, 2004: 12:25 PM EDT NEW YORK (CNN/Money) - Honda's gas-electric hybrid Insight edged out the Toyota hybrid Prius for the most fuel-efficient vehicle, according to th e government mileage readings, while the Dodge Ram pickup was measured a s the worst gas guzzler. The mileage readings from the Environmental Protection Agency says that t he manual transmission version of the hybrid Insight could get 61 miles per gallon in city driving and 66 mpg on the highway, compared to 60 mpg in the city and 51 mpg on the highway for the Prius, which is unusual f or getting better city mileage due to its greater reliance on its electr ic motor during stop-and-go driving. Different versions of hybrid vehicles from Honda and Toyota hold the top seven spots on the list of most efficient vehicles. The manual transmission diesel version of the Volkswagen Jetta Wagon is just behind at No. It didn't use much more fuel than the hybrid version on the highway, ge tting 36 mpg in the city and 44 mpg on the highway in its manual transmi ssion version. The new hybrid version of the Ford Escape, the first sport/utility vehicl e to use a hybrid engine, is No. At the other end of the ratings, the two-wheel-drive version of the Dodge Ram Pickup got only 9 mpg in the city and 12 mpg on the highway. That e dged out the high-end sports car Lamborghini L-147/148 Murcielago, which got 9 mpg in the city and 13 mpg on the highway. CORRECTION graphic The original version of this article got the name of the Honda vehicle at top of the fuel efficiency list wrong. Vehicles more than 8,500 pounds gross vehicle weight, such as the Hummer H2, are exempt from federal fuel economy requirements. The other vehicles among the top gas guzzlers are other high-end vehicles from Lamborghini, Ferrari and Bentley, as well as the manual transmissi on version of the Dodge Ram pickup, which came in at No.
Cache (8192 bytes)
Click Here One can academically appreciate the fact that every electric motor in the world works because of magnetism. One can reflect on the fact that ever y transformer uses it, too. CRT monitors, TVs, brushless DC motors, powe r meters, modern welding gear and engine ignition timing systems, variou s vending machines and umpteen other gadgets use permanent magnets. And electromagnetism has a hand somewhere in making practically every other electronic device work. To get a true, visceral appreciation for magnetism, though, you can't bea t playing with magnets. Really terrifyingly freakin' strong magnets, by preference. When I was a kid, I had a bag full of black ferrite magnets of various sh apes and sizes. "These", in this case, are neodymium iron boron (NIB) magnets. They're co mmonly just referred to as "neodymium" or "rare earth" magnets, and this composition is both more powerful and cheaper than the previous king of the permanent magnet world, samarium cobalt. Well, the smallest magnets in the above picture are little gold-plated cy linders only 64 by 23mm in size. The field extending from these magnet s doesn't reach far with any strength, because the smaller a magnet of a given intrinsic strength is, the smaller will be the volume of space it can fill with a magnetic field of a given strength. Magnetic field stre ngth drops off as the inverse cube of the distance from the magnet, too; get twice as far away and the field strength drops by a factor of about eight. So two of these little tackers barely notice each other over a d istance of more than an inch. But just tossing one of these tiny 'uns in the vague vicinity of another will cause them to click together end-to-end. And they do it strongly en ough that I can hold the one on the end of a full stick of 12 of 'em and twirl the rest around as fast as I can, without them letting go. Spherical magnets, mercifully, tend to push your fingers out of the way as they head for each other, so the se ones aren't too prone to pinching people. But they can certainly do i t, and playing with them without letting them smack together so hard tha t they damage themselves is easier said than done. These magnets are all covered with protective metal plating, but careless play will flake it off quite quickly, and the NIB material itself is quite fragile. ForceFiel d, who seem to be pretty much the only really serious sellers of a prope r range of cheap surplus rare earth magnets on the Internet at the momen t They're still not dirt cheap, mind you. I bought three $US20 grab bags on Ebay, and got 81 magnets as a result; they'd all easily fit inside a ci garette packet, if not for the fact that they'd roll it up into a ball i f you tried. There's an assortment of rods, discs, rectangular prisms and spheres, and there are only three of the usual flat-banana-shaped surplus hard drive magnets. Those magnets are massively strong, just like any other neodym ium, but their odd shape makes them unusually fragile. The ForceField grab bags seem to be the cheapest way to get hold of these things, short of getting old dead hard drives for free and ripping them to bits. Field strength Any time people talk about super-powerful magnets they have to show pictu res of big metal things dangling, so here some are. Tool mobile That's a 15 inch spanner hanging there along with the other ironware. The sphere holding the main string of tools is only about two thirds of the way to holding its maximum load. Magnetic field strength is measured with two units, the Gauss and the Tesla . The earth's natural magnetic field is about 05G, depending on where you are - it's weaker at the equator and stronger at the poles. It's also sl owly declining at the moment, which is something that it does periodical ly; geological evidence shows that it's actually reversed several times over the planet's life. The strongest cheap ferrite magnets have a field strength at their poles of around 1000G, or 01T. NIB rare earth magnets, on the other hand, hav e surface field strength of about 1T. The size of a magnet has a lot to do with the perceived strength of its f ield, though. None of these magnets are very big, so that inverse-cube-l aw field strength reduction bites into their power quite quickly. Chisel the huge ferrite disc magnet off the back of a large dead speaker (if it wasn't dead before you started chiselling, it sure will be when y ou've finished) and you'll have a magnet with only about 1000G field str ength, measured at the peak strength areas on its poles. But big speaker magnets commonly weigh more than a kilogram and are sever al inches across. The peak strength areas at the poles are thus already a few inches away from the middle of the magnet's field. In this case, y ou can move another few inches away and still have 1/8th field strength. So if you wave one of these big magnets over a pile of nails, they'll lea p up to stick to it from several inches away. Take a 1-Tesla-field-strength neodymium magnet the size of a button, thou gh, and the peak field areas on the outside of the magnet will only be a couple of millimetres away from the middle of the field. Now moving jus t another couple of millimetres away gives you 1/8th field strength. Magnetic Resonance Imaging (MRI) machines only have about 15 Tesla f ield strength, for comparison. The reason why an MRI machine is a giant contraption that needs liquid ni trogen cooling, rather than a neat little metal-plated lump that you can buy over the Internet, is twofold. It's partly because the MRI machine is also a sensitive radio receiver, detecting the radio-frequency energy emitted by the magnetic nuclei in the patient's body when they interact with a strong magnetic field. But it's mainly because a 15 Tesla MRI m achine is creating a 15 Tesla field over a large enough volume that a p atient can be stuck into said field. By the same token, junkyard car-lifting electromagnets only have about 1T field strength, but they generate that field over a big enough volume t hat their total lifting capacity, for conveniently steel-bodied cars, is massive. The coils under their protective armour draw at least a few ki lowatts, and maybe considerably more - 20kW isn't out of the question fo r a big car-lifter. You're not going to be lifting any Toyotas with a five buck magnet from a nywhere. Nails will hop up only about an inch to hit the strongest of th e magnets in the ForceField grab bags. Because of their limited field size, small neodymium super-magnets like t hese ones aren't actually much of a problem to deal with, at least as fa r as messing up your monitors and erasing your credit cards and wiping y our video tapes and being hit by flying spanners goes. Yes, when I had one in my back pocket, I at one point found myself unexpe ctedly attached to the washing machine. But the rapid diminution of the field strength means that you can hold the strongest of these magnets - the three spheres end-to-end, for instance - in your hand and wave them around a mere foot and a half from a computer monitor, and notice only s light image distortion and discolouration. Touch those same magnets directly to the screen, mind you, and they'll ma gnetise the heck out of the shadow mask and leave you degaussing until p ractically all of the world's cows have come home, had a nice sleep and gone away again. but I am not confident enough of my skill with it to deliberately Magn a-Doodle all over a monitor just so you can see what it looks like. Sorr y Quite big rare earth magnets can be had, if you want more field range. this one, for instance, which only has about 11 times the vol ume of a ping-pong ball, but which ForceField warn you not to buy unless you're confident that you know how not to crush, blind or mangle yourse lf with it. As far as terrestrial magnetic fields go, 1T is quite strong, but it ain' t much by the standards of the universe. pulsars ( which are spinning neutron stars) have magnetic fields. If they were mad e of nothing but neutrons then they wouldn't, but they've also got super conducting superfluid protons and various other exotic forms of matter, so they have. They get just...