Explaining Magnetism with The Domain Theory

In the early 20th century, before scientists properly understood the structure of atoms and how they work, they came up with an easy-to-understand idea called the domain theory to explain magnetism. A few years later, when they understood atoms better, they found the domain theory still worked but could itself be explained, at a deeper level, by the theory of atoms. All the different aspects of magnetism we observe can be explained, ultimately, by talking about either domains, electrons in atoms, or both. Let’s look at the two theories in turn.

Explaining Magnetism with The Domain Theory

Explaining Magnetism with The Domain Theory

Imagine a factory somewhere that makes little bar magnets and ships them out to schools for their science lessons. Picture a guy called Dave who has to drive their truck, transporting lots of cardboard boxes, each one with a magnet inside it, to a different school. Dave doesn’t have time to worry which way the boxes are stacked, so he piles them inside his truck any old how. The magnet inside one box could be pointing north while the one next to it is pointing south, east, or west. Overall, the magnets are all jumbled up so, even though magnetic fields leak out of each box, they all cancel one another out.

The same factory employs another truck driver called Bill who couldn’t be more different. He likes everything tidy, so he loads his truck a different way, stacking all the boxes neatly so they line up exactly the same way. Can you see what will happen? The magnetic field from one box will align with the field from all the other boxes… effectively turning the truck into one giant magnet. The cab will be like a giant north pole and the back of the truck a huge south pole!

What happens inside these two trucks is what happens on a tiny scale inside magnetic materials. According to the domain theory, something like an iron bar contains lots of tiny pockets called domains. Each domain is a bit like a box with a magnet inside. See where we’re heading? The iron bar is just like the truck. Normally, all its onboard “boxes” are arranged randomly and there’s no overall magnetism: the iron is not magnetized. But arrange all the boxes in order, make them all face the same way, and you get an overall magnetic field: hey presto, the bar is magnetized. When you bring a magnet up to an unmagnetized iron bar and stroke it systematically and repeatedly up and down, what you’re doing is rearranging all the magnetic “boxes” (domains) inside so they point the same way. magnetic modules for wind power

This theory explains how magnetism can arise, but can it explain some of the other things we know about magnets? If you chop a magnet in half, we know you get two magnets, each with a north and south pole. That makes sense according to the domain theory. If you cut a magnet in half, you get a smaller magnet that’s still packed with domains, and these can be arranged north-south just like in the original magnet. What about the way magnetism disappears when you hit a magnet or heat it? That can be explained too. Imagine the van full of orderly boxes again. Drive it erratically, at really high speed, and it’s a bit like shaking or hammering it. All the boxes will jumble up so they face different ways and the overall magnetism will disappear. Heating a magnet agitates it internally and jumbles up the boxes in much the same way.

Explaining magnetism with the atomic theory

The domain theory is easy enough to understand, but it’s not a complete explanation. We know that iron bars aren’t full of boxes packed with little magnets—and, if you think about, trying to explain a magnet by saying it’s full of smaller magnets isn’t really an explanation at all, because it immediately prompts the question: what are the smaller magnets made of? Fortunately, there’s another theory we can turn to.

Back in the 19th century, scientists discovered they could use electricity to make magnetism and magnetism to make electricity. James Clerk Maxwell said that the two phenomena were really different aspects of the same thing—electromagnetism—like two sides of the same piece of paper. Electromagnetism was a brilliant idea, but it was more of a description than an explanation: it showed how things were rather than explaining why they were that way. It wasn’t until the 20th century, when later scientists came to understand the world inside atoms, that the explanation for electromagnetism finally appeared. bonded injection magnets classification

Explaining magnetism with the atomic theory

Explaining magnetism with the atomic theory

We know everything is made of atoms and that atoms are made up of a central lump of matter called the nucleus. Minute particles called electrons move around the nucleus in orbit, a bit like satellites in the sky above us, but they also spin on their axis at the same time (just like spinning tops). We know electrons carry electric currents (flows of electricity) when they move through materials such as metals. Electrons are, in a sense, tiny particles of electricity. Now back in the 19th century, scientists knew that moving electricity made magnetism. In the 20th century, it became clear that magnetism was caused by electrons moving inside atoms and creating magnetic fields all around them. Domains are actually groups of atoms in which spinning electrons produce an overall magnetic field pointing one way or another.

Like the domain theory, atomic theory can explain many of the things we know about magnets, including paramagnetism (the way magnetic materials line up with magnetic fields). Most of the electrons in an atom exist in pairs that spin in opposite directions, so the magnetic effect of one electron in a pair cancels out the effect of its partner. But if an atom has some unpaired electrons (iron atoms have four), these produce net magnetic fields that line up with one another and turn the whole atom into a mini magnet. When you put a paramagnetic material such as iron in a magnetic field, the electrons change their motion to produce a magnetic field that lines up with the field outside.

What about diamagnetism? In diamagnetic materials, there are no unpaired electrons, so this doesn’t happen. The atoms have little or no overall magnetism and are less affected by outside magnetic fields. However, the electrons orbiting inside them are electrically charged particles and, when they move in a magnetic field, they behave like any other electrically charged particles in a magnetic field and experience a force. That changes their orbits very slightly, producing some net magnetism that opposes the very thing that causes it (according to the classic bit of electromagnetic theory know as Lenz’s law, which is related to the law of conservation of energy). As a result, the weak magnetic field they produce opposes the magnetic field that causes it—which is exactly what we see when diamagnetic materials try to “fight” the magnetic field they’re placed in.

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