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What the heck is voltage, anyway?
Posted by
CharlieWorton
on 09 Jun, 2017 04:02
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Hi, all... I'm enrolled in a distance education electronics course, and I'm right back at the starting gate.
Amps, I've got figured out. 1 coulomb of electrons is 6.24*10^18 electrons. That many electrons moving past a point in one second is one amp. So I think I've got that one sorted. Similarly, Ohms is resistance... the more ohms, the more resistance, the fewer electrons make it through.
But volts... that one just isn't jelling for me. My texts are telling me that it is 'energy' or 'potential'. But that's not helping me understand it. If electrons all have the same energy, then the only way to increase the rate of energy delivery is to increase the number of electrons moving past a point... which means raising the amps. I'm kinda visualizing the finish line at a racetrack. As a few electrons pass the finish line, we have a low amp rate. As more electrons pass the line, we have a higher amp rate. And really, that ought to be the end of the story. The only parameters there are amps - the number of electrons passing the finish line - and resistance, which I guess would be the width of the track.
I don't see how volts even figure into that picture.
Can anyone help me get a clearer picture of this?
Thanks - Charlie
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#1 Reply
Posted by
IanB
on 09 Jun, 2017 04:23
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Voltage is pressure. Force. Tension.
In the old days, people called voltage "electrical pressure". High voltage supplies were called "high tension supplies".
The electrons don't move unless you force them, push them along. Voltage is what does the pushing.
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#2 Reply
Posted by
rstofer
on 09 Jun, 2017 04:27
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I'm not fond of the water analogy and it breaks down pretty quick but here it is:
Volts is the pressure kicking the electrons through the resistance. Like water pressure in that, for a given hose length and diameter (resistance), the only way to get more water out is to increase the pressure (volts).
But volts has to be a potential difference, not some absolute value. If you consider a hose coming down the mountain, the pressure at the bottom is related to the difference in the water level in the hose. But it is also the pressure difference taken at a couple of points somewhere along the hose, not necessarily the top and bottom.
So, for now, just consider voltage to be a potential difference between any two arbitrary points and move along.
Ohm's Law really tells you everything you need to know: Voltage = Current * Resistance : E=I*R
ETA
Siince you have no problem visualizing current flow then rearrange the equation as Current = Voltage / Resistance. As the voltage increases, the current increases. As the resistance decreases, current increases. Both of these make sense intuitively.
But the trick is to remember that voltage is always a DIFFERENCE in potential between two points. This will be important when you get to Kirchoff's Laws.
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#3 Reply
Posted by
Sweeney
on 09 Jun, 2017 04:34
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I like the analogy.
The question is how would you "just raise the amps" in your picture? Or in other words: Why would the electrons move in the first place and not just be stationary? To go with your picture, it's the voltage that's motivating the electrons to move in the first place. If you wanted for more electrons to cross the finish line in your analogy, you could either widen the track (reduce resistance) or push the little electrons harder (increase the voltage).
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#4 Reply
Posted by
IanB
on 09 Jun, 2017 04:43
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The question is how would you "just raise the amps" in your picture? Or in other words: Why would the electrons move in the first place and not just be stationary? To go with your picture, it's the voltage that's motivating the electrons to move in the first place. If you wanted for more electrons to cross the finish line in your analogy, you could either widen the track (reduce resistance) or push the little electrons harder (increase the voltage).
This is basically correct.
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#5 Reply
Posted by
helius
on 09 Jun, 2017 04:46
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Some particles have an inherent charge. Electrons and quarks are among those fundamental particles which have charge. The explanation for why is a subject for graduate level physics (supersymmetry and/or string theory).
All charged particles interact through what is called the electric field (E). In physical terms, they are surrounded by a sea of "virtual photons" which carry force to and from them in accordance with this field. Photons are the force-carriers of the electric field.
The electric field is also influenced by moving magnetic fields (the Maxwell-Faraday equation). These fields exist everywhere in space; they are not objects that can be reasoned about mechanistically, but properties of the space that fills the universe. Sometimes this has been called "spooky action at a distance" since no physical contact is involved.
Voltage is just the electric field between two points. One of the laws of physics is that any path from one point in the electric field to a second point represents an equal change in electric potential (the energy gained or lost by an object of a certain charge). This change in potential is called the voltage between the two points.
Potential can be thought of as height. In the gravitational field, there is a kind of "ground height" that is defined locally as height of the ground, or can be defined globally as the mean sea level. Raising objects requires adding to their gravitational potential energy, and dropping them releases that energy. If you raise a weight in one location, move it horizontally, and then drop it somewhere else, you effectively move energy from the first location to the second. The energy is being stored as gravitational potential energy in the weight. With a weight with twice the mass, twice as much energy is being moved: we could take the mass out and talk about the "gravitational potential" as the difference in the gravitational field between different heights. This is related to the strength of the field: near the surface it is ~ 9.81 m2/s2. At high altitudes, the strength is less, until interplanetary space where it becomes very small.
The situation with charges is equivalent. When a charge is moved into an electric field of the same polarity, its electric potential energy is increased by an amount equal to the magnitude of charge times the voltage between the two points: it is like being "raised" to an "electric height". In a single alkaline cell, the voltage between the terminals is 1.5 V, so a single electron entering the + side and coming out the - side acquires a potential energy of 1.5 eV. This is potential energy entirely caused by the charge's position in the E field, it has nothing to do with velocity. Conductors have low resistance, which means that they are like ideal equipotential surfaces: the electric field is the same throughout the conductor. So we can in effect extend the battery terminals with wires to any length we like. Now all the electrons in the wire connected to the - terminal have the same potential energy relative to the + side: 1.5 eV. If we connect a resistor between the wires, the E field will be 1.5 V across the resistor. Electrons will move through the resistor while losing their potential energy (in this case it is transformed into heat). The resistance limits the rate at which the electrons can enter the resistor, and a very high resistance will dissipate heat at a low power, while a low resistance will heat up as it releases heat quickly. Physically the resistor can be made by winding a wire around a bobbin: the longer the length of wire, the greater the resistance since the electrons have a longer path to follow. There are other ways of making resistors, but it doesn't change this description.
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#6 Reply
Posted by
IanB
on 09 Jun, 2017 04:52
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I'm not sure that such a long and technical explanation will help much.
Electronics is a branch of physics. Physics is where you deal with force, energy, charge, fields and so on.
If you try to study electronics at a theoretical level without being well versed in physics you will be totally lost.
Therefore if you do not have that education in physics, you either need to take some physics courses, or you need to stick with simple analogies.
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#7 Reply
Posted by
forrestc
on 09 Jun, 2017 05:55
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Amps, I've got figured out. 1 coulomb of electrons is 6.24*10^18 electrons. That many electrons moving past a point in one second is one amp. So I think I've got that one sorted. Similarly, Ohms is resistance... the more ohms, the more resistance, the fewer electrons make it through.
Let me see if I can try a different approach to what everyone else has used.
It's probably useful to introduce ohms law at this point:
E=I*R
So I is current or as you stated.. how many electrons which pass a given point.
R is resistance, or how difficult it is to push electrons around a circuit.
E is voltage. Or EMF... Electro-Motive-Force. Volts just happens to be the unit.
If you think of it this way:
Current is the actual movement of electrons.
Resistance is how hard it is to move the electrons.
Voltage is how much force is being applied to the electrons to move them.
Then things logically come out of this:
For a circuit with a given resistance, increasing the voltage will result in more electrons moving or more current, since there is more force being applied to the electrons to force them "through" the resistance.
For a circuit with a given voltage, reducing the resistance will increase the current since there is less opposition to the electrons moving. With the same amount of force being applied you logically will push more electrons.
Another way to think about this that just occured to me is if you think about pushing a car up a hill..... (or other rolling object),
Volts is how hard you push the car - think: more people and/or pushing harder would push the car faster.
Resistance is how much brake you apply - the harder you push the brake down the more difficult it is to push the car.
Current would be how far you move the car in a certain amount of time.
(and so you think about it when you get there: Watts is a combination of how hard you push the car and how far you move it - just multiply the two).
You could also think about this with a rock (the bigger rock the more resistance, how hard you push the rock is voltage, and how far the rock moves is current) or similar.
One other free resource that may be useful for you:
https://www.khanacademy.org/science/electrical-engineering (And I just looked and they use yet another way to try to describe voltage in an intuitive manner - it may work for you instead).
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#8 Reply
Posted by
EEVblog
on 09 Jun, 2017 06:23
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Voltage is a potential difference between two points.
Take the example of a tube with a ball bearing in it. Put this tube flat on the ground so there is no height difference between the ends.
Does the ball move either direction? No, it stays still. Why? Because there is no potential (height) difference between one end and the other.
Think of the height difference as the voltage that can cause the ball (current) to move.
Having the tube flat is the same as having 0 volts at the end of each tube.
Or 10 volts at the end of each tube, it doesn't matter, it's the difference in electrical potential (voltage) that allows current to flow based on the resistance across it (ohms law)
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#9 Reply
Posted by
MrAl
on 09 Jun, 2017 06:39
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Hi,
There is a difference between understanding the basic nature of something and just being able to use the fact that it has an effect on something else.
Imagine that you did not know what "wind" (the movement of air) was. You're sitting on the porch and you see your flag start to move, it starts to move in a certain way because the wind picks up, but you dont know what wind is and you do not have the resources to find out, but you do see the flag move. What could you do to understand wind, without any other input?
One thing you could do is measure how much the flag moves and in what direction, then make conclusions about what this new thing called 'wind' is doing when the flag moves. You could then understand this thing 'wind' just by knowing how the flag moves, even though you dont really know what wind itself is in an intrinsic way. You do know the effects it has on the flag though and that tells you a lot.
After your study of the flag (and not the wind itself) you might get a call from a friend, and they might ask you to come over and bring your flag so they could measure the wind in their back yard. You take your flag with you, then tell your friend they can buy a flag and do the same thing themselves. They do that, then they can measure the wind any time they want to know how it is behaving, even though they dont know what wind actually is yet either.
So we dont have to know exactly what voltage 'is' but we can measure it's effects on other objects anyway, and use that for all sorts of useful things. The story of voltage starts with a "point charge". We can draw conclusions about how this acts when we introduce another "test charge", where the test charge acts like the flag. We thus understand how things act even if we dont know what they are.
If you know how voltage 'acts' in various circuits, you'll know a lot about electronics even if you dont know exactly what voltage as an object actually is itself. This is the way a lot of physics works, it's based on experiments and conclusions about what causes the actions that take place. Since charges either attract or repel, when various concentrations of charge come near to each other they will cause some charge to move and that of course is current. So voltage itself as we usually know it is the accumulation of charges of differing concentrations in different local regions.
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#10 Reply
Posted by
FrankBuss
on 09 Jun, 2017 07:00
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For an intuitive understanding, It makes all sense in the context of ohm's law, which can be visualized like this:
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#11 Reply
Posted by
ggchab
on 09 Jun, 2017 08:13
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I love this drawing
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#12 Reply
Posted by
Brumby
on 09 Jun, 2017 08:36
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If electrons all have the same energy, then the only way to increase the rate of energy delivery is to increase the number of electrons moving past a point... which means raising the amps.
You are so close with your understanding at this point - but you missed one critical factor ...
Why would the electrons move at all? .... and, if so, how strongly and in which direction?
You need something to push them along - and that's where the volts come in. Voltage is electrical pressure. The higher the volts, the more the pressure - which is
how you raise the amps.
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#13 Reply
Posted by
kalel
on 09 Jun, 2017 08:51
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For an intuitive understanding, It makes all sense in the context of ohm's law, which can be visualized like this:
Wow, awesome! P.S. poor mister Amp even seems to be sweating from the heat generated by friction caused by Mr Ohm?
I guess the height/slope consideration can help with voltage as mentioned by Dave. Oddly enough, they definitely discussed movement of electrons and potential differences in elementary school (although voltage specifically wasn't mentioned), and yet I still didn't get voltage much after. The only difference is that electrons move from - to + (I guess that's called electron flow), while we consider current to flow from + to - (I guess that's called conventional current), e.g. here:
https://www.mi.mun.ca/users/cchaulk/eltk1100/ivse/ivse.htm - Although you might understand that already, but if not, it can be helpful to avoid some confusion later.
P.S. I still don't really understand voltage, so take anything I write about it with a grain of salt.
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#14 Reply
Posted by
KL27x
on 09 Jun, 2017 09:13
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If I stand next to you and drop a penny on your head, you will feel it. If you stand under the Eiffel tower and I climb to the top and I drop a penny on your head, you will feel it. Same thing, just on the order of ~1 x 10^-28 coulombs worth of electron-pennies... which is one. But with a little difference in voltage.
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#15 Reply
Posted by
EEVblog
on 09 Jun, 2017 09:26
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For an intuitive understanding, It makes all sense in the context of ohm's law, which can be visualized like this:
Wow, awesome! P.S. poor mister Amp even seems to be sweating from the heat generated by friction caused by Mr Ohm?
And
that is Power dissipation!
Power = Voltage X Current
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#16 Reply
Posted by
tooki
on 09 Jun, 2017 13:57
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As for the confusion of voltage existing even when there is no current flow at all, consider that voltage is pressure, and that pressure exists in the absence of movement. Think of a gas cylinder: it's under pressure (voltage), but with the valve fully closed (essentially infinity ohms), no gas flow (current) occurs.
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#17 Reply
Posted by
Shock
on 09 Jun, 2017 14:18
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Wow, awesome! P.S. poor mister Amp even seems to be sweating from the heat generated by friction caused by Mr Ohm?
Watt?
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#18 Reply
Posted by
Brumby
on 09 Jun, 2017 14:20
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There's always one in the crowd.
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If electrons all have the same energy, then the only way to increase the rate of energy delivery is to increase the number of electrons moving past a point... which means raising the amps.
You are so close with your understanding at this point - but you missed one critical factor ... Why would the electrons move at all? .... and, if so, how strongly and in which direction?
You need something to push them along - and that's where the volts come in. Voltage is electrical pressure. The higher the volts, the more the pressure - which is how you raise the amps.
you are close. people talk about pressure... but where the pressure comes from? its from the charge imbalance between 2 points... how to make such charge imbalance? through chemistry (in battery) or some electromotive force, mechanical or electromagnetic... mechanical nature (such as thunder and electrostatics) can be well demonstrated with van de graff ball...
https://en.wikipedia.org/wiki/Electromotive_forcehttps://en.wikipedia.org/wiki/Van_de_Graaff_generator
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#20 Reply
Posted by
Brumby
on 09 Jun, 2017 15:01
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Voltage is the electrical pressure. This seems to be the fundamental the OP needs to grasp. The water analogy works extremely well at this level.
How that pressure is created comes afterwards.
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#21 Reply
Posted by
rstofer
on 09 Jun, 2017 16:06
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There are some pretty good descriptions above. The cartoon says it all! (So why am I writing this...)
The thing to remember is that when we measure volts, we are measuring the potential difference between any two points. Consider placing 3 AA batteries (just say they are 1.5V) on a bench, end to end with the + ends all pointing in the same direction. You can measure between the two ends (an arbitrary 2 points) and get about 4.5 volts. You can measure across just two batteries (again, 2 arbitrary points) and get about 3 volts and, of course, you can measure across just one battery (again, 2 arbitrary points) and get 1.5V. For this test, the black probe goes on the - end of the string and the red probe goes on the + end.
Cool, just what we expect! We connect batteries in series all the time - think flashlights...
Now, let's take battery on the right end of the string and turn it around. Now we get only 1.5V across the 3 batteries! Not only does voltage have a magnitude, it has a direction or polarity. We consider voltage to increase (gain) as current flows from the - to + terminals. So, we assign a + direction if the current flow is from the - terminal to the + and a - direction when the current flows the other way. So where we had 1.5 + 1.5 + 1.5 = 4.5, we now have 1.5 + 1.5 - 1.5 = 1.5
The 'any arbitrary 2 points' allows us to measure voltages in our circuit across any component we wish. In very short order, you will be tracing current flow through circuits with multiple resistors in series or parallel. When you do, you will be calculating the voltage across and the current through each resistor using Kirchoff's Laws. If we know the current, we calculate voltage. If we know the voltage, we calculate current.
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#23 Reply
Posted by
Ratch
on 09 Jun, 2017 18:21
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Hi, all... I'm enrolled in a distance education electronics course, and I'm right back at the starting gate.
Amps, I've got figured out. 1 coulomb of electrons is 6.24*10^18 electrons. That many electrons moving past a point in one second is one amp. So I think I've got that one sorted. Similarly, Ohms is resistance... the more ohms, the more resistance, the fewer electrons make it through.
But volts... that one just isn't jelling for me. My texts are telling me that it is 'energy' or 'potential'. But that's not helping me understand it. If electrons all have the same energy, then the only way to increase the rate of energy delivery is to increase the number of electrons moving past a point... which means raising the amps. I'm kinda visualizing the finish line at a racetrack. As a few electrons pass the finish line, we have a low amp rate. As more electrons pass the line, we have a higher amp rate. And really, that ought to be the end of the story. The only parameters there are amps - the number of electrons passing the finish line - and resistance, which I guess would be the width of the track.
I don't see how volts even figure into that picture.
Can anyone help me get a clearer picture of this?
Thanks - Charlie
Voltage is the energy density of the charge, defined as joules/coulomb in the MKS system. It takes energy to move two charged particles of the same polarity close together. Even more energy if they are moved closer together. Still more energy if several charged particles are gathered. If it takes one joule of energy to corral one coulomb of charge together at a location, that location is defined to have an energy density per charge unit of one volt with reference to where the charges came from. The same polarity charges don't like to be together, and will scatter if a conduction path exists. The resistor and wire provide the conduction path. A higher energy density per unit charge (voltage) at one end of the resistor will cause the charges to move through the resistor to the lower voltage at the opposite end. While traveling through the resistor, the charges will encounter repulsive forces from the ionic cores of the resistor atoms and various fields from other charged particles of matter within the resistor. The charges will lose energy in the form of heat, and will arrive at the other end of the resistor with less energy than they started with. The same number of charges exit the resistor as entered, but with less energy. Therefore, the energy density per unit charge (voltage) will be less. This loss of voltage is called the voltage drop caused by the energy dissipation (heat) loss while transiting the resistor. The wire has much less resistance so the voltage drop is very low and is usually neglected.
Ratch
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#24 Reply
Posted by
bson
on 10 Jun, 2017 02:04
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The only parameters there are amps - the number of electrons passing the finish line - and resistance, which I guess would be the width of the track.
Why would electrons want to move in the first place, and why in one direction of wire instead of the other? They're being pushed by an
electromotive force, EMF, measured in Volt.