how electromagnets work?

[asad118]

dear users,

i have read that in any metal, all the positive and negative charges are randomly scattered. as a result these charges cancel the magnetic feild of each other. but when an electric coil is warped around a metal piece and electric current is passed through it, these charges get aligned, in this way they dont cancel the magnetic field of each other and that metal convert into a magnet.

 

i would like to ask two questions.

1. is my above explanation correct?

2. what is actually meant by alignment? does it mean that the positive and negative charges move in opposite directions? does it mean

that the electromagnetic effect pull opposite charges away from each other?

[swansont]

dear users,

i have read that in any metal, all the positive and negative charges are randomly scattered. as a result these charges cancel the magnetic feild of each other. but when an electric coil is warped around a metal piece and electric current is passed through it, these charges get aligned, in this way they dont cancel the magnetic field of each other and that metal convert into a magnet.

 

i would like to ask two questions.

1. is my above explanation correct?

2. what is actually meant by alignment? does it mean that the positive and negative charges move in opposite directions? does it mean

that the electromagnetic effect pull opposite charges away from each other?

 

Moving charges cause magnetic fields. When the charges are not in net motion, i.e. no current, there's no field. It's not that there's a huge field and it cancels. There's no net field of the protons and electrons. (the particles each have a small field, but random orientation will cancel this on a small scale)

 

Looking at it more deeply, the result is one of special relativity and how fields look in different reference frames, but that's more advanced and can be confusing if you're not familiar with relativity.

[daniton]

 

When the charges are not in net motion, i.e. no current, there's no field. It's not that there's a huge field and it cancels. There's no net field of the protons and electrons.

I think asad118 is trying to say moving electrons in metal atom assumed in the whole metal.

[Enthalpy]

Positive charges, that is atom nuclei, make no significant contribution to electromagnets, permanent magnets, nor magnetic materials. Only electrons do, which are the negative charges.

 

Each electron creates a small magnetic field, through its spin and through its orbital momentum. This ultimately makes the field in a ferromagnetic material, and it needs no movement by the electrons, at least not a movement a usually conceived.

 

Most electrons in matter come in pairs, which cancels out both sources of field, spin and orbital. That's not even random.

 

In some materials (iron, oxygen...) a few electrons are not paired and create individual magnetic fields.

 

In the ferromagnetic subset of these materials, the nonpaired electrons interact strongly enough that their orientation is not random. More surprisingly, this interaction can result in a net global field instead of a global cancellation.

 

One simple case: the material alternates regularly stronger and weaker individual magnetic fields (hence of orbital origin). Magnets side-by-side align spontaneously in opposite directions, but if strong and weak magnets alternate, it can result in all strong ones pointing in one direction, outwhelming all weak ones pointing in the opposite direction.

 

In this case, the material gets spontaneously magnetized. Though, this works only in microscopic volumes called Weiss domains. At a bigger scale, the Weiss domains orient randomly or even against an other to cancel out the field.

 

Now, in some materials called permanent magnets, they don't cancel out. Or in the core of an electromagnet, the external influence of the current in the coil lets the well-oriented Weiss domains grow, taking volume from the badly-orientated Weiss domains, so that the the core is magnetized as long as the external current flows. This is a ferromagnetic material.

 

Depending on how easily Weiss domains can grow and shrink (or equivalently, the Bloch boundaries who separate them can move), the material is "magnetically soft" (electromagnet's core: easy to orient, but loses this orientation) or "magnetically hard" (permanent magnet: hard to orient, but keeps the orientation). There is no clear limit between both, with some materials being semi-hard; as well, soft materials still keep some little remanence, in a variable amount very important to transformers or motors.

 

Magnetic hardness depends a lot on the composition, thermal history and deformation history of the material; for iron, magnetically hard goes with mechanically hard - hence the name - but not for all materials.

[swansont]

Ah, I see. The question is about the core, and not the electromagnet itself. The latter half of Enthalpy's post touches on this: instead of the random alignment of the local fields canceling, as they do when there is no external field present, they tend to align with the external field, adding to it.