Actually, I would go a tad further and say that none of the three exists without the other two. >> - You can have V without I How would you measure a voltage without having any current flowing -- i.e. without relocating some charge carriers? If you can't, how can you "have" it? In order to have a voltage without a current, you would have to postulate a resistance with the value "infinity" -- something possible in theory, but not really in practice. (At least nobody's seen one so far...) And then /knowing/ that it is there definitely would require you to take out /some/ charge carriers -- and bingo, there's the current you don't want to have. >> - You can have R without I Again, how can you measure a resistance without moving any current (charge carriers) through the resistance? Keep in mind that the concept of a fixed resistance (independent of anything else) is a simplification, helping in quick and dirty engineering, but is not correct. A resistance is only the proportional constant between the current and the voltage. There is no resistance without a current (and a voltage), as the resistance is only defined by the relationship between those two. A 5% carbon film resistor without a voltage applied to it is not an electrical resistance. Only after applying a voltage and a current, it becomes a resistance of a certain value (which is more or less close to the one that it's supposed to have, depending on the applied voltage and current, among other variables). >> - You CAN'T have I without V and R Now this one is the only one that's correct. BTW, there's also conductivity: I = G * V :) (Just another form to express the proportion between current and voltage.) > Hey don't feel bad at all. It's probably best that you get this confusion > cleared up right now or it will haunt your circuits forever. What you are > saying is still basically incorrect though. Voltage can (sort of) exist > without current, but not without resistance. When you're reading a voltage > off two pins, remember that they are effectively separated by an infinite > resistance of air. Without this resistance, there could be no voltage. In > Ohm's law, if R=infinity, then current drops to zero. Resistance can exist > without either current or voltage (as in a resistor sitting there on your > desk), but its arguable whether its actually resistance unless it is > resisting something. Without a current to resist it's no better than a clod > of dirt or something. Keep in mind exactly what current is... It's the > flow of electrons, electricity in it's most fundamental form. Current > exists in all electrical circuits. Without electrons flowing to the battery > terminals, there would be no voltage on them. Without current flowing > through a resistor, there would be no voltage. Current, in its most basic > form, can (sort of) exist without any voltage. All you need is some > organized flow of electrons through a negligible resistance. If you haven't > considered the following analogy to fluid dynamics it might help your > understanding. > > Think of voltage as a pressure in a system of pipes, current as the amount > of water flowing, and resistance as a necking down of the pipe. Large > resistance equate to large pipe-area reductions. It should now become clear > that with an infinite resistance the pressure (voltage) assumes some > constant state. For a pressure/voltage drop to occur there must be some > resistance, since if there is no resistance the water/current will just flow > freely. It would be silly to say that there is some pressure in an empty > pipe, and likewise it would be silly to say that there is some voltage > without a current backing it. Voltage is more the willingness of a current > to overcome resistance than a measure of electricity. Every day you incur > thousands of volts upon your body just walking around on the carpet, but > obviously this does not really do anything (besides zap that expensive new > chip you're working on.) > > I'm sure there's a better rendition of the analogy out there somewhere since > I just pulled this out of my arse. Search on google if you think it will > help. > > Jeeze I hope that helped some, surely there's gotta be something in there > that sparks a lightbulb? > > ----- Original Message ----- > From: "Rick Regan" > To: > Sent: Friday, April 16, 2004 7:47 PM > Subject: Re: [EE]: Why isn't Ohm's Law written I=V/R? > >>> No. Like I said before, if your power supply contains bipolar >>> transistors, then it's all BASED on current, since bipolar transistors >>> are current-controlling devices. >>> >>> There's this big transistor on the back, probably on a heat sink. The >>> power supply allows the base current of that transistor to rise >>> (increasing the collector current as well, but the beta factor) until >>> it senses that a reference voltage has reached a correct value. In CV >>> mode, the reference voltage is just the output voltage In CC mode, the >>> reference voltage is the voltage across some current-sensing resistor. >>> Your linear regulators (7805, etc) work the same way. The internal >>> components are controlling current, rather than voltage directly. >>> >>> A transistor is a fine example of a device that permits current to be >>> controlled directly. Bipolar transistors are current-controlled, FETs >>> are voltage controlled, but in each case it's the current that is being >>> manipulated. (Come to think of it, tubes work this way too...) >> >> It's probably my fault for not being clear enough >> about what I was asking, so I will make one more attempt >> (I promise - this is my last post!). >> >> I am not saying the algebra is wrong. I am not saying >> that at any given time it's not convenient to put any one >> of the three variables on the left hand side of the >> equation. I was just trying to understand how current >> could be an entity in its own. This was my understanding >> and I guess you're saying I'm wrong: >> >> - You can have V without I >> - You can have R without I >> - You CAN'T have I without V and R >> >> Thanks for the responses. (I will now unsubscribe from the [EE] tags and > stick with [PIC] ;)). -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics