lecture 10

# lecture 10 - 21.1 Magnetic Fields Magnetism has been...

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21.1 Magnetic Fields Magnetism has been observed since roughly 800 B.C. Certain rocks on the Greek peninsula of Magnesia were noticed to attract and repel one another. Hence the word: Magnetism. So just like charged objects, magnetized objects can exert forces on each other – repulsive or attractive. A magnet has two poles , a North and a South: Like poles repel and opposite poles attract: D N S Similar to electric charges, but magnetic poles always come in pairs. They never exist as a single pole called a magnetic monopole . N S N e get two magnets each with S S N Break in half We get two magnets, each with two poles!

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Electric charges produce electric fields and magnets produce magnetic fields . We used a small positive charge (test charge) to determine what the electric field lines looked like around a point charge. Can we do a similar thing to determine what the magnetic field lines look like around a magnet??? Yes! We can use a small magnet called a compass ! The compass needle is free to pivot, and its tip (the North pole) will point toward the South pole of another magnet. D So the field lines around a bar magnet look like this: B We represent the Magnetic Field with a capitol B:
B 1. They point away from North poles and point toward South poles. The compass needle will line up in the direction of the field! Properties of Magnetic Field Lines: 2. The magnetic field at any point in space is tangent to the field line at that point. 3. The higher the density of field lines, the stronger the field. Thus, the strongest field is near the poles!!! 4. The field lines must form closed loops, i.e. they don’t start or top in mid space stop in mid space. N There are no magnetic monopoles! Since a compass needle points North on the surface of the earth, the earth must have a magnetic field , and its South pole , called agentic orth must be in the northern hemisphere Magentic North , must be in the northern hemisphere.

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## This note was uploaded on 08/04/2009 for the course PHYS 2002 taught by Professor Blackmon during the Spring '08 term at LSU.

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lecture 10 - 21.1 Magnetic Fields Magnetism has been...

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