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Magnetism

Magnetism. Magnets. A magnet has polarity - it has a north and a south pole; you cannot isolate the north or the south pole (there is no magnetic monopole) Like poles repel; unlike poles attract. Magnets.

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Magnetism

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  1. Magnetism

  2. Magnets • A magnet has polarity - it has a north and a south pole; you cannot isolate the north or the south pole (there is no magnetic monopole) • Like poles repel; unlike poles attract

  3. Magnets • A compass is a suspended magnet (its north pole is attracted to a magnetic south pole); the earth’s magnetic south pole is within 200 miles of the earth’s geographic north pole (that is why a compass points "north")

  4. Magnets • Some metals can be turned into temporary magnets by bringing them close to a magnet; magnetism is induced by aligning areas called domains within a magnetic field • Domains strong coupling between neighboring atoms of ferromagnetic materials to form large groups of atoms whose net spins are aligned • Unmagnetized substance  domains randomly oriented

  5. Magnets • When an external magnetic field is applied the orientation of the magnetic fields of each domain may change to more closely align with the external magnetic field • Domains already aligned with the external field may grow at the expense of others

  6. Magnets • Materials can be classified as magnetically hardor soft • Soft – like iron - are easily magnetized, but lose magnetism easily  once an external field is removed, the random motion of the particles in the material changes the orientation of the domains  the material returns to an unmagnetized state

  7. Magnets • Hard – like cobalt and nickel – difficult to magnetize, but retain their magnetism  domain alignment persists after an external field is removed  the result is a permanent magnet

  8. Magnetic Fields • The concept of a field is applied to magnetism as well as gravity and electricity. • A magnetic field surrounds every magnet and is also produced by a charged particle in motion relative to some reference point. • B = F____ q0(v*sinq)

  9. Magnetic Fields • The direction of a magnetic field, B, at any location is defined as the direction in which the north pole of a compass needle points at that location

  10. Magnetic Fields • To indicate direction on paper we use the following conventions:  Arrows show direction in the plane of the page X Crosses represent the tail of an arrow and show direction into the page . Dots represent the tips of arrows and show direction out of the page

  11. Magnetic Force • A charge moving through a magnetic field experiences a force Fmagnetic =qv(sinq)B  q –magnitude of charge, in Coulombs (C)  v –velocity of charge, in m/s and must have a component perpendicular to the field  B –magnetic field strength, in Teslas (1T=Ns/Cm)  no magnetic force acts on a stationary charge

  12. Magnetic Force • Use the right-hand rule to find the direction of the magnetic force • Magnetic force is always perpendicular to both v and B • Place your fingers in the direction of B with your thumb pointing in the direction of v • The magnetic force on a positive charge is directed out of the palm of your hand • If q is negative, find the direction as if q were positive and reverse the direction

  13. The Circular Trajectory • Consider a positively charged particle moving perpendicular to a magnetic field • Since the magnetic force always remains perpendicular to the velocity the magnetic force causes the particle to move in a circular path • The force according to the RHR is directed to the center of the circular path

  14. The Circular Trajectory • Since Fmag = qvB and Fc = mv2/r then qvB = mv2/r and r = mv/qB

  15. Magnetic Fields Produced by Currents • A current carrying wire produces a magnetic field of its own • Discovered by Hans Christian Oersted in 1820 • Marked the beginning of electromagnetism • r  radial distance • μ0 permeability of free space = 4π x 10-7 Tm/A

  16. Magnetic Field of a Current Carrying Wire • The direction of this field can be determined using the right-hand rule.  Grasp the wire in the right hand with your thumb in the direction of the current  Your fingers will curl in the direction of the magnetic field

  17. Magnetic Field of a Current Loop • You can use the right-hand rule to determine the field around a current carrying loop • Regardless of where you are on the loop the magnetic field inside of the loop is always the same direction - upward

  18. Magnetic Field of a Current Loop • Solenoids – produce strong magnetic fields by combining several loops of wire together  are important in many applications because they act as a magnet when it carries current  magnetic field can be increased by inserting an iron rod through the center of the coil creating an electromagnet

  19. Magnetic Force on a Current-Carrying Conductor • Current electricity is charged particles in motion • Since charged particles moving in a magnetic field experience a force, likewise a current-carrying wire placed in a magnetic field also experiences a force

  20. Magnetic Force on a Current-Carrying Conductor • Fmagnetic = BILsinө • B  Magnetic field strength in Teslas (T) • I  Current • L  length of conductor within B

  21. Magnetic Force on a Current-Carrying Conductor • To find the direction of the magnetic force on a wire we again use the right-hand rule • You place your thumb in the direction of the current (I) in the wire rather than the velocity (v) • Your fingers as before are in the direction of the magnetic field B • The magnetic force comes out of your palm

  22. Magnetic Force on a Current-Carrying Conductor • Current-carrying wires placed close together exert magnetic forces on each other  when current runs in the same direction the wires attract one another  when current runs in opposite directions the wires repel one another

  23. Magnetic Force on a Current-Carrying Conductor • Loudspeakers use magnetic force to produce sound • Most speakers consist of a permanent magnet, a coil of wire and a flexible cone • A sound signal is converted to a varying electrical signal and is sent to the coil • The current causes a magnetic force to act on the coil

  24. Magnetic Force on a Current-Carrying Conductor • When the current reverses direction, the magnetic force on the coil reverses direction, and the cone accelerates in the opposite direction • Alternating force on the coil results in vibrations of the attached cone, which produces variations in the density of air in front of it, or sound waves

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