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Magnetism: Crash Course Physics #32


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Duration: 09:47
Uploaded: December 01, 2016
Information as of 2017-01-13 17:30



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This episode was sponsored by Prudential. Go to http://Raceforretirement.com and see how quickly 1% can add up.

You’re probably familiar with the basics of magnets already: They have a north pole and a south pole. Two of the same pole will repel each other, while opposites attract. Only certain materials, especially those that contain iron, can be magnets. And there’s a magnetic field around Earth, which is why you can use a compass to figure out which way is north. In this episode of Crash Course Physics, Shini takes us into the world of magnetism!


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[intro music]

This episode is supported by Prudential

(0:06) So it's friday, April 21,1820. A physics professor in Denmark named Hans Christian Oersted is in the middle of a lecture and he's using a compass and an electric wire for a demonstration.

(0:17) He turns on the current that runs through the wire and he notices that the needle in the compass starts to move and when he turns the current off, the needle moves back to where it was. Then he runs the current through the wire in the opposite direction and sees the sees the needle move the other way.

(0:29) What Oersted demonstrated that day was a fundamental discovery, the connection between electricity and magnetism, and it changed the field of physics forever. The relationship between electricity and magnetism not only explains what Oersted and his students witnessed in 1820, it also makes possible much of the technology that's used today, from hydroelectric dams to your smart phone.

(0:49) It even explains why Earth's magnetic field is essentially keeping you from being cooked alive right now. And all you really need to understand the basics of magnetism and electricity is this.

[intro plays]

(1:11) You're probably familiar with the basics of magnets already. They have a north and south pole. Two of the same pole will repel each other while opposites attract. Only certain materials, especially those that contain iron, can become magnets. It depends on their molecular properties and other metals including cobalt, nickel, and iron are attracted to magnets even though they aren't magnets themselves like the metal in your refridgerator door. And there's a magnetic field around Earth, which is why you can use a compass to figure out which way is north. The magnet in the compass aligns itself with the Earth's magnetic field

(1:40) just as we use electric field lines to represent the electric field created by charges, we can draw magnetic field lines to represent magnetic fields created by magnets. And as with electric fields the more crowded the lines are, the stronger the magnetic field.

(1:54) The lines point from the north pole to the south pole like how electric field lines point from the positive to the negative charge, but there's a key difference. You can have an electric field spreading outward from a single electric charge but that can't happen with magnets because you can't isolate one magnetic pole.

(2:09) If you chop a bar magnet in half, you don't end up with one north magnet and one south magnet. You end up with two magnets each with it's own north pole and south pole. This means that the magnetic field lines surrounding a magnet always form closed loops.

(2:22) We measure magnetic fields using a unit called the Tesla, which is one Newton per amp per metre and one Tesla is a very strong magnetic field. The fields from some of the strongest superconducting magnets in the world are only 10 Teslas.

(2:35) Now when Oersted was doing his demonstration on that fateful day in 1820, he was using a regular compass magnet. But when he brought the magnet close to a wire carrying a current, the magnetic field from that current exerted a force on the needle, moving it to point in a different direction.

(2:48) Oersted had discovered one of the fundamental principles of electromagnetism: an electric current produces a magnetic field. After a few more months of experimenting, Oersted figured out that when a current runs through a wire, the magnetic field that it produces surrounds the wire. Expressed with field lines, the magnetic field would appear as circle, with the wire at their center.


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