Thread: The Physics of Electricity

Firstly, I have just finished year 12, completing subjects in physics, chemistry, biology, mathematics, and english. Over the years, I realised that what I had been taught about electricity made little sense and contained alot of contradictions. Originally we are taught that electricity is a flow of particles called electrons from a source to a load. These particles contain energy, and they flow through wires just like water flows through pipes. When they arrive at the load, the particle's energy is given to the load, and then the particle flows around the circuit back to the source to replenished with more energy. This is sometimes called the "freight cars" analogy. Well, this is obviously totally wrong.

If this analogy were true, the electrons in the wire would have to physically move at close to the speed of light, which means they could not move around corners or bends in the wire becuase they would just shoot straight out of it. They would also have an extremely low chance of ever reaching the load, because they would collide with the lattice of the conductor and continuosly bounce around like a pinball which in turn would generate enormous heat in the wire which could cause it to explode.

However, I could never find a suitable explaination for what electricity actually was in any of my high school textbooks. They never explained the mechanics of electricity, they just rattled off a few little facts about it, such as how a magnetic field can induce a current in a wire and so on. Then after that, we dived straight into equations such as F = nBIAsin*, also with little explaination. The point is: we are never properly taught what electricity actually is. So I had to pursue an answer myself. I found out that electricty is actually like a mechanical wave: a conductor is full of free moveable charges (electrons) which repel eachother. In a conducting wire which has no external field or force being applied to it, the electrons are all in electrostatic equilibrium. If a magnet is brought close to one end of the wire and rotated once, the charges up that end are repelled, and then attracted back. Electrons repell eachother through their electrostatic fields that surround them, which operates at the speed of light. Thus, when the magnet is rotated, a sinusoidal wave motion of electrons inside the wire occurs. The first electron moves closer to the second electron which causes it to be repelled towards the next electron and so on. Within the wire, there are compressions and rarefactions of electrons. This is how electricity works: the electrons themselves do not move very far distances at all, the energy moves from the source to the load through what could be called "electronic waves".

Surprisingly, I was never taught this simple fact anywhere throughout my schooling life, which certainly made understanding the concepts in the electricity area much more difficult. Even my teacher agrees with me that our education system is in desperate need of a serious revamp.

I have done an extensive amount of research into electricity over the last few years in this field, I have been collecting information from many sources and writing my own defintions of terms used in the field of electricity. Even entering the realms of Nikola Tesla and studying many of his patents. I have come a long way since those years, and I feel that I am infinitely ahead of my peers now because they never questioned their knowledge or sought to improve it. Some of them may have acheived better marks than me in their exams, but I dont care because I know about the much more important concepts that are fundamental to the field which sadly they may never come to know.

I have also compiled a short list of some the things that I think need changing in our understanding of electricity:

1. Electrons should be positively charged. I know it doesnt really matter either way, but it would make much more sense if it was made this way. This error dates back to Benjamin Franklin. We have known for a long time that things move from where there is a higher concentration to where there is a lower concentration (pressure flow). If electrons were said to be positively charged, we could then say that they move from positive to negative. This is all about voltage (potential difference), if there are a group of electrons together, they will move from where there is a surplus (+), to where there is a deficency (-). Of course we would then have to label protons as negative charges.

2. Voltage is simply the electrostatic field strength, it acts as a "pressure" that can cause electrons to move. If there is a difference in electrostatic field strength in a conductor, the electrons will move to try to achieve equilibrium.

3. Does the energy move inside or outside the wire? Personally im not quite sure at present, but many engineers are telling me that it does. They say that the energy travels virtually directly from the source to the load via electromagnetic waves. The electrons basically act as a waveguide for them. However im not totally convinced, does anyone here have an opinion on that?

4. I highly doubt that superconductivity is a quantum mechanical phenomenom. I think that once an even better understanding of electricity and matter is obtained, we will be able to explain it without QM.

Why do people like you keep harping on the + and - convention. It's just a name. Give it up. It won't change no matter how much you dislike it.

Also, electrons do flow at about half the speed of light (IIRC) through copper wire. You can measure this by carefully measuring how long it takes for a light to come on when you flip the switch. You have to be sure to take in to account the time it takes the light to light once it starts getting electricity and the time it takes the light to reach your sensor too though.

Superconductivity cannot be explained by the normal properties of matter. This comes from people who know a lot more about the normal properties of matter than you do. Seriously, you just graduated high school and you think you know better. Come back after you've completed 12 years in a serious college.

QM and relativity work over their range of validity regardless of whether you agree with them or not, whether you like them or not or whether you understand them or not.

Originally Posted by MagiMaster

Why do people like you keep harping on the + and - convention. It's just a name. Give it up. It won't change no matter how much you dislike it.

Also, electrons do flow at about half the speed of light (IIRC) through copper wire. You can measure this by carefully measuring how long it takes for a light to come on when you flip the switch. You have to be sure to take in to account the time it takes the light to light once it starts getting electricity and the time it takes the light to reach your sensor too though.

Firstly, I never said they would change it, I simply said that it would have been more logical if electrons were labelled as positive in the first place, and could eliminate some of the misconceptions in understanding electricity.

Secondly, electrons do not flow around a circuit at around half the speed of light. The energy that they carry does, which leads me to believe that you didnt read my post very thoroughly. The best measurements have shown that electrons only move a few millimeters per second, which to my knowledge is called their drift velocity. Another concept in disagreement with your understanding is alternating current. In this type of current, the elctrons have no net displacement, and move back and forth inside the wire, but the energy moves in one direction from the source to the load.

Im not sure about your position, age or education, but you dont seem to know much about electricity at all. Even though I have only just graduated from high school, I can still have a very good knowledge of physics and I am going straight into university next year to study nanotechnology, which I have also researched alot.

Originally Posted by Waveman28

Originally Posted by MagiMaster

Why do people like you keep harping on the + and - convention. It's just a name. Give it up. It won't change no matter how much you dislike it.

Also, electrons do flow at about half the speed of light (IIRC) through copper wire. You can measure this by carefully measuring how long it takes for a light to come on when you flip the switch. You have to be sure to take in to account the time it takes the light to light once it starts getting electricity and the time it takes the light to reach your sensor too though.

Firstly, I never said they would change it, I simply said that it would have been more logical if electrons were labelled as positive in the first place, and could eliminate some of the misconceptions in understanding electricity.

Secondly, electrons do not flow around a circuit at around half the speed of light. The energy that they carry does, which leads me to believe that you didnt read my post very thoroughly. The best measurements have shown that electrons only move a few millimeters per second, which to my knowledge is called their drift velocity. Another concept in disagreement with your understanding is alternating current. In this type of current, the elctrons have no net displacement, and move back and forth inside the wire, but the energy moves in one direction from the source to the load.

Im not sure about your position, age or education, but you dont seem to know much about electricity at all. Even though I have only just graduated from high school, I can still have a very good knowledge of physics and I am going straight into university next year to study nanotechnology, which I have also researched alot.

I know that in AC the electrons don't move much. I wasn't talking about AC. Anyway, I'll give you that I got the speed of the current and the speed of the electrons mixed up.

Well, good luck with your studies. All I can say is that while it's good to question things, you shouldn't be too sure of yourself.

Originally Posted by Waveman28

Originally we are taught that electricity is a flow of particles called electrons from a source to a load. These particles contain energy, and they flow through wires just like water flows through pipes. When they arrive at the load, the particle's energy is given to the load, and then the particle flows around the circuit back to the source to replenished with more energy. This is sometimes called the "freight cars" analogy. Well, this is obviously totally wrong.

No, it is perfectly correct.

If this analogy were true, the electrons in the wire would have to physically move at close to the speed of light, which means they could not move around corners or bends in the wire becuase they would just shoot straight out of it.

Wrong. The individual electrons do not have to move at close to the speed of light.

They would also have an extremely low chance of ever reaching the load, because they would collide with the lattice of the conductor and continuosly bounce around like a pinball which in turn would generate enormous heat in the wire which could cause it to explode.

Current flowing in a wire does generate heat. Why does it have to be an "enormous amount"?

Electrons repell eachother through their electrostatic fields that surround them, which operates at the speed of light.

Correct. Then why did you say above that the individual electrons have to move at the speed of light?

Thus, when the magnet is rotated, a sinusoidal wave motion of electrons inside the wire occurs.

This is only true for alternating current.

1. Electrons should be positively charged.

Yawn.

