So lets think in two coils separated one of the other. We run a current(With positive charged particles) in one of them and therefore we create a magnetic field, which induce a current in the other coil opposing this magnetic field. My question is: will be positive or negative charged particles in the second coil moving?

And If those depend on the material, then: If our second coil has positively charged particles and the particles moves in the contrary direction as in the first coil. What if we have another second coil with negatively charged particles, will them move in the same direction as the positive?


The "charged particles" you refer to are socalled charge-carriers.

My question is: will be positive or negative charged particles in the second coil moving?

As you mention in the next sentence, it depends on material. But in the metal-wire coils I assume you are thinking of here, the charge-carriers are electrons and thus negatively charged. There are several types of charge-carriers:

  • Negative electrons as in usual metal wires in circuit wiring.
  • Positive holes in semiconductors, which are just "missing electrons". A missing negative charge in a regular pattern corresponds to a positive charge, which is the reason that the hole is positive.
  • Negative and positive ions in solutions. If a salt like NaCl is dissolved in water, it splits into $\mathrm{Na}^+$ and $\mathrm{Cl}^-$ ions, and therefore when a potential is set up over the solution, the positively charge ions move to the lower potential and the negatively charged to the higher potential.

To answer the second part of your question, remember that the charge-carrier doesn't have any influence on the direction of the current!

In your induction experiment, if you need a current to go upwards that could be achieved with any sign of the charge-carrier:

  • If it was holes or other positive carriers, they would move upwards so that the current flows upwards, but
  • if it was electrons or other negative carriers, they would move downwards so the current would still be directed upwards.

Current is defined as the direction a positive carrier would have - for convenience. Because now we don't have to worry about what carriers we are dealing with in your experiment.

  • $\begingroup$ Sorry, I am a little confused right now. You are saying that if in a circuit we have for example, a diode and a light bulb and we have positive charge-carrier, and afterwards negative charge-carrier, then the light bulb will work in both cases. $\endgroup$ – Andre Oct 7 '16 at 20:34
  • $\begingroup$ @Andre You have to establish a circuit. If a circuit is working and the current can run steadily, it means that negative charge moves in one direction around it (corresponding to positive charge moving the opposite way around). The type of charge-carrier doesn't matter. $\endgroup$ – Steeven Oct 7 '16 at 21:02
  • $\begingroup$ For example, if a semiconductor p-type is a part of a wire circuit and a salt bath (an electrolyte) is as well a part of the circuit, then current will still move in one direction around the circuit. The positive charge-carriers (the holes) in the semiconductor piece then move the opposite way, but that still just corresponds to more electrons moving the other way. And the positive ions move to one end of the salt bath, catching the incoming electrons at the electrode, while... $\endgroup$ – Steeven Oct 7 '16 at 21:04
  • $\begingroup$ ... negative ions move to the other end, delivering their excess charge (electrons) to that electrode which is connected to the circuit again. All in all, current flows and the light-bulb in the circuit can glow. $\endgroup$ – Steeven Oct 7 '16 at 21:06

In any electrical conductor, there's some motility associated with both the positive and negative charge carriers. In metallic conductors, the positive atomic nuclei are essentially fixed in place on the crystal lattice, while each atom has a conduction electron that's more or less free to move. On the other hand, in electrolyte solutions, it can be the case that a positive ion has less mass and greater motility than the associated negative ion.

No matter what the source of the electromotive force in your circuit, within each material component the charge carriers with greater motility do more of the moving than the other charge carriers. This isn't something that you can control, really: it's a property of the material. So if your coil were made of p-doped semiconductor in the conducting region, it'd be the positive holes that move; but if your coil is ordinary metal it'll be the negative charges that move, regardless of your voltage source.

  • $\begingroup$ @ rob - Why do you use the term "motility" for the charge carriers here, which seems to be more related to medical sciences of e.g. the gastrointestinal tract. The usual term in conductors, semiconductors or electrolytes is "mobility". The mobility of a charge carrier is the ratio of the mean speed of its movement in an applied electric field to the strength of this field. $\endgroup$ – freecharly Oct 7 '16 at 19:57
  • $\begingroup$ @freecharly I suppose you're right; language is strange. "Mobility" has several meanings; "motility", perhaps because of its biological usages, conjures a mental image of swimming corpuscles. If you think the other language is more appropriate, feel free to edit the answer and delete these comments. $\endgroup$ – rob Oct 7 '16 at 20:10
  • $\begingroup$ I still have a question related to this. This mobility is determined by internal forces(structure of the atom) and also by external force, in this case the electrical force. This says to us that in an electromagnetic induction the only external force, which should be taken into account is the electrical force. $\endgroup$ – Andre Oct 7 '16 at 20:49
  • $\begingroup$ @Andre That's right. In general, forces internal to a complicated system excactly cancel each other out --- that's actually Newton's Third Law. $\endgroup$ – rob Oct 7 '16 at 20:51

No matter what the charge sign of the charge carriers in your coil where the induction takes place, if the charge carriers in your coil are positive they move in the direction of the induced electric field and if they are negative, they move against it. Thus the direction of the induced current is independent of the charge carrier signs. There could even be both positive and negative charge carriers present.The negative charge carriers will never run in the same direction as the positive charge carriers.


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