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# L25_a - MAGNETIC FORCE MAGNETIC FIELD(Chapter 22 Read...

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MAGNETIC FORCE & MAGNETIC FIELD (Chapter 22) Read historical background in 22.a1 Magnetism known of for over 2000 years. Magnets known to have two poles o North points to Earth’s North Pole (which is a south magnetic pole!) o South points to Earth’s South Pole (which is a north magnetic pole!) Poles cannot be separated – no magnetic monopoles (so far) Connection between magnetism and electricity dates to 1819 (Oersted) Important contributions in early 1820’s from Ampére, Faraday, Henry Relationship between magnetic field and changing electric fields and vice versa formulated by Maxwell in 1861

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MAGNETIC FIELD B r Magnetic field is a VECTOR (important) Source can be permanent magnet OR nearby electric current Can use MAGNETIC FIELD LINES to represent how magnetic field depends on location in space N S
Magnetic Field Lines – RULES o Lines start and stop on poles of a magnet OR lines form closed loops around current-carrying wires No such thing as a magnetic monopole For magnet – lines start at North pole and end at South pole o MAGNITUDE of magnetic field | | B r indicated by DENSITY of lines o DIRECTION of magnetic field at a point is tangent to field lines at point Direction of magnetic field at some location is the direction that a compass needle would point at that location. N S

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MAGNETIC FORCE B F r on a moving charged particle (charge q , velocity v r ) Experimental observations show: 1 st : For magnitude, find q F B | | r and v F B | | r BUT 0 = B F r if v r parallel to B r 2 nd : For direction, find that if v r and B r not parallel, then B F r
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