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)

MagnetismMagnetism

MagnetsMagnets

► A A magnet magnet has polarity - it has a has polarity - it has a north and a south pole; you cannot north and a south pole; you cannot isolate the north or the south pole isolate the north or the south pole (there is no magnetic monopole) (there is no magnetic monopole)

► Like poles repel; unlike poles attract Like poles repel; unlike poles attract

MagnetsMagnets

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

MagnetsMagnets► Some metals can be turned into Some metals can be turned into

temporary magnets by bringing them temporary magnets by bringing them close to a magnet; magnetism is close to a magnet; magnetism is induced by aligning areas called induced by aligning areas called domainsdomains within a magnetic field within a magnetic field

► DomainsDomains strong coupling between strong coupling between neighboring atoms of ferromagnetic neighboring atoms of ferromagnetic materials to form large groups of materials to form large groups of atoms whose net spins are alignedatoms whose net spins are aligned

► Unmagnetized substance Unmagnetized substance domains domains randomly orientedrandomly oriented

MagnetsMagnets

►When an external magnetic field is When an external magnetic field is applied the orientation of the magnetic applied the orientation of the magnetic fields of each domain may change to fields of each domain may change to more closely align with the external more closely align with the external magnetic fieldmagnetic field

►Domains already aligned with the Domains already aligned with the external field may grow at the expense external field may grow at the expense of othersof others

MagnetsMagnets

►Materials can be classified as magnetically Materials can be classified as magnetically hardhard or or softsoft

► SoftSoft – – like ironlike iron - are easily magnetized, but lose - are easily magnetized, but lose magnetism easilymagnetism easily once an external field is removed, the once an external field is removed, the random motion of the particles in the material random motion of the particles in the material

changes the orientation of the domainschanges the orientation of the domains the material returns to an unmagnetized the material returns to an unmagnetized statestate

MagnetsMagnets

► HardHard – – like cobalt and nickellike cobalt and nickel – difficult – difficult to magnetize, but retain their to magnetize, but retain their magnetismmagnetism

domain alignment persists after an domain alignment persists after an external field is removedexternal field is removed

the result is a the result is a permanent magnetpermanent magnet

Magnetic FieldsMagnetic Fields

►The concept of a field is applied to The concept of a field is applied to magnetism as well as gravity and magnetism as well as gravity and electricity.electricity.

►A A magnetic fieldmagnetic field surrounds every surrounds every magnet and is also produced by a magnet and is also produced by a charged particle in motion relative to charged particle in motion relative to some reference point. some reference point.

►B = B = F____ F____

qq00(v*sin(v*sin))


►The direction of a The direction of a magnetic fieldmagnetic field, , BB, , at any location is defined as the at any location is defined as the direction in which the north pole of a direction in which the north pole of a compass needle points at that locationcompass needle points at that location


►To indicate direction on paper we use To indicate direction on paper we use the following conventions:the following conventions:

ArrowsArrows show direction in the plane show direction in the plane of of the page the page

X CrossesX Crosses represent the tail of an represent the tail of an arrow arrow and show direction and show direction into the pageinto the page

. Dots. Dots represent the tips of arrows and represent the tips of arrows and show direction out of the pageshow direction out of the page

Magnetic ForceMagnetic Force

►A charge moving through a magnetic A charge moving through a magnetic field experiences a forcefield experiences a force

FFmagneticmagnetic =qv(sin =qv(sin)B)B q –magnitude of charge, in Coulombs (C)q –magnitude of charge, in Coulombs (C) v –velocity of charge, in m/s and must have a v –velocity of charge, in m/s and must have a

component perpendicular to the fieldcomponent perpendicular to the field B –magnetic field strength, in Teslas B –magnetic field strength, in Teslas

(1T=Ns/Cm) (1T=Ns/Cm) no magnetic force acts on a stationary no magnetic force acts on a stationary chargecharge

Magnetic ForceMagnetic Force►Use the right-hand rule to find the Use the right-hand rule to find the

direction of the magnetic forcedirection of the magnetic force►Magnetic force is always perpendicular to Magnetic force is always perpendicular to

both both vv and and BB►Place your fingers in the direction of Place your fingers in the direction of BB

with your thumb pointing in the direction with your thumb pointing in the direction of of vv

►The magnetic force on a positive charge The magnetic force on a positive charge is directed is directed outout of the palm of your hand of the palm of your hand

► If q is negative, find the direction as if q If q is negative, find the direction as if q were positive and reverse the directionwere positive and reverse the direction

The Circular TrajectoryThe Circular Trajectory

►Consider a positively charged particle Consider a positively charged particle moving perpendicular to a magnetic fieldmoving perpendicular to a magnetic field

