Inducing electric current in a wire (a comparison)

[Mitko Gorgiev]

Consider the following experiment: from a lacquered copper wire we cut off twenty to thirty pieces of about 10 cm. From them we form a bundle of parallel wires and connect the two ends with one more wire each. The other ends of these two wires are connected to a sensitive analog ammeter. We hold the bundle horizontally and move quickly a strong and broad magnet downwards on its left side. The pointer of the instrument will make a deflection to one side. If we now move the magnet quickly downwards on the right side of the bundle, the instrument will make a deflection to the opposite side. The magnetic flux that we have produced in the wire is now in the opposite direction to the one in the first case, which is why the deflection is in the opposite direction. The motion of the magnet produces current even if we only approach it to the bundle from one side without lowering it below the bundle. In this case the current is somewhat weaker. But if we now move the magnet down to the middle of the bundle, the instrument won’t show any current, because the left and the right halve of the magnet act on opposite sides of the bundle, canceling each other out.
We can do the experiment with only a single wire instead of a bundle, as long as we have a very strong magnet and a very sensitive ammeter.
You can imagine that inside this wire there is a propeller or there are many propellers in a row. When you turn a propeller manually from the left side, then it is turning in one direction and it is blowing on one side (plus), but it is suctioning on the other side (minus). When you turn the propeller from the right side, then it is turning in the contrary direction and the air current is in the opposite direction. But you cannot turn the propeller from above. Exactly the same picture we have with the magnet and the wire.

After we have lowered the magnet down and have produced a current in one direction, then we can move it back upward. In that case we produce a current in the contrary direction, just as we will produce an air-current in the contrary direction if we turn the propeller from down up.

 

[berkeman]

Welcome to the PF.

It would be good if you studied a bit more about how induction works. One concept you are missing so far is that the important thing is the change of magnetic flux in a coil of wire, not near a long straight wire. And having a bundle of wires connected in parallel makes no difference -- you want more coils to generate a larger output voltage and current.

https://en.wikipedia.org/wiki/Electromagnetic_induction

 

[anorlunda]

Comparing electric current with flow of air is also berry misleading. You will understand faster if you forget about electrons.

 

[vanhees71]

I think to "forget about electrons" is a bad advice. Simple (classical) fluid models for the charge carriers are of great help to particularly understand induction, which is usually the most difficult conceptual topic of the four Maxwell equations (particularly since there's so much confusion about it in elementary textbooks). You can even get quite far with a non-relativistic treatment (though one has to take it with a grain of salt when it comes to electrodynamics in moving media, where a relativistic treatment is usually needed) since the Newtonian limit is only valid up to first order in $v/c$ (at best).

 

[anorlunda]

I think to "forget about electrons" is a bad advice. Simple (classical) fluid models for the charge carriers are of great help to particularly understand induction, which is usually the most difficult conceptual topic of the four Maxwell equations (particularly since there's so much confusion about it in elementary textbooks).

Sorry to disagree, but you are thinking about advanced students. For beginners struggling to learn circuits, I'll stick with my advice. I base it on the many threads we get here on PF from people who tie themselves into mental knots trying to make sense of basic electricity. Many of them are math phobic, and would never study fields or Maxwell's equations. They attempt to find math-free rationalizations, and get stuck in the tar. Forget electrons is advice directed at them.

 

[Mitko Gorgiev]

Welcome to the PF.

It would be good if you studied a bit more about how induction works. One concept you are missing so far is that the important thing is the change of magnetic flux in a coil of wire, not near a long straight wire.

Thank you.
I try to find the principle and this comparison serves pretty good. In finding the principle I don't have to stick imperatively to a coil of wire.
Speaking of coils, here is a comparison for it: when we move a magnet in and out of a solenoid, we can compare that to the "push and spin" mechanisms, which can be found in small toy carousels, in some ashtrays or spinning top toys.

 

[vanhees71]

Sorry to disagree, but you are thinking about advanced students. For beginners struggling to learn circuits, I'll stick with my advice. I base it on the many threads we get here on PF from people who tie themselves into mental knots trying to make sense of basic electricity. Many of them are math phobic, and would never study fields or Maxwell's equations. They attempt to find math-free rationalizations, and get stuck in the tar. Forget electrons is advice directed at them.

For me particularly at this level of studying circuit theory without the higher vector calculus of the Maxwell equations (without which of course no true understanding is possible to be honest) the idea of an electron fluid drifting through the wires is very intuitive and explaines with a very simple heuristic argument, how resistors (Ohm's Law from the frictional motion of the electrons driven by the electric field), capacitors (charges flowing to and from the plates), and coils (induction of the magnetic field through the motion of the electrons) work.