Originally Posted by Waveman28

Firstly, I have just finished year 12, completing subjects in physics, chemistry, biology, mathematics, and english. Over the years, I realised that what I had been taught about electricity made little sense and contained alot of contradictions. Originally we are taught that electricity is a flow of particles called electrons from a source to a load. These particles contain energy, and they flow through wires just like water flows through pipes. When they arrive at the load, the particle's energy is given to the load, and then the particle flows around the circuit back to the source to replenished with more energy. This is sometimes called the "freight cars" analogy. Well, this is obviously totally wrong.

There is nothing wrong about either of these analogys. In the water analogy, you have water in a pipe. If you open a value to apply pressure, the water starts to flow pretty much instantanously throughout the pipe. The water behind pushes the water ahead of it. The water itself does not have to move very fast for the the effect to be felt very quickly over the length of the pipe.

The same is true for the freight car example. You have a string of freight cars. If you push on the last one, it pushes on the one ahead of it, etc. Again, you see the effect at the end much faster than the time it would take for the last freight car to move the distance from one end to the other.

What could be clearer than that?

I agree with the OP, electricity needs to be better taught in physics classes. They were using a straw analogy in physics 2 classes at my college, but teaching it so poorly people thought that the electrons were flowing through the center of wires.

To the OP: You really should post this type of question in the electrical forum, all electrical engineers are physicists, but not all physicists are electrical engineers.

For your point number three, the answer is both. Picture an insulated wire made out of solid copper. Inside of the wire, there is no field. On the outer surface of the copper you have many point fields along a wave front propagating down the length of the wire. Inside of the insulation, you are dissipating the component of the field able to travel outside of the wire by x% which can be expressed in terms of skin depth. Then, outside of the insulation, you are propagating in air, with an attenuation of about 20 dB/decade (in ideal systems). Normally, the insulation is thick enough to stop wave from propagating outside of the wire.

School as a whole (K - undergrad at least) needs a lot of work. The general strategy is to teach simplifications first and details later. That's fine, in and of itself. After all, a 7th grader isn't likely to understand the quantum mechanical description of the flow of electrons no matter how good the teacher is. The problem comes when 1) the fact that they're just learning simplifications is never explained and 2) they never get to a high enough level to learn the details.

Originally Posted by alienmindsinc

They were using a straw analogy in physics 2 classes at my college, but teaching it so poorly people thought that the electrons were flowing through the center of wires.

Okay, I'll bite. What's wrong with that?

The analogy they use is that if you pinch a straw you restrict current flow and increase resistance of the wire. If you pinch a straw you are not decreasing surface area.

That, and the electricity propagates along the surface of the conductor.

Originally Posted by Harold14370

Originally Posted by Waveman28

Originally we are taught that electricity is a flow of particles called electrons from a source to a load. These particles contain energy, and they flow through wires just like water flows through pipes. When they arrive at the load, the particle's energy is given to the load, and then the particle flows around the circuit back to the source to replenished with more energy. This is sometimes called the "freight cars" analogy. Well, this is obviously totally wrong.

No, it is perfectly correct.

If this analogy were true, the electrons in the wire would have to physically move at close to the speed of light, which means they could not move around corners or bends in the wire becuase they would just shoot straight out of it.

Wrong. The individual electrons do not have to move at close to the speed of light.

They would also have an extremely low chance of ever reaching the load, because they would collide with the lattice of the conductor and continuosly bounce around like a pinball which in turn would generate enormous heat in the wire which could cause it to explode.

Current flowing in a wire does generate heat. Why does it have to be an "enormous amount"?

Electrons repell eachother through their electrostatic fields that surround them, which operates at the speed of light.

Correct. Then why did you say above that the individual electrons have to move at the speed of light?

Thus, when the magnet is rotated, a sinusoidal wave motion of electrons inside the wire occurs.

This is only true for alternating current.

1. Electrons should be positively charged.

Yawn.

No, the freight cars analogy above as I was taught was wrong, because the individual electrons that start at the source often never reach the load (they might if you leave the light bulb on for an hour or so, because the slow drift velocity of the electrons would eventually get them there). Im just curious as to why teachers dont teach us the basics of waves from about year 3 or even year 2 onward, its very simple and it will make it so much easier for the students to learn important concepts like electricity.

They would, because they have made no reference to waves or one electron pushing the next and so on, just a system where particles circulate around the wire obtaining energy and then moving to the load and dropping it off.

