Ohm's Law Problem
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From: Tauno Voipio on 16 Feb 2010 16:04 Joerg wrote: >> > > That is because you were not a ham radio operator on a tight budget. We > had glass turning mildly liquid, being sucked in and ending up snug on > the plates. That was the time to turn off the rig. A few seconds more in > transmit and there'd be a loud bang. > The Haig Dimple whisky bottle looks like a 6146 or an EL 500 after a contest. Been there, tried that. -- Tauno Voipio, OH2UG (for nearly 50 years) tauno voipio (at) iki fi
From: Tauno Voipio on 16 Feb 2010 16:10 Bill Bowden wrote: > Ohm's Law Problem: > > Find the voltage at the 2 junctions of a 3 element voltage > divider across a supply voltage of 8.4 volts. The two > junctions of the divider both supply external current of 5mA. > > +8.4 > | > R1 = 240 > | > .---------> 5 mA > | > R2 = 570 > | > .---------> 5 mA > | > R3 = 100 > | > | > GND > > > A more basic problem can be solved using Thevenin's > idea with only 2 resistors of say 570 and 240 ohms > and a 6.9 volt supply, and 5mA of current from > the single junction. > > +6.9 > | > R1 =240 > | > .----------> 5 mA > | > R2 =570 > | > GND > > The output impedance at the junction is the > 2 resistors in parallel, or about 169 ohms. > The open circuit voltage ignoring the 5mA is > about 2.044 volts, and so the voltage drop > on R1 (240) is 2.044 - (169 * .005) = 1.2 volts. > > But I don't see an easy way to apply Thevenin to > the other case where there are 3 or more resistors > and junctions with known currents from the junctions. > > Any ideas? > > -Bill De Thevenin works also here: 1. Cut the circuit below the upper 5 mA drain. The power supply and the top resistor with the drain form a new supply with an EMF of 8.4 V - 5 mA * 240 Ohms. 2. Perform the same operation with the lower drain. The EMF of step 1 will be dropped by 5 mA to 240 + 570 Ohms, and the internal resistance of the new supply is 240 + 570 Ohms. 3. Connect the bottom resistor to the supply of step 2 and solve your result. The current drains have been marked as constant-current elements, so their infinite impedance does not enter the impedances. -- Tauno Voipio tauno voipio (at) iki fi
From: Joerg on 16 Feb 2010 17:41 Tauno Voipio wrote: > Joerg wrote: >>> >> >> That is because you were not a ham radio operator on a tight budget. >> We had glass turning mildly liquid, being sucked in and ending up snug >> on the plates. That was the time to turn off the rig. A few seconds >> more in transmit and there'd be a loud bang. >> > > > The Haig Dimple whisky bottle looks like a 6146 or an EL 500 > after a contest. > > Been there, tried that. > Wow, I've never made a 6146 suck in its glass. I had the well past cherry-red though. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Jon Kirwan on 16 Feb 2010 18:23 On Tue, 16 Feb 2010 09:10:06 -0800, Joerg <invalid(a)invalid.invalid> wrote: >Jon Kirwan wrote: >> On Mon, 15 Feb 2010 13:29:30 -0800, Joerg >> <invalid(a)invalid.invalid> wrote: >> >>> <snip> >>> It's much more important to >>> experiment, experiment, experiment, get a "feel" for what works, _then_ >>> dive into the theory. Not the other way around. Just my 2 cents. >> >> It's hard for most of us to get a feel for what works without >> first having some idea of what to expect. Theory is primary >> to interpreting and understanding experimental result. >> >> What isn't known through theory defines the word 'random.' > >That would be the professor's thought process. To us back then things we >didn't understand were not random at all. For example, you simply "knew" >that the Q of a power matching network had to be at least 10 or you'd >get into EMI troubles. Or that grid-bias tube stages were way more >stable by nature. Or that certain modes of operation in a transistor >could lead to a phssst ... *BANG* (later I learned about the concept of >a SOA), and so on. I think you missed my meaning. It's not that theory must exist for the observation itself. It's that theory is primary to interpreting and understanding experimental results. I've used the concept of a small-angle pendulum swing here, before. I added then the point about "observing"; that the period itself, when better timing becomes available to notice (itself, depending upon prior theory), is not well-enough predicted. That's an example of interpreting and understanding experimental results. What remains (let's say, the idea that the diameter of the hole and the different diameter of the pin must be accounted in some way) is consistent -- at that moment -- with the idea of 'random.' The values may then be subjected to _other_ theory, the idea of Poisson events and its integral (Gaussian distribution) to see if the distribution has a skew to it for which some rule of thumb might then be applied, though otherwise ignorantly until better theory is developed. No one is prevented by what I wrote from accounting for an issue by making up rules that "seem to work until something better comes along." A bias, or skew, or some other element can certainly be found. But take note, using _other_ theory to do so and inform that choice. Your example takes place amidst a great deal of existing theory of one kind or another, as well. All of which helps to inform some rule of thumb, as well. Which makes the point well. To strip away all of the veneer of cultural training, imagine a neanderthal posed along a mountain ridge, looking into the distance at a slightly curving horizon. Without any concept of "sphere" in mind. With that theory firmly held, _we_ are _able_ to "observe" that curve and adduce it to a spherical Earth 'theory.' We can "see" it. However, the neanderthal probably wouldn't have any way of even noticing the effect. I might argue that theory not only informs observation, in many regards the lack of it prevents such observation from even taking place. Now, let's add a "theory of a flat world surface" to this neanderthal's mind. Now, the neanderthal _can_ observe the curve, because it _differs_ from existing theory. And might even be able to formulate some rule of thumb about the deviation observed vs relative height on a mountain, perhaps. But again, observation through theory. >If it was all random we'd have had much more failing parts and homebrew >devices, but we didn't. Well, I hope the point I was making is clearer. Or not. ;) Jon
