physics d--chapter 21 magnet forces and and magnetic fields

Physics D--Chapter 21Physics D--Chapter 21

Magnet ForcesMagnet Forces

and and

Magnetic FieldsMagnetic Fields

MagnetsMagnets

• North and South poleNorth and South pole

– Like poles repelLike poles repel

– Opposite poles attractOpposite poles attract


• Surround magnetized materialSurround magnetized material

• Direction is the direction a Direction is the direction a compass pointscompass points– Away from N, towards SAway from N, towards S

• Strongest at the polesStrongest at the poles

New vector symbolsNew vector symbols

• Into the pageInto the page

• Out of the pageOut of the page

–

–

Earth’s Magnetic FieldEarth’s Magnetic Field

Charged Particles in a Charged Particles in a Magnetic FieldMagnetic Field

• Stationary chargeStationary charge

• Moving perpendicular to fieldMoving perpendicular to field

• Moving Parallel to fieldMoving Parallel to field

• A charge only experiences a force if a A charge only experiences a force if a component of its velocity is component of its velocity is perpendicular to the fieldperpendicular to the field

–No forceNo force

–Maximum forceMaximum force

–Force is zeroForce is zero

Right-Hand Rule #2 (RHR2)Right-Hand Rule #2 (RHR2)

• Used to Used to determine the determine the direction of the direction of the magnetic forcemagnetic force

Magnitude of Magnetic Magnitude of Magnetic FieldField

• Force exerted on a moving test Force exerted on a moving test charge at a certain pointcharge at a certain point

• Symbol => BSymbol => B• Units=>Tesla(T)=>(NUnits=>Tesla(T)=>(Nsec)/(Csec)/(Cm)m)

– 1 gauss(G) = 101 gauss(G) = 10-4 -4 TeslaTesla

€

B =FM

q0(v sinθ )

Example ProblemExample Problem

• An electron experiences a An electron experiences a downward force of 3.0*10downward force of 3.0*10-16-16 N while N while traveling through a magnetic field traveling through a magnetic field of 3.8*10of 3.8*10-3-3 T west. What is the T west. What is the direction and velocity of the direction and velocity of the electron?electron?


€

F = 3.0 ×10−16N

B = 3.8 ×10−3T

q = −1.602 ×10−19C

v = ?

€

v =3.0 ×10−16N

3.8 ×10−3T(−1.602 ×10−19C)

€

v = 4.9 ×105m /s

€

v = 4.9 ×105m /s North€

B =FM

q0(v sinθ )

€

v =FM

Bq0(sinθ )

Path of chargePath of charge

• If a charge If a charge moves moves perpendicular perpendicular to a magnetic to a magnetic field, it will field, it will travel in a circletravel in a circle

Path of chargePath of charge

€

Fc =mv 2

r

€

qvB =mv 2

r

€

FM = Bq(v sinθ )

€

r =mv

qB

Magnetic Force on a Magnetic Force on a Current Carrying WireCurrent Carrying Wire

• Current is a flow of charge particlesCurrent is a flow of charge particles• A magnetic force is exerted on a A magnetic force is exerted on a

moving chargemoving charge• Then a magnetic force is exerted Then a magnetic force is exerted

on current in a wireon current in a wire

€

FM = ILBsinθ

€

FM = Bq(v sinθ )

€

×ΔtΔt


• Use your right Use your right hand rule to hand rule to find the force find the force on the wireon the wire


• A 4.0 m wire runs perpendicularly A 4.0 m wire runs perpendicularly through a 0.050 T field and carries through a 0.050 T field and carries a current of 3.0 A. The field points a current of 3.0 A. The field points north and the current flows west. north and the current flows west. What is the force on the wire?What is the force on the wire?


€

l = 4.0m

B = 0.050T

I = 3A

F = ?

€

FMagnetic = 0.050T 3A( ) 4.0m( )

€

FMagnetic = 0.60N

€

FMagnetic = 0.60N @ Down€

FM = ILBsinθ


• Most speakers work based on this Most speakers work based on this conceptconcept

Torque on a Current Torque on a Current Carrying WireCarrying Wire

• A loop of current carrying wire will A loop of current carrying wire will experience a torque when placed experience a torque when placed properly in a magnetic fieldproperly in a magnetic field

•Maximum Torque when the normal of the loop is perpendicular to the field•Minimum Torque (zero) when the normal of the loop is parallel to the field


€

FM = ILBsinθ

€

θ =90°

€

τ =Fl

€

τ =2ILB(w

2sinφ)

€

τ =F(w

2sinφ)

€

τ =IABsinφ

€

τ =NIABsinφ


€

τ =NIABsinφ

€

magnetic moment = NIA

The greater the magnetic moment of a coil, the greater the torque that is applied will be.

