| Electronics > Beginners |
| Negative voltage ? |
| << < (3/5) > >> |
| helius:
The reason that positive and negative voltage regulators are different parts is that they regulate the input voltage by shunting current to ground. In a LM7805, the input and output are positive with respect to ground; in a LM7905, the input and output are negative with respect to ground. The ground is a single pin that is on both the input and output sides of the regulator, which makes them non-isolated. This is the critical reason they must come in positive and negative versions, unlike "lab power supplies", which are isolated from their inputs. The better analogy to understand voltage is "repository of potential energy", similar to height for gravitational potential energy. When Galileo dropped his weights from the tower, they were forced downward because their height above the ground (above "zero") gave them potential energy that could be released. After they were lying on the ground, they could fall no farther because no potential energy above the ground remained. Yet if there were a well, they could fall down into it because moving to a negative height releases yet more potential energy. Potential energy can't be seen or measured directly, so it naturally may be negative, like a book-keeping entry. The position chosen to be called "zero" is somewhat arbitrary. With electrical fields, too, the zero is arbitrary*, or better call it a selected reference point. A charge at a region where the field is high feels a force towards regions where it is low: at the moment you connect a wire across the terminals of a 9V battery, the field is 9V higher at the (+) side compared to the (-) side. We select which point to call our "zero": we can measure with the reference point at the (+) side, in which case the (-) side is -9 volts; or we can measure with the reference point at the (-) side, in which case the (+) side is +9 volts; or we can measure with the reference point in the exact middle of the wire, in which case the (+) side is +4.5 volts and the (-) side is -4.5 volts. Only the voltage difference between two points has any effect on the movement of charges (current). * It is possible to measure the electric field absolutely using electrometers, but this type of measurement is specialized and not often required. There is an analogous "absolute zero height" in gravitation, which is far out in interstellar space: again, not often used. |
| rstofer:
--- Quote from: ebastler on February 20, 2019, 04:44:34 pm --- --- Quote from: wraper on February 20, 2019, 04:37:00 pm ---Actually it's not a proper analogy at all. --- End quote --- So -- which part of a circuit (which is in scope of the "water analogy" at all) can't be explained by a "negative pressure" model? Nothing wrong with a simple mechanical model where a pump is "sucking" at the pipe. Where does that simple model work for positive pressure, but collapse for "negative pressure" (negativ erelative to the environment)? --- End quote --- Well, it fails when you try to lift water more than about 30'. |
| Nerull:
Voltage is a measure of electrical potential. In an analog to gravitational potential, it matches well with 'height'. If I'm climbing up a cliff and stop halfway and decide I'm going to reference myself as 'zero height', than anything above me is positive height and anything below me is negative height. If you drop something from above me, it will fall and reach me. If you drop something from below me, it will not. Now, its a bit reversed in electricity because the most common charge carrier is the electron, which 'falls up' and goes from negative to positive, but the idea is the same. Electrons will flow across a potential difference, but what point you pick as 'zero' is arbitrary. Anything above it is then positive, and anything below it is negative. '0' is simply a chosen reference, like the origin in a CAD drawing. All other voltages are given relative to the chosen zero point. |
| ebastler:
--- Quote from: rstofer on February 20, 2019, 07:41:02 pm --- --- Quote from: ebastler on February 20, 2019, 04:44:34 pm ---So -- which part of a circuit (which is in scope of the "water analogy" at all) can't be explained by a "negative pressure" model? Nothing wrong with a simple mechanical model where a pump is "sucking" at the pipe. Where does that simple model work for positive pressure, but collapse for "negative pressure" (negativ erelative to the environment)? --- End quote --- Well, it fails when you try to lift water more than about 30'. --- End quote --- Look, I'm not trying to be difficult or stubborn here. I know that this "water model" for electrical circuits has limitations. But in fact, all models used in physics do. And nevertheless they are used, and are useful because -- as long as they are used within their limitations -- (a) they allow you to predict reality, and (b) they allow you to visualize things intuitively. At some point, under some boundary conditions, in some parameter range a model "breaks down" and does no longer predict reality correctly. In that regime a new, more advanced model is needed. But still, physicists (and engineers) will happily use the simplistic model in the range where it works, since it is nice and simple. So why not use the water model, and even the colloquial term "negative pressure", in the regime where it works nicely to explain voltages, currents, and resistances? As I said in earlier posts, your explanations are "more correct", i.e. they describe a model that will work in a wider range of conditions. (But not under all conditions either.) But that does not make the simpler models wrong or useless within their domain. |
| Doctorandus_P:
--- Quote from: ebastler on February 20, 2019, 08:03:57 pm ---... under some boundary conditions, in some parameter range a model "breaks down" and does no longer predict reality correctly. --- End quote --- ... and when that happens, you should stop using the analogy. Vacuum is a pretty hard limit when trying to create a negative pressure and that is where this analogy breaks down and you shout STOP USING IT and switch to something else. To OP: Though the concepts are simple, they do need time to sink in. So indeed take your time and let it sink in. In electronics there are also a significant number of parts that have a mirror image. for example NPN and PNP transistors. I used to think for quite some time that NPN transistors were Normal, and the ohters were Perverted. Very similar with N-channel and P-channel MOSfets. Inductors and Capacitors are also very much alike if you change currents and voltages. If you put a voltage over an inductor, then current will start to flow. If you put a curent through a capacitor then voltage will start to rise. (Both are quite lneairly actually untill some limits are reached). If you ever get down to raw math and differential equeations youl also start seeing analogies between mechanical and electrical systems. They look different on the outside, but on the inside the math is the same. |
| Navigation |
| Message Index |
| Next page |
| Previous page |