Why doesn't direct current exhibit self-induction?

[scientist91]

Ok, I reformed my question, and I will be very happy if you answer me. I know that the electrons have magnetic field. So if the electrons are moving through a conductor (in direct current) than logically the magnetic field will move also. So there will be also self-induction. Can you give me good, deep and simple explanation for my question. Thank you.

 

[Mentz114]

then logically the magnetic field will move also

No, bad logic. Magnetic fields come from moving charges or changing electric fields. You can get a 'good, deep' explanation by studying Maxwell's equations. Or perhaps someone with a lot of time and patience will explain here.

 

[Xezlec]

Moving charges create magnetic fields. Moving charges do not necessarily create moving magnetic fields. If all of the electrons in the conductor are moving at (on average) a constant speed, then the magnetic field is constant. It doesn't move.

Think about this. For every electron that moves forward, another electron behind it moves up to take its place. So the magnetic field never goes away.

Does that help?

 

[lalbatros]

When you switch on a current, a magnetic field appears.
Therefore, any circuit has a (self-)inductance. (see the definition)
Self-inductance is not a property of a current (in first approximation), but it is a property of a circuit.

However, for very high-frequencies, the current cannot flow anymore in the core of the conductors, and instead it flows in "the skin" of the conductors. This is called "the skin effect". The thickness of the skin depends on the frequency of the current. When the skin effect appears, the inductance of a circuit will also depend on the frequency of the current flowing trhough the conductor. In this case, the self-inductance becomes really a property of the system {circuit+current flowing through it}. This also shows you that inductances are essentially related to circuits. When the current densities within the wires have to be taken into account, or when there is no real wire but only bulk current (eddy current, currents in a tokamak plasma, current within the sun, ...) the concept of inductance becomes less clear and the full Maxwell's equations are needed to give a complete description of such a system. The Lenz law is still valid and described quantitatively by the Maxwell's equations.

Now, in steady-state conditions (constant current, "DC") the self-inductance of a circuit has no effect on anything. The electromotive forces depend on the inductances and only on the time-derivative of the currents (and not on the currents themselves). Therefore when the currents are constant there are no electromotive forces, no "induction". But this does not mean there is no inductance ...

Remember the Lenz law:

(Electromotive force in a loop) = - d(PHI)/dt

where PHI is the flux of the magnetic field through this loop.
When the magnetic field is constant, the flux is constant and therefore the electromagnetic force is zero: no "induction".

 

[scientist91]

Moving charges create magnetic fields. Moving charges do not necessarily create moving magnetic fields. If all of the electrons in the conductor are moving at (on average) a constant speed, then the magnetic field is constant. It doesn't move.

Think about this. For every electron that moves forward, another electron behind it moves up to take its place. So the magnetic field never goes away.

Does that help?

Ok, thank you very much for the effort that you put into help me. This which Xezlec said helped me a lot to understand the real thing. I think the speed doesn't matter. In the direct current, the electrons can also have different speed. I think that the strong of the magnetic field depends from the speed.

 

[scientist91]

Explain please, how do the magnetic field is moving (does it change or move?) when the speed of the current is different. Thank you.

 

[Hootenanny]

From Ampere's law we can show that magnetic field of a long straight wire is directly proportional to the current flowing through the wire. In other words, the greater the current, the greater [stronger] the resultant magnetic field.

 

[scientist91]

From Ampere's law we can show that magnetic field of a long straight wire is directly proportional to the current flowing through the wire. In other words, the greater the current, the greater [stronger] the resultant magnetic field.

And when the magnetic field is changing?

 

[Hootenanny]

And when the magnetic field is changing?

Provided the current is constant, the magnetic field is stationary.

 

[Xezlec]

Explain please, how do the magnetic field is moving (does it change or move?) when the speed of the current is different. Thank you.

First of all, when the magnetic field changes, it usually moves. And when it moves, it usually changes. So I use "change" and "move" to mean the same thing. "Change" is probably a better word.

When the current is greater, the field is also greater. The faster an electron moves, the stronger the magnetic field it creates.

In direct current, the average speed of all the electrons in the wire doesn't change in time. So, the magnetic field is constant (because it is the combination of the magnetic fields due to all the electrons).

If you increase the current, then you increase the average speed of the electrons, so the magnetic field gets stronger. A magnetic field getting stronger is a change. During that change, while you are increasing the current, self-induction happens.

 

[scientist91]

First of all, when the magnetic field changes, it usually moves. And when it moves, it usually changes. So I use "change" and "move" to mean the same thing. "Change" is probably a better word.

When the current is greater, the field is also greater. The faster an electron moves, the stronger the magnetic field it creates.

In direct current, the average speed of all the electrons in the wire doesn't change in time. So, the magnetic field is constant (because it is the combination of the magnetic fields due to all the electrons).

If you increase the current, then you increase the average speed of the electrons, so the magnetic field gets stronger. A magnetic field getting stronger is a change. During that change, while you are increasing the current, self-induction happens.

So let's realize, the magnetic field is not attached on the electron so the magnetic field is not moving with the electron, it stays in one position. All electrons make one magnetic field, and it is created when there is current inside the conductor (electric field) and it is changing (moving) when there is different speed of the electrons.

 

[Hootenanny]

So let's realize, the magnetic field is not attached on the electron so the magnetic field is not moving with the electron, it stays in one position. All electrons make one magnetic field, and it is created when there is current inside the conductor (electric field) and it is changing (moving) when there is different speed of the electrons.

Sounds about right to me

 

[scientist91]

And on all things that can be magnetized, are the magnetic fields are created around the electrons?

 

[rcgldr]

Wouldn't the magnetic field components be simply following the flow of individual eletrons? It would only be the sum of these components that wouldn't be flowing.

As an analogy, imagine a process where energy was converted into electrons at one end of a evacuated tube, that the electrons traveled to the other end of a tube where they were converted back into energy. You'd have both tiny magnetic and gravitation fields moving along with the electrons. Only when the density of this stream of electrons got sufficiently high enough would these tiny fields sum up to appear as a single continuous non-moving field.