I know heat is produced in a circuit naturally because of the resistance in the wire (a.k.a. electrical impedence), but its not particularly high in real circuits because the electrons themselves do not have a high velocity, so not much energy is dissipated this way through lattice collisions. By old analogies, they are basically saying that plain old wires are like particle accelerators, the electrons themselves move at extremely high velocities, so therefore if they collided with the lattice, a huge amount of energy would be released as heat, so with millions of these collisions occuring, the wire would quickly vaporise.

I said that electrons dont have to move at the speed of light because of the electrostatic field that surrounds them which allows energy to be transfered through waves. On the contrary, the simplification we are taught makes no reference to this field or waves, or anything that bears the slightest resemblence to what is really happening in a circuit. It suggests that these particles are physically transporting energy from source to load and then moving back around to be "refilled" with energy. Thats why I made the comparisson.

True, which is what I indicated, but DC instead of sending a sinusoidal wave would send a wave pulse (the top half of a sin wave).

Understand?

This is a nitpick, but I do know that the only time you get the half sine wave is from rectified AC. Something like a battery will give a constant current.

Originally Posted by Waveman28

No, the freight cars analogy above as I was taught was wrong, because the individual electrons that start at the source often never reach the load (they might if you leave the light bulb on for an hour or so, because the slow drift velocity of the electrons would eventually get them there). Im just curious as to why teachers dont teach us the basics of waves from about year 3 or even year 2 onward, its very simple and it will make it so much easier for the students to learn important concepts like electricity.

True, which is what I indicated, but DC instead of sending a sinusoidal wave would send a wave pulse (the top half of a sin wave).

Understand?

It seems natural to start teaching electricity with direct current, which is simpler in concept. Direct current is not the top half of a sine wave. The waveform could be perfectly flat, and in the simplest d-c circuit, the kind you would start explaining to a beginner, it is. There isn't any wave involved, unless you want to deal with fourier transforms, or get into quantum theory, and I don't think a beginning student is ready for that. There is nothing inherent in the freight car analogy that says an individual freight car will end up at any particular point. Maybe you just read that into it.

Originally Posted by alienmindsinc

The analogy they use is that if you pinch a straw you restrict current flow and increase resistance of the wire. If you pinch a straw you are not decreasing surface area.

That, and the electricity propagates along the surface of the conductor.

If you pinch a straw, you decrease the cross-sectional area where the water is flowing. Why isn't that analogous to decreasing conductor size?

When you say the electricity propagates on the surface, I suppose you mean the skin effect?
http://en.wikipedia.org/wiki/Skin_effect

The skin effect is the tendency of an alternating electric current (AC) to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core.

Not all the current is at the surface. For d-c and most ordinary a-c purposes, it is a good approximation that the conductance of a wire is proportional to its cross-sectional area.

Originally Posted by MagiMaster

This is a nitpick, but I do know that the only time you get the half sine wave is from rectified AC. Something like a battery will give a constant current.

Youve got a point there. But I must admit that I am a bit confused in terms of the output of a simple DC generator, some sources say it is a straight line output, other suggest it is a positive sine curve such as in this picture I found:


It seems that the voltage output of a DC generator cannot be constant, because the rotating coil's angle with the stator constantly changes.

Originally Posted by Waveman28

Originally Posted by MagiMaster

This is a nitpick, but I do know that the only time you get the half sine wave is from rectified AC. Something like a battery will give a constant current.

Youve got a point there. But I must admit that I am a bit confused in terms of the output of a simple DC generator, some sources say it is a straight line output, other suggest it is a positive sine curve such as in this picture I found:


It seems that the voltage output of a DC generator cannot be constant, because the rotating coil's angle with the stator constantly changes.

It depends on whether or not the generator includes a filter and regulator.

A filter can be as simple as a large capacitor across the output. During the pulses the capacitor charges and then discharges to supply voltage to fill in the "gaps". This will give you a DC voltage with a small AC component riding on it.

A regulator, which can be as simple as a zener diode, then maintains a steady output voltage something below the level where the AC component rides. This provides your load with a steady DC current as long as the current draw doesn't excede the systems design limits.

The same type of system is used to provide DC current from an AC source. First the AC current is rectified to pulsed DC and then run through a filter/regulator.

