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EPS 102 lecture 14

# EPS 102 lecture 14 - EPS 102 lecture 14 Tuesday March 10th...

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EPS 102 lecture 14 Tuesday March 10 th , 2009 Magnetic fields: this topic is going to take us from thinking about internal processes and heat transport and shift us to the climate. Magnetic fields are generated on the interior of the earth and are generated because the planet cools and because we have plate tectonics. They also protect us from solar wind and cosmic rays. This is one of the oldest scientific pursuits: the modern view of magnetic fields is that it is produced in the internal Earth. “The Earth itself is a magnet, and it was just like a sphere with a gigantic bar magnet inside of it.” The Earth had a dipolar field structure, but we know that the interior is way too hot for a bar magnet to be sitting there. Magnetic field is an example of a vector field. It has a direction and intensity/amplitude. It produces some magnetic field and to get it’s orientation, we put our compass in various places in space and the needle will align with the direction of the magnetic field. If you have 2 North poles, they repel each other because you are pressing field lines together which resist. The more field lines you have per unit area the stronger the field is. There are forces associated with magnetic fields. If you are an observer (like at mid latitudes) you would see the magnetic field come in at an angle but if you were at the equator they would be horizontal. You can tilt the axis horizontally to get a dip compass where the needle pivots about a horizontal axis so in the mid latitudes, it would point down. Explorers carried both types of compasses. Francis Drake explored the Western Coast of North America and he carried both types, as did James Cook. The intensity of the field observations came way after the 1500’s. Gauss in the 1830’s measured this intensity of the magnetic field. If you really wanted to know if you crossed the equator, the best thing to have would be a dip compass. Pop it out and it will go from being in one direction to the other direction. If you stood on the magnetic north pole, the dip compass would point straight down. You could use magnetic fields to navigate and to find out which direction was North. We could also use astronomical observations for your latitude. But determining longitude was the problem. The magnetic field had bumps and jags and features in it that were deviations from a perfect bar magnet. If you had the magnetic inclination, or the direction the dip compass would point as a function of position. At the equator, it would point perfectly horizontal but if you were in England and wanted to sail to the new world, you would be at a declination of 64 degrees and sail west until you hit 70 degrees and stop. When you went back and measured the field later, it changed. The magnetic equator shifts up and down as does the rest of it as a function of time. It was almost useless for navigation but great for science because we can figure out about the processes that generate them. In a sense there can

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EPS 102 lecture 14 - EPS 102 lecture 14 Tuesday March 10th...

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