PHYA22 Chapter 33.doc

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Physics and Astrophysics
Brian Wilson

Chapter 33 The Magnetic FieldMagnetismMagnetism is not the same as electricityMagnetism is a longrange forceMagnets have two poles the north and the south poles Two like poles exert repulsive forces on each other and two opposite poles exert attractive forces on each otherMagnetic Materials materials that are attracted to or that a magnet sticks to iron nickel and cobalt and is attracted to pole poles of a magnetMagnetic Dipole a permanent north and south pole cutting a magnet in half yields two weaker but still complete magnetsMagnetic Monopole an isolated magnetic pole never been observed yetThe geographic north pole is actually a south magnetic pole and is offset slightly from the geographic poles of the earths rotation axisThe Discovery of the Magnetic FieldAn electric current causes magnetismRightHand Rule relates orientation of the compass needles to the direction of the currentoPoint your right thumb in the direction of the currentoCurl your fingers around the wire to indicate a circleoYour fingers point in the direction of the magnetic field lines around the wirex current into the page current out of the pageMagnetic Field BoA magnetic field is created at all points in space surrounding a currentcarrying wireoMagnetic field at each point is a vector It has both a magnitude B and a directionoMagnetic field exerts forces on magnetic poles The force on a north pole is parallel to B and the force on the south pole is opposite BThe pair of opposite forces exerts a torque on the needle rotating the needle until it is parallel to the magnetic field at that pointMagnetic forces cause a compass needle to become aligned parallel to a magnetic field with the north pole of the compass showing the direction of the magnetic field at that pointBecause compass needles align with the magnetic field the magnetic field at each point must be tangent to a circle around the wireMagnetic Field Lines imaginary lines drawn through a region of space so that a tangent to a field line is in the direction of the magnetic field and the field lines are closer together where the magnetic field strength is largerThe Source of the Magnetic Field Moving ChargesMoving charges are the source of the magnetic field2BiotSavart Law Bm4pqvsinqr direction given by the rightpoint charge0hand rule where q is the charge v is the velocity r is the distance from the charge and q is the angle between v and rThe SI unit of magnetic field strength is tesla T and 1 T1 NAm6Permeability Constant m m1257 x 10 TmA00RightHand Rule in finding B
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