►Since the magnetic force always remains Since the magnetic force always remains perpendicular to the velocity the perpendicular to the velocity the magnetic force causes the particle to magnetic force causes the particle to move in a circular pathmove in a circular path

►The force according to the RHR is The force according to the RHR is directed to the center of the circular pathdirected to the center of the circular path

The Circular TrajectoryThe Circular Trajectory

►Since FSince Fmagmag = qvB and F = qvB and Fcc = mv = mv22/r then/r then

qvB = mvqvB = mv22/r/r

andand

r = mv/qBr = mv/qB

Magnetic Fields Produced by Magnetic Fields Produced by CurrentsCurrents

► A current carrying wire produces a magnetic A current carrying wire produces a magnetic field of its ownfield of its own

► Discovered by Hans Christian Oersted in 1820Discovered by Hans Christian Oersted in 1820► Marked the beginning of Marked the beginning of electromagnetismelectromagnetism

► r r radial distance radial distance► μμ00 permeability of free spacepermeability of free space = 4 = 4ππ x 10 x 10-7-7

Tm/ATm/A

r

IB

20

Magnetic Field of a Current Magnetic Field of a Current Carrying WireCarrying Wire

►The direction of this field can be The direction of this field can be determined using the determined using the right-hand right-hand rulerule..

Grasp the wire in the right hand Grasp the wire in the right hand with your thumb in the direction of the with your thumb in the direction of the currentcurrent

Your fingers will curl in the direction Your fingers will curl in the direction of the magnetic fieldof the magnetic field

Magnetic Field of a Current Magnetic Field of a Current LoopLoop

►You can use the You can use the right-hand ruleright-hand rule to to determine the field around a current determine the field around a current carrying loop carrying loop

►Regardless of where you are on the Regardless of where you are on the loop the magnetic field inside of the loop the magnetic field inside of the loop is always the same direction - loop is always the same direction - upwardupward

Magnetic Field of a Current Magnetic Field of a Current LoopLoop

►SolenoidsSolenoids – produce strong magnetic – produce strong magnetic fields by combining several loops of wire fields by combining several loops of wire togethertogether

are important in many applications are important in many applications because they act as a magnet when it because they act as a magnet when it carries currentcarries current

magnetic field can be increased by magnetic field can be increased by inserting an iron rod through the center inserting an iron rod through the center of the coil creating an of the coil creating an electromagnetelectromagnet

Magnetic Force on a Current-Magnetic Force on a Current-Carrying ConductorCarrying Conductor

►Current electricity is charged particles Current electricity is charged particles in motionin motion

►Since charged particles moving in a Since charged particles moving in a magnetic field experience a force, magnetic field experience a force, likewise a current-carrying wire placed likewise a current-carrying wire placed in a magnetic field also experiences a in a magnetic field also experiences a forceforce


►FFmagneticmagnetic = BI = BILsinLsinөө

► B B Magnetic field strength in Teslas (T) Magnetic field strength in Teslas (T)► I I Current Current► L L length of conductor within B length of conductor within B


►To find the direction of the magnetic force To find the direction of the magnetic force on a wire we again use the right-hand ruleon a wire we again use the right-hand rule

►You place your thumb in the direction of You place your thumb in the direction of the current (I) in the wire rather than the the current (I) in the wire rather than the velocity (v)velocity (v)

►Your fingers as before are in the direction Your fingers as before are in the direction of the magnetic field Bof the magnetic field B

►The magnetic force comes out of your The magnetic force comes out of your palmpalm


►Current-carrying wires placed close Current-carrying wires placed close together exert magnetic forces on each together exert magnetic forces on each otherother

when current runs in the same direction when current runs in the same direction the wires attract one anotherthe wires attract one another

when current runs in opposite when current runs in opposite directions directions the wires repel one anotherthe wires repel one another


►Loudspeakers use magnetic force to Loudspeakers use magnetic force to produce soundproduce sound

►Most speakers consist of a permanent Most speakers consist of a permanent magnet, a coil of wire and a flexible magnet, a coil of wire and a flexible conecone

►A sound signal is converted to a varying A sound signal is converted to a varying electrical signal and is sent to the coilelectrical signal and is sent to the coil

►The current causes a magnetic force to The current causes a magnetic force to act on the coilact on the coil


►When the current reverses direction, When the current reverses direction, the magnetic force on the coil reverses the magnetic force on the coil reverses direction, and the cone accelerates in direction, and the cone accelerates in the opposite directionthe opposite direction

►Alternating force on the coil results in Alternating force on the coil results in vibrations of the attached cone, which vibrations of the attached cone, which produces variations in the density of produces variations in the density of air in front of it, or sound wavesair in front of it, or sound waves

Documents

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)