From: Joerg on 16 Feb 2010 19:43
Jon Kirwan wrote: > On Tue, 16 Feb 2010 09:10:06 -0800, Joerg > <invalid(a)invalid.invalid> wrote: > >> Jon Kirwan wrote: >>> On Mon, 15 Feb 2010 13:29:30 -0800, Joerg >>> <invalid(a)invalid.invalid> wrote: >>> >>>> <snip> >>>> It's much more important to >>>> experiment, experiment, experiment, get a "feel" for what works, _then_ >>>> dive into the theory. Not the other way around. Just my 2 cents. >>> It's hard for most of us to get a feel for what works without >>> first having some idea of what to expect. Theory is primary >>> to interpreting and understanding experimental result. >>> >>> What isn't known through theory defines the word 'random.' >> That would be the professor's thought process. To us back then things we >> didn't understand were not random at all. For example, you simply "knew" >> that the Q of a power matching network had to be at least 10 or you'd >> get into EMI troubles. Or that grid-bias tube stages were way more >> stable by nature. Or that certain modes of operation in a transistor >> could lead to a phssst ... *BANG* (later I learned about the concept of >> a SOA), and so on. > > I think you missed my meaning. It's not that theory must > exist for the observation itself. It's that theory is > primary to interpreting and understanding experimental > results. > > I've used the concept of a small-angle pendulum swing here, > before. I added then the point about "observing"; that the > period itself, when better timing becomes available to notice > (itself, depending upon prior theory), is not well-enough > predicted. That's an example of interpreting and > understanding experimental results. > > What remains (let's say, the idea that the diameter of the > hole and the different diameter of the pin must be accounted > in some way) is consistent -- at that moment -- with the idea > of 'random.' The values may then be subjected to _other_ > theory, the idea of Poisson events and its integral (Gaussian > distribution) to see if the distribution has a skew to it for > which some rule of thumb might then be applied, though > otherwise ignorantly until better theory is developed. No > one is prevented by what I wrote from accounting for an issue > by making up rules that "seem to work until something better > comes along." A bias, or skew, or some other element can > certainly be found. > > But take note, using _other_ theory to do so and inform that > choice. > Ok, but: Lets say the guy with the pendulum never really had a chance to visit a school, very common in medieval days. But he wants to make clocks. Good clocks. So he might decide to conduct numerous long term experiments to see which pendulum works best without actually fully understanding why it does. IOW, not having a grasp on the theory does not preclude him from making clocks of superb quality and consequently a nice chunk of money. > Your example takes place amidst a great deal of existing > theory of one kind or another, as well. All of which helps > to inform some rule of thumb, as well. Which makes the point > well. > > To strip away all of the veneer of cultural training, imagine > a neanderthal posed along a mountain ridge, looking into the > distance at a slightly curving horizon. Without any concept > of "sphere" in mind. With that theory firmly held, _we_ are > _able_ to "observe" that curve and adduce it to a spherical > Earth 'theory.' We can "see" it. However, the neanderthal > probably wouldn't have any way of even noticing the effect. > Or he does notice, starts walking up to the curved horizon and the dang thang keeps moving away from him. Hence, the horizon must be afraid of him :-) > I might argue that theory not only informs observation, in > many regards the lack of it prevents such observation from > even taking place. > True. > Now, let's add a "theory of a flat world surface" to this > neanderthal's mind. Now, the neanderthal _can_ observe the > curve, because it _differs_ from existing theory. And might > even be able to formulate some rule of thumb about the > deviation observed vs relative height on a mountain, perhaps. > > But again, observation through theory. > Or he might decide "Ah, what the heck, let's get something to eat" :-) >> If it was all random we'd have had much more failing parts and homebrew >> devices, but we didn't. > > Well, I hope the point I was making is clearer. Or not. ;) > There are two ways to arrive at good product. Theoretical and empirical, or a combination thereof. I've heard many old masters respond to questions "Well, that's just how it is" and these guys were real masters in their profession. I grew up in medical ultrasound. Back in the 80's a lot of stuff around PZT-based transducers was not understood. Yet the front end parts and transducers of the old machines are not that much different in performance from the ones today, where we have oodles of computer horsepower to simulate and calculate just about anything. If we had said "Oh shoot, we don't understand the theory so let's not build but keep studying until we do" a lot of people in hospitals would have needlessly died. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |