Peter T. Daniels ran out of electrical outlets; states rewiring entire house to be the only feasible solution.
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From: Hatunen on 31 Mar 2010 18:05 On Wed, 31 Mar 2010 13:07:06 -0400, Tak To <takto(a)alum.mit.eduxx> wrote: >Btw, I find the arrangement of the typical US high school >curriculum, in which biology ("life science"), chemistry >and physics were divided into separate years of study >totally insane. Why not study a little bit of everything >each year? Ditto for the division of mathematics courses. >An entire year of "Geometry" sandwiched between "Algebra >I" and "Algebra II"? Absurd! It's hard to learn a difficult subject a little bit at a time. Physics and math both require a sort of continuity as the concepts develop. -- ************* DAVE HATUNEN (hatunen(a)cox.net) ************* * Tucson Arizona, out where the cacti grow * * My typos & mispellings are intentional copyright traps *
From: PaulJK on 1 Apr 2010 01:38 Adam Funk wrote: > On 2010-03-30, Hatunen wrote: > >> On Mon, 29 Mar 2010 19:40:33 +0100, Adam Funk >> <a24061(a)ducksburg.com> wrote: >> >>> On 2010-03-29, Doctroid wrote: >>> >>>> In article <ruOdna7WF6m71C3WnZ2dnUVZ7o5i4p2d(a)westnet.com.au>, >>>> Peter Moylan <gro.nalyomp(a)retep> wrote: >>> >>>>> It's true that a great many resistors are nearly linear, i.e. they can >>>>> be said to obey Ohm's law to a reasonable degree of accuracy, provided >>>>> that you don't go beyond their design limits. >>> >>> Well, another way to look at it is that the resistance changes in >>> various situations, such as, erm, higher voltages. >> >> The most familiar example is the common incandescent lamp, which >> has a low resistance when first turned on. See >> http://www.sylvaniaautocatalog.com/sylvania/imgs/resistance_vs_temp.gif > > AIUI, this is part of the reason (the low initial resistance > aggravates the thermal shock) that lamps generally blow when switched > on from cold --- right? The resistance is often not quite evenly distributed along the length of the wolfram wire. Initially, the overall resistance is low so the initial split second current is many times higher then the current flowing through the bulb during the normal steady working state. Every small section of wire with slightly higher resistance will cause higher voltage gradient along its length and consequently higher power generation and heat increase. Because the wire's resistance increases sharply with temperature, it results in significantly sharper increase of resistance in the short hoter section(s) of the wire, which in turn..and so on...you get the idea. Eventually, the rest of the wire length heats up and the current is reduced to normal working condition. Whenever the light is switched on, each slightly higher-resistance section of the wolfram wire undergoes extra stress of momentary overheating. Every such time these sections loose a few molecules of metal from their surface which usually settles on the inner surface of the glass bulb. This loss of metal makes the stressed wire section even more resistant and eventually we reach the stage when during one switch-on its temperature rises too far before the rest of the wire warms up to reduce the current and take its share of the load and the stressed section burns with a spectacular flash. pjk
From: PaulJK on 1 Apr 2010 01:51 Otto Bahn wrote: > "PaulJK" <paul.kriha(a)paradise.net.nz> wrote i >>>>>>>>>> A homework for Doctroid and P.Moylan >>>>>>>>>> http://www.allaboutcircuits.com/worksheets/diode1.html >>>>>>>>>> >>>>>>>>>> just trying to help you to stop embarrassing yourself with >>>>>>>>>> high school physics. >>>>>>>>> >>>>>>>>> P'raps you should ask Doctroid what he is a doctroid of. >>>>>>>>> >>>>>>>>> BW >>>>>>>> >>>>>>>> See, the thing about high school physics is, it's taught by high >>>>>>>> school >>>>>>>> teachers out of high school textbooks to high school students. >>>>>>>> >>>>>>>> High school physics teachers usually do not have advanced training >>>>>>>> in >>>>>>>> physics. Often neither do the authors of high school physics >>>>>>>> textbooks. >>>>>>>> Even when they do, they recognize the necessity of presenting a >>>>>>>> simplified picture to high school students, who are not ready for >>>>>>>> differential equations, surface integrals, and deep questions about >>>>>>>> the >>>>>>>> meaning of physical law. >>>>>>>> >>>>>>>> So what gets taught is "Ohm's law is V = IR". The ones who go on to >>>>>>>> earn degrees in physics are taught there's more to it than that. >>>>>>>> (And >>>>>>>> some of them learn it.) The ones who major in English Lit or >>>>>>>> Business >>>>>>>> or Electrical Engineering? Might not. >>>>>>> >>>>>>> Since R=V/I is a definition, care to elaborate on when the >>>>>>> relationship >>>>>>> does not hold? >>>>>>> >>>>>>> Your Nobel awaits. >>>>>> >>>>>> I refuse to enter into further public discussion with people with no >>>>>> reading comprehension skills. As before, you can write me at rsholmes >>>>>> dot physics dot syr dot edu, if you are genuinely interested in >>>>>> learning. >>>>>> >>>>> >>>>> From Fundementals of Physics by Halliday and Resnick: >>>>> >>>>> "The relationship V = i/R remains as the derfinition of the resistance >>>>> of >>>>> a conductor whether or not the conductor obeys Ohm's law." >>>> >>>> So the conductor might not obey Ohm's law. Some of these mokes have >>>> been claiming that everything always obeys Ohm's law. >>> >>> Just because there's a few misbehaving conductors doesn't mean >>> the rest of us shouldn't obey Ohm's law. >> >> But it applies even to the conductor of Boston Philharmonic. > > If you are altnernating the conductors, then you have to call > it impedence. Alternator alternates conductors. The one that doesn't alternate is Dynamo. Potential is measured in promises, Current in years, Capacity in seats, and Frequency in performances per season. pjk
From: PaulJK on 1 Apr 2010 02:10 Otto Bahn wrote: > "Adam Funk" <a24061(a)ducksburg.com> wrote > >>>>>> It's true that a great many resistors are nearly linear, i.e. they can >>>>>> be said to obey Ohm's law to a reasonable degree of accuracy, provided >>>>>> that you don't go beyond their design limits. >>>> >>>> Well, another way to look at it is that the resistance changes in >>>> various situations, such as, erm, higher voltages. >>> >>> The most familiar example is the common incandescent lamp, which >>> has a low resistance when first turned on. See >>> http://www.sylvaniaautocatalog.com/sylvania/imgs/resistance_vs_temp.gif >> >> AIUI, this is part of the reason (the low initial resistance >> aggravates the thermal shock) that lamps generally blow when switched >> on from cold --- right? > > Yup. If you always warm up you light bulbs before turning > them on, they last longer, but it's cheaper just to buy a > new bulb now and then. :-) I am not quite sure what kind of device you'd use to warm the bulb's filament to at least 1200°C. It might be cheaper to use a variable resistor to increase the power in about a quarter of a second, or use the commercially available light dimmers. It's still cheaper to change each 50¢ - $1.00 bulb once every few years. pjk
From: PaulJK on 1 Apr 2010 02:16
Otto Bahn wrote: > "PaulJK" <paul.kriha(a)paradise.net.nz> wrote > >>>>>>>>> Resistance is the opposition offered by a body or substance to >>>>>>>>> the passage through it of an electric current. >>>>>>>> >>>>>>>> Quantitative definition, please, not just a description. Here >>>>>>>> is a 1N4002 diode: How would you determine its resistance? >>>>>>> >>>>>>> Slowly increase the voltage across it until it goes "pop". >>>>>>> It's resistance is now pretty much infinite for any value >>>>>>> of voltage you're likely to apply. >>>>>> >>>>>> "Pretty much infinite". Is that an engineering term? >>>>> >>>>> Yes, when we don't feel like calculating at what voltage arcing >>>>> across the gap might occur. >>>> >>>> There's no gap in a semiconductor diode.... >>> >>> Even after it's gone "pop"? >> >> No, there's usually no gap. >> The semiconductor diods I used to torture to death >> usually exhibited near zero resistance in both directions >> after they've gone "pop". Of course the "pop" was usually >> accompanied by a more spectacular and loud "kha-boom" >> from the nearby fuses. > > The red one's I popped most definitely had a visible gap > inside the plastic and did not conduct electricity. Who is the red one? |