To claim you can learn about physics and engineering without math is cheating the students. I know this tendency to "avoid math" in the recent decades of socalled "didactical development". It's the greatest sin ever done to the young generation's education!

 

[Mitko Gorgiev]

Comparing electric current with flow of air is also berry misleading.

Why is it misleading? Can you elaborate it?

 

[anorlunda]

Why is it misleading? Can you elaborate it?

That's a difficult question to answer at B level. I'll try.

The drift velocity of electrons in a wire carrying power is very low. About 1 cm/sec. But any motion of an electron creates a change in the elctromagnetic field. Those changes move down the wire at the speed of light, effecting other electrons all through the wire to the far end almost instantaneously. Therefore, when you flip the light switch, power from a power plant thousands of miles away can light your lamp instantaneously, even though electrons flowing from that power plant may take 5 years to reach your house.

A poor analogy, but better than none, is that every electron is connected to every other electron by a little spring. That is very different than a gas where molecules move freely except when they collide.

Another problem with air flow or water flow analogies to electricity is that they lead people to visualize electrons to be like little packets of energy. When they get to the far end, they burst and deliver their energy to the load. Electric power P is voltage times current. P=VI. Delivering current to the far end does not deliver power unless there is a voltage drop at the far end. Delivering water to a faucet fills your glass with water regardless of pressure differences inside the water glass. Again, air and water flows are not like electricity.

Stay curious and good luck with your future studies. Many things in physics are different than our simple intuitive guesses. First, you'll study circuits where it is best to ignore electrons and fields. Then you'll study electrostatics, electrodynamics, and fields and learn how to treat charged particles and the motion of particles. Conduction in a wire and resisance also requires study of molecular level structures and quantum mechanics, and those are post graduate topics.

 

[vanhees71]

It's a very good exercise to evaluate the electromagnetic field for a DC power line. For a very long coaxial wire this can be done completely analytically, and there you get out that the electrical energy is "transported" from the source ("battery") to the other end via the electromagnetic field with the energy flowing perpendicular to the wire (which is easy to see without calculation too, using that the electric field is almost along the wire (in the usual approximation of Ohm's Law neglecting the magnetic interaction of the conduction electrons exactly along the wire) and the magnetic field is curling around the wire, so that the Poynting vector (energy flow of the electromagnetic field) is radially directed to the wire. Indeed the drift velocity of electrons is very slow. They crawl at an order of magnitude of about $1\text{mm}/\text{sec}$, which is due to the enormous amount of electrons per unit volume inside the wire.

 

[anorlunda]

magnetic field is curling around the wire, so that the Poynting vector

Oh please. Do you not see the B level tag on this thread?

 

[vanhees71]

What's the problem to answer the question posed? Since when is the Poynting vector not B-level anymore?

 

[Dadface]

What's the problem to answer the question posed? Since when is the Poynting vector not B-level anymore?

I think people interpret "B level" in terms of what is taught in the high schools in the country they are living in. In the UK the syllabi have regularly changed over the years but as far as I can recall not one of them has ever required a knowledge of the Poynting vector.

 

[vanhees71]

Well, ok. Then we cannot answer B-level questions in a useful way anymore. If somebody asks about "where the energy" sits, you must be allowed to discuss energy density in continuum systems. Otherwise already the question doesn't make sense. If I look what happened to the high-school-physics curricula in Germany over the last 30 years, I can only cry the whole day too!

 

[Dadface]

Well if in your opinion the best way to answer the question is by reference to the Poynting vector you could advise the student to look it up. In the coursework component of UK courses students were encouraged to go beyond the syllabus requirements and do some research.

 

[vanhees71]

Great! I also guess that students who subscribe to a forum like this are those who are interested in the subject and want to get answers beyond the (at least in Germany) sometimes idioscyncratic plans made by some didactical agenda which is not much driven by the logical structure of the subject itself but some ideological ideas about what didactical. The most disturbing idea in recent years was the euphemism called "competence orientation", which (again I can only speak for Germany) is to teach high-school students to solve blindly some standard-type problems without a real understanding about the concepts, which imho is the opposite of what's aimed at in teaching of (and also research in!) the natural sciences.

 

[Mitko Gorgiev]

That's a difficult question to answer at B level. I'll try.

The drift velocity of electrons in a wire carrying power is very low. About 1 cm/sec. But any motion of an electron creates a change in the elctromagnetic field. Those changes move down the wire at the speed of light, effecting other electrons all through the wire to the far end almost instantaneously. Therefore, when you flip the light switch, power from a power plant thousands of miles away can light your lamp instantaneously, even though electrons flowing from that power plant may take 5 years to reach your house.

A poor analogy, but better than none, is that every electron is connected to every other electron by a little spring. That is very different than a gas where molecules move freely except when they collide.

Another problem with air flow or water flow analogies to electricity is that they lead people to visualize electrons to be like little packets of energy. When they get to the far end, they burst and deliver their energy to the load. Electric power P is voltage times current. P=VI. Delivering current to the far end does not deliver power unless there is a voltage drop at the far end. Delivering water to a faucet fills your glass with water regardless of pressure differences inside the water glass. Again, air and water flows are not like electricity.

Stay curious and good luck with your future studies. Many things in physics are different than our simple intuitive guesses. First, you'll study circuits where it is best to ignore electrons and fields. Then you'll study electrostatics, electrodynamics, and fields and learn how to treat charged particles and the motion of particles. Conduction in a wire and resisance also requires study of molecular level structures and quantum mechanics, and those are post graduate topics.

As far as I have understood your reply, you find two flaws in the analogy. The first is the speed of propagation of the action. You actually say that the air flow can't achieve the speed of the electricity flow.
Let's say we have a torus, a very long torus filled with air. Inside the torus there is fan, as big as the cross section of the torus. It begins to turn. Wouldn't all the air at every point inside the torus start to move instantaneously?
The second flaw I couldn't quite understand.

 

[davenn]

Wouldn't all the air at every point inside the torus start to move instantaneously?

no, it movement propagates along the tube where as, as anorlunda pointed out, the EM field
propagates along the wire at near the speed of light taking into account the velocity factor of the wire

The second flaw I couldn't quite understand.

not sure what you were referring to ?

 

[anorlunda]

As far as I have understood your reply, you find two flaws in the analogy.

Analogies are always flawed.

The language of physics is mathematics. I tried and failed to make you understand without the math. Too bad. Nature is not obligated to make things work in ways that are easy to understand.

The next step should be for you to get out there and learn the math, and the calculus needed to take a real physics course on electrostatics and electrodynamics.

 

[Mitko Gorgiev]

The language of physics is mathematics.

Before mathematics come, first one has to truly visualize what's going on. And the visualization has nothing to do with mathematics.

 

[anorlunda]

Normally

Before mathematics come, first one has to truly visualize what's going on. And the visualization has nothing to do with mathematics.

Perhaps @vanhees71 can help you with that.

 

[vanhees71]

No, because if you forbid to use math, one cannot communicate adequately about physics!

 

[weirdoguy]

Before mathematics come, first one has to truly visualize what's going on.

Well then good luck with learning quantum physics... And probably most of the physics. I would say that in most advanced topics it's the other way around - first you learn the maths and then based on that you build your visualisation.

 

[Mitko Gorgiev]

No, because if you forbid to use math, one cannot communicate adequately about physics!

I don't forbid using of maths, I only say what comes first and what next.

 

[weirdoguy]

I only say what comes first and what next.

And what you say is in general wrong, based on experience of millions of physicists throughout the years. Some topics can be visualised without math, some can't. You have to deal with it.

 

[vanhees71]

You need the math to analyze the physical situation and then to find an intuitive picture (where it's even subjective what you consider intuitive, and also it changes with more experience what you feel to be intuitive). The socalled "common sense" may work for very simple issues in classical mechanics, though even within classical mechanics there are sometimes quite surprising results (only think about the spinning top ;-)).

 

[anorlunda]

I don't forbid using of maths, I only say what comes first and what next.

The best strategy for learning is to follow the lead of your teachers, and textbook authors. They put in the trouble of planning in what order topics should be presented, and what prerequisites should be required before starting.

Setting out to do it your way, not only in what to learn, but even what order to learn things, can sometimes succeed, but more often results in disappointment. Trust your teachers and textbook authors, and follow their lead. They are trying to help you.

 

[rude man]

As far as visualizing how charges move slowly yet the field moves at a goodly percentage of the speed of light is a curio comprising a number of identical, lined-up metal spheres each suspended by equal-length threads. The spheres are in contact at rest. You swing one of the end spheres some angle and let it swing back like a pendulum until it impacts its neighbor. (All threads are in full tension). The spheres are not even seen to move at all except the one on the far end which swings outward almost as far as the inward swing did, and apparently instantaneously. Quite dramatic.
Does anyone know the name of this device?

 

[Dadface]

The device is commonly known as Newtons cradle. The impulse travels through at the speed of sound in the metal.

 

[rude man]

The device is commonly known as Newtons cradle. The impulse travels through at the speed of sound in the metal.

Thanks @Dadface