• Question: i want to know about the origin of magnetic field. so many times i scratched my head but didn't got any answer.

    Asked by nuclearamit to Chris, Dave, David, Fiona, Jack on 19 Jun 2013.
    • Photo: Jack Miller

      Jack Miller answered on 19 Jun 2013:


      You’re right to scratch your head — this question puzzled a large number of people for a very long time, and there are many things about it we still don’t understand! To be honest, it’s taken me about five years to get a grip on magnetism — it’s a complicated property of matter, and it has its origins in quantum mechanics. I’ll try and explain it as best I can, and this will probably be a complete wall of text, so bear with me!

      Electromagnetism is one of four fundamental forces in nature — the other three being gravity (the weakest), and two nuclear forces (the strong force, which holds quarks together inside protons and neutrons, and the weak force, which is responsible for some times of radioactive decay). The reason “electromagnetism” is called “electromagnetism” is because electricity and magnetism really are two different sides of the same coin. Let’s think of a wire carrying a current next to a small charge that’s moving. You’ll know (and can see by experiment) that the current moving through the wire generates a magnetic field, and that magnetic field exerts a force on the moving charge.

      Let’s look at things a bit differently. If there wire’s carrying a current, then we can think of it as having lots of charges moving along the wire at a certain speed. Let’s say we’ve got a lot of positive charges, moving to the right, whilst an equal number of negative charges remain at rest. If the wire is electrostatically neutral, the distance between adjacent positive charges must be the same as the distance between adjacent negative charges. Now, here comes the tricky bit: let’s say that we’ve got a positive charge moving to the right at the same speed as all of those positive charges flowing in the wire. Then if we imagine putting a little video camera on that moving positive charge — so that we see things from the frame of reference of that charge — we’d see something a bit different. We’d see lots of _negative_ charges flowing to the _left_, and it turns out that the wire as a whole would look negatively charged. The charge would therefore experience a purely _electrostatic_, rather than magnetic attraction to the wire (opposite electrical charges attract). Since all we’ve done in this different description is look at things from a different perspective, and nature doesn’t care from what angle we look at it, we have to conclude that electricity and magnetism are the same.

      This then sets us up to understand a whole lot more about how electromagnetic fields come about in nature. Electrons are charged, and they move. Protons are charged, and they move too, albeit nowhere near as much as electrons. Let’s imagine an electron orbiting an atom. It’s a charge moving around, and it generates a magnetic field. Now, if you imagine another electron moving around, it’ll generate a magnetic field too. The strength of the overall field in space will depend on the position of the fields produced by the two moving charges. You can imagine that it’s possible to have a smaller resulting field, or a larger one depending on the overall orientation of these two spinning charges.

      In a ferromagnet, i.e. a bar magnet you’d find at school, it turns out that there are lots of small domains where all of these little fields produced by moving electrons add up constructively, and produce a “magnetic domain” pointing in a particular direction. There are then lots of other domains pointing in other directions, and, by having an excess of domains pointing in one direction, it’s possible to get a net magnetic moment overall, producing a bar magnet. This behaviour is “emergent”, in that it spontaneously arises from the smaller bits of matter.

      There are other types of magnetism than ferromagnetism, however you’re unlikely to come across them in everyday life. Most matter is either paramagnetic or diamagnetic, which means that it behaves itself like a magnet does, _but only when you put it IN a magnetic field_. The difference between paramagnetism and diamagnetism is essentially the direction the matters’ “acting” magnet is pointing — in a diamagnetic material the generated magnetisation is opposite that of the applied, external magnetic field (‘dia’ meaning ‘across’ in greek, like a diagonal line…or, I guess, Diagon Alley…). Paramagnetic materials are the opposite — the generated magnetisation is in the same direction as the field.

      The best example of diagmagnetism in action I can probably think of is this: !

      Hope this helps — it’s a complicated subject, and not everything is really known!

      — Jack

    • Photo: Chris Mansell

      Chris Mansell answered on 19 Jun 2013:


      Jack’s answer is superb. I’ve clicked on the “like” button. If I can think of anything helpful to add, I will but for the moment, Jack’s answer seems very comprehensive.

    • Photo: Fiona Coomer

      Fiona Coomer answered on 19 Jun 2013:


      The simplest way to think of magnetism is simply from the movement of electric charge in a loop. Have you ever made an electromagnet by coiling some wire around a piece of iron, and passing a current through it? In this case, when the current is switched on in the wire, the (negatively charged) electrons move around the wire, and this movement of charge creates a magnetic field. This then magnetises the piece of iron.

      A permanent magnet (like a bar magnet) contains atoms or ions in it with an odd number of electrons (or an even number but ‘unpaired’). As the unpaired electrons move in their orbital about the nucleus of the atom, this movement of charge (we call this spin) means that the atom (or ion) acts like a tiny electromagnet. In a permanent magnet, the direction of the magnetic field (we call this the magnetic moment) on each atom lines up with the one on the next atom forming domains where all the moments point in the same direction (we call this a ferromagnet). These magnetic fields add up to give a magnetic field big enough to measure, or even pick up paper clips!!

      The earth’s magnetic field arises in a similar way – from the motion of molten iron alloys in the outer core of the earth.

    • Photo: David Freeborn

      David Freeborn answered on 19 Jun 2013:


      I agree with Chris- that was a really good explanation from Jack. I don’t think there’s much more to add there! 😀

      You can watch a cool video of the frog levitating here: https://www.youtube.com/watch?v=A1vyB-O5i6E

      And here is an amazing video of a superconductor being “locked” into a magnetic field here. This is one of the amazing properties of superconductors:

    • Photo: Dave Farmer

      Dave Farmer answered on 19 Jun 2013:


      I’m going to leave it to my favourite scientist to answer, take it away Richard Feynman:

      Stick with it, he rambles a bit a the start…

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