In the past month or two I've been teaching myself about electronics from the ground up with various textbooks (mainly Grob's Basic Electronics). But I'm stumped when it comes to potentiometers/voltage dividers. The concept of reducing voltage, especially through resistors, doesn't make any sense to me. I was under the impression that resistance only affects current, not voltage. But apparently pots are the exception to this rule... I just don't understand why.
Help!
Help!
If you have a resistance and the current changes then the voltage must change too. Its just ohms law.
I understand the mathematics, but the actual process makes no real sense to me. I can predict what a potentiometer will do in a circuit, but no idea how it does it. How in the world is voltage variated?
The wiper of the potentiometer can be set anywhere along its length. The entire voltage is across the potentiometer, so the wiper can be set to tap off a particular voltage.
Maybe imagining the potentiometer as two separate resistors, and the comparative values can be adjusted, with the wiper connecting to the node between the resistors. If the resistor values are equal, the voltage at the node is V/2; if the upper resistor is twice the value of the lower one the voltage at the node is V/3. If the upper resistor is half that of the lower the voltage is V/0.67. The voltage isn't "variated," only the point referenced by the wiper is changed.
With a rheostat, the current is actually changed.
Maybe imagining the potentiometer as two separate resistors, and the comparative values can be adjusted, with the wiper connecting to the node between the resistors. If the resistor values are equal, the voltage at the node is V/2; if the upper resistor is twice the value of the lower one the voltage at the node is V/3. If the upper resistor is half that of the lower the voltage is V/0.67. The voltage isn't "variated," only the point referenced by the wiper is changed.
With a rheostat, the current is actually changed.
Aha! I was listening to Yes while drawing some diagrams off the top of my head, and suddenly it hit me like a rock. A potentiometer is a gate for a separate pathway. The "main" pathway is independent of the alternate pot pathway, but the pot pathway is dependent on the main pathway's current.
If a primary voltage source is providing 100V, and there's a pot with a resistance of 10 ohms, the current is therefore 10 amps. When the wiper is placed directly in the middle of the resistor, it creates a potential difference of 50V between pot's first and second terminal, because the current stays the same!
After really thinking about it and actually working with it on paper, it's fairly obvious. The current can't change in this context, and there's a variable amount of resistance. The voltage of the electrical source is irrelevant, which is what I was having trouble grasping.
If a primary voltage source is providing 100V, and there's a pot with a resistance of 10 ohms, the current is therefore 10 amps. When the wiper is placed directly in the middle of the resistor, it creates a potential difference of 50V between pot's first and second terminal, because the current stays the same!
After really thinking about it and actually working with it on paper, it's fairly obvious. The current can't change in this context, and there's a variable amount of resistance. The voltage of the electrical source is irrelevant, which is what I was having trouble grasping.
I think your confusion arose because you only thought of a resistor as something which when given a voltage creates a current. A resistor is also something which when given a current creates a voltage. These two views, both of which must be kept in mind, are actually the two opposite ends of a complete continuum.
When two resistors are in series (as in a potential divider) then neither of them is at the end of the continuum. They each help set the voltage and current for the other one. The net result has to be what Ohm's Law tells us.
When two resistors are in series (as in a potential divider) then neither of them is at the end of the continuum. They each help set the voltage and current for the other one. The net result has to be what Ohm's Law tells us.
Imo, it is this kind of "understanding" that tend to lead to confusion. Ohm law is very simple: V=IR, but it is very surprising that people understanding of this can sometimes be very low, even among EE. This is very true when students with "theory" and limited practical experience have to deal with the concept in real world.
V=IR is Math. It means that I=V/R and that R=V/I. Pretty easy when no conceptual thinking is involved. But try to ask whether 1V can kill or not.
I prefer to look at the Ohm law as "V causing I". I think that is the basics regarding energy, electron movement, etc. Of course, one must have the basics knowledge of Physics, which might have been forgotten by non technical students.
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I'm sure you have, many times.
There are severe penalties for breaking Ohm's Law, ranging from "flash, bang, ouch" up to the death penalty.Understanding, when correct and complete, is extremely useful. It can lead to places that people just plugging numbers into formulas will never notice. It can be useful to remember that Maxwell's equations do not specify cause and effect, but merely equate things. Similarly for Ohm's Law. If you force a current through a resistor then you will find that it develops a potential difference. You may wish to think of this as electrons getting blocked in their progress and so building up at one end.Jay said:
I think my biggest pitfall was not really understanding what voltage really is. From what I now understand (and correct me if I'm wrong), voltage is the amount of force it takes to move a current through a resistive medium. If a (totally theoretical) circuit were to have absolutely no resistance, then the electrons would flow from negative to positive with no "help". When resistance is added to the circuit, suddenly the current faces opposition, and the current needs to exert more force to get through the resistor. This reduces the flow but increases the pressure. Voltage is essentially pressure, current is essentially flow, and resistance is essentially... well, resistance. And all of it is really the same big concept, broken down into smaller concepts to help us understand what's going on.
My understanding is that even though it's sometimes called EMF (electromotive force), voltage isn't really a force. It's only a potential. I'm far from understanding all the physics, but my analogy would be to pick up a rock and hold it in your outstretched hand, at shoulder height. Neither the rock, ground, or gravity has changed, only the relative positions. The energy used in lifting the rock is now kinetic energy, and potential for work, available by dropping the rock. And in much the same way, it's the position of the electrons that creates a potential for work, the voltage.
When thinking of the 'fluid in a pipe' analogy voltage is analogous to pressure. The actual force trying to accelerate an electron is not the voltage, but the rate of change of voltage with position. Its like the difference between height and slope of a hill. Voltage is like height. Electric field (which gives the force) is like slope.
Good analogies can get you started, but eventually they can lead you astray, so remember that they are just analogies.
Good analogies can get you started, but eventually they can lead you astray, so remember that they are just analogies.
Yeah, I realize now that voltage is just a measure of difference.
I came up with a way to express Ohm's Law as I understand it:
Voltage is the amount of work it takes to move charge from point A to point B.
Current is the speed at which charge moves.
Resistance is opposition to current.
Therefore:
Greater current requires greater voltage and/or less resistance.
I came up with a way to express Ohm's Law as I understand it:
Voltage is the amount of work it takes to move charge from point A to point B.
Current is the speed at which charge moves.
Resistance is opposition to current.
Therefore:
Greater current requires greater voltage and/or less resistance.
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