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### Course: Physics library>Unit 13

Lesson 1: Magnets and Magnetic Force

# Introduction to magnetism

An introduction to magnetism. Created by Sal Khan.

## Want to join the conversation?

• When a bar magnet is broken in the middle, what is the effect on its intensity?
• the magnetic intensity would decrease as the distance between the poles decreases.
• at Sal says a magnet is created when the electrons line up. how were lodestones' electrons lined up in the first place?
• Lodestones were igneous rocks, which means that they were originally lava. When the lava came up through a volcano, the rock took a significant time to cool. In this time, the Earth's magnetic field automatically aligned the domains, or atoms, of the lava. Then, when the rock cooled, the atoms were set in place, making the rock magnetic. This is the process that factories use to make bar magnets today.
• when you rub a nail across a magnet many times, the nail becomes magnetized.
why does this work?
• This is because of magnetic induction. Nail is made of iron. Iron is magnetized easily. But you should remember that in iron there are atomic magnets which line up with each other in groups called domains. In an un-magnetized piece of iron the magnetic domains are pointing out in all directions and so cancel out each other. So when we rub the nail with a magnet it becomes magnetized. This is because all the tiny N-Poles add up at one end and all the S-Poles add up at the other end.
• Sal talks about with electrostatics we find that a point charge whether it is an electron or proton creates its own electric "vector" field and it is a monopole. But then as he is talking about magnets he says if you were to cut a magnet in half, the two halves would create a dipole again and again. He also says even if you were to keep cutting it till you had only an electron left it would still remain a dipole. How is this possible if he stated earlier an electron has a monopole?
• The electrostatic charge of an electron is a monopole. The magnetic field of a spinning electron is a dipole.
• okay so what your saying is that the earth is a big huge magnet????
• Yes. Electromagnetism is one of the most common sources of power on the earth. The earth is constructed with a North and South Pole and a Inner core made up of metallic composites presumably and largely iron. The Magnetic field around the earth is driven largely by this source of electromagnetism. In general the poles and the core of the earth are a lot like a battery on a massive scale. This magnetic field around the earth suffers interference from solar particles released from the sun with large electrical charges and cause the Northern Lights (Aurora Borealis). I might be wrong... and I got a little off topic.
• Why can there never be a monopole for a magnet?
• The indications we have from some of the String Theories have given a mass for a magnetic monopole so high that we would need is way beyond anything we could produce, it predicted mass is about a Planck mass.

In quantum field theory the difference between there being electric monopoles but not magnetic ones is because of a broken symmetry.
• what is the difference between electric and magnetic field ??
• Electric fields are made by charges or changing magnetic fields and create force on charge.
Magnetic fields are made by current (moving charge) or changing electric fields and create force on moving charges only.
Magnetic and electric fields are two sides of the same coin but viewed from different frames of reference.
• How can I explain simple magnetic force to children ages 2 1/2-4 years of age? For example when they play with trains that have magnets that connect them, when they don't connect how can I say, well it's the magnet type that doesn't allow them to connect or does let them connect.
• at that age, it make be sufficient for them to experience the phenomenon.

If you can find some magnets for them play with external to the toy train then, as they play, you might find some discussion starts to emerge.
If they are sufficiently engaged, I would encourage use of proper terminology (north and south) since positive and negative can be a) just difficult to comprehend and b) confusing later in lfe

Cheers
• At Sal says magnetism always comes in the form of a dipole, but after watching this video I read a wikipedia article that said the moon does not currently have a dipolar magnetic field. Would this be because the moon's magnetic field is so weak in comparison to ours? Or something else?