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 ► 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")

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

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

Magnets ► Materials can be classified as magnetically hard or 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

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

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____ q 0 (v*sin  ) q 0 (v*sin  )

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

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

Magnetic Force ► A charge moving through a magnetic field experiences a force F magnetic =qv(sin  )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

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

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

The Circular Trajectory ► Since F mag = qvB and F c = mv 2 /r then qvB = mv 2 /r and r = mv/qB

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 ► μ 0  permeability of free space = 4π x Tm/A

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

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

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

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

Magnetic Force on a Current- Carrying Conductor ► F magnetic = BILsinө ► B  Magnetic field strength in Teslas (T) ► I  Current ► L  length of conductor within B

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

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

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

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