That's why car alternators produce three phase AC, then use six diodes for full wave rectification.

When you add three sine waves together spaced 120 degrees apart, you get a perfectly strait line.

Also why USA power transmission is three phase.

Originally Posted by Wild Cobra

That's why car alternators produce three phase AC, then use six diodes for full wave rectification.

When you add three sine waves together spaced 120 degrees apart, you get a perfectly strait line.

Also why USA power transmission is three phase.

Nope, not quite. The three phases add up to zero, but the three phase bridge rectifier has some ripple in it.
http://en.wikipedia.org/wiki/File:3_...fication_2.png

So, would it be correct to say that in a simple DC generator and circuit setup whereby the DC generator consists of only a single rotating wire in a stationary magnetic field, the voltage will be variable throughout the circuit? Of course the voltage would only be positive, but it may still change. Heres another thing, in terms of the electrons in the circuit, would it be right to say that electron compressions move through the circuit in one direction and rarefactions move in the other? This would "complete the circuit".

would it be right to say that electron compressions move through the circuit in one direction and rarefactions move in the other?

I think the convention is to talk about the virtual movement of positive ions in the opposite direction. I could be wrong though.

Originally Posted by MagiMaster

The problem comes when 1) the fact that they're just learning simplifications is never explained and 2) they never get to a high enough level to learn the details.

This. Especially 1) is a big problem. It is teacher dependent, and good teachers will get to that.
Some students are of course not interested enough to pick it up, which can be a consequence of talent, previous teachers or (sub)culture.

That said: I don't see a lot of problems with any of the examples you give. I agree with about everything Harold said in this thread. For almost all purposes, it doesn't even matter whether electrons are positive or negative. It's btw perfectly possible to have currents were positive particles move (ions in batteries or plasma e.g.). It also doesn't matter how fast these particles move (although mentioning it is good to define the boundaries of the model).
There are of course some area's where it does matter, such as Electro Discharge Machining, Scanning Tunnelling Microscopy, CRT-screens... (the convention still doesn't matter btw), but those are hardly the things you want to bother starting students with.

The DC generator you showed is also about the worst source of DC I've seen. For accurate measurements, nothing beats chemical batteries and twisted or shielded wires.

Originally Posted by Waveman28

So, would it be correct to say that in a simple DC generator and circuit setup whereby the DC generator consists of only a single rotating wire in a stationary magnetic field, the voltage will be variable throughout the circuit? Of course the voltage would only be positive, but it may still change.

Yes. But there very few practical uses for such pulsed DC other than lighting a bulb or running a heater element. Electronics in particular need a steady unvarying voltage to work. This why they use regulated power supplies.

Heres another thing, in terms of the electrons in the circuit, would it be right to say that electron compressions move through the circuit in one direction and rarefactions move in the other? This would "complete the circuit".

I don't really see how this visualization is useful. You don't have to have pulsed DC to have a complete circuit. Also, considering the fact that the propagation speed is a sizeable fraction of the speed of light, in practical application you can treat the voltage as changing instantaneously throughout the circuit.

Consider a 3600 rpm generator, giving a "peak to peak" period of 1/120 of a sec. In that time, the leading edge of your "compression" could have traveled 2500 km. In reasonably sized circuit, the "compression" would fill the circuit.

Originally Posted by MagiMaster

Why do people like you keep harping on the + and - convention. It's just a name. Give it up. It won't change no matter how much you dislike it.

Also, electrons do flow at about half the speed of light (IIRC) through copper wire. You can measure this by carefully measuring how long it takes for a light to come on when you flip the switch. You have to be sure to take in to account the time it takes the light to light once it starts getting electricity and the time it takes the light to reach your sensor too though.

MagiMaster, that is not quite correct. When I turn on a shower, the water that falls initially falls on my head is not coming to me all the way from the well. It's the water that was already in the pipes, getting displaced by water from the well. (Actually it's water getting displaced by water getting displaced by water getting displaced.... etc.... all the way back to the water coming from the well. )

Similarly to the water sitting in my pipes, there are always inert electrons in a wire. If I add electrons to one end of that wire, they will push on the inert electrons nearest them, which push on the next ones, which push on the next ones, ..... etc.... and the first electrons that flow into that light bulb will be the ones that were already sitting in the wire nearest it, not the ones I added.

Right. Someone else already pointed out that mistake.