GalvanometerGalvanometer

GalvanometerGalvanometer

• Used to measure a currentUsed to measure a current• Consists of a loop of wire within a Consists of a loop of wire within a

magnetic field and a spring to magnetic field and a spring to resist motionresist motion

• Current through the loop causes a Current through the loop causes a force to rotate the loopforce to rotate the loop– Direction of rotation depends on Direction of rotation depends on

direction of currentdirection of current

MotorsMotors

•Current is supplied and the magnetic field Current is supplied and the magnetic field puts a force on the loopputs a force on the loop

Brushes allow current to be reversed every 180 degrees. This allows full rotation.

Magnetic Field of a Magnetic Field of a Current Carrying WireCurrent Carrying Wire

• Concentric Concentric circles of a circles of a magnetic field magnetic field form around a form around a current carrying current carrying wirewire


Right-Hand Rule #1 Right-Hand Rule #1 (RHR1) (RHR1)

Shows direction of Shows direction of magnetic fieldmagnetic field

Thumb:Thumb: direction of I direction of I

Fingers: Fingers: direction of Bdirection of B


• Increased current yields increased Increased current yields increased magnetic field strengthmagnetic field strength

• Field weakens with distanceField weakens with distance• Insert constantInsert constant

• Permeability of free spacePermeability of free space

€

B ∝I

r

€

B =μ0I

2πr

€

μ0 = 4π ×10−7T⋅m /A

Magnetic Field in a Current Magnetic Field in a Current LoopLoop

• Field formed by a loop can be Field formed by a loop can be found using the right hand rulefound using the right hand rule– Inside of loop is always one directionInside of loop is always one direction– Outside of loop is always the opposite Outside of loop is always the opposite

directiondirection• Forms N and S poleForms N and S pole

Magnetic Field in a Current Magnetic Field in a Current LoopLoop

SolenoidsSolenoids

• Several closely spaced parallel Several closely spaced parallel loopsloops

• Fields of individual loops combineFields of individual loops combine• Iron core is inserted to make field Iron core is inserted to make field

strongerstronger– ElectromagnetElectromagnet

SolenoidsSolenoids

Reverse Right-Hand Rule #1:Reverse Right-Hand Rule #1:

(rRHR1): determines field(rRHR1): determines field– Fingers:Fingers: curl in direction of curl in direction of

conventional currentconventional current– Thumb:Thumb: points to N pole of solenoid points to N pole of solenoid

Magnetic DomainsMagnetic Domains

• Magnetism is caused by the spin of Magnetism is caused by the spin of electronselectrons– Usually paired up and cancels outUsually paired up and cancels out

• Microscopic magnetic region Microscopic magnetic region composed of a group of atoms composed of a group of atoms whose magnetic fields are alignedwhose magnetic fields are aligned

Magnetic DomainsMagnetic Domains

• Hard magnetic materials the Hard magnetic materials the domain alignment resists changedomain alignment resists change

• Soft magnetic materials the Soft magnetic materials the domain alignment is easily domain alignment is easily changedchanged

Permanent MagnetsPermanent Magnets

• Hard Magnetism -Hard Magnetism -

• Soft Magnetism -Soft Magnetism -

–Hard to magnetize but retain Hard to magnetize but retain magnetismmagnetism

•Cobalt and NickelCobalt and Nickel

–Easy to magnetize, but do not Easy to magnetize, but do not retain magnetismretain magnetism

•IronIron

Two Parallel WiresTwo Parallel Wires

• Each wire carries a currentEach wire carries a current• Each wire creates a magnetic fieldEach wire creates a magnetic field• Each wire’s magnetic field will Each wire’s magnetic field will

produce a force on the other wireproduce a force on the other wire

Two Parallel WiresTwo Parallel Wires

• Current traveling in same directionCurrent traveling in same direction

• Current traveling in opposite Current traveling in opposite directionsdirections

–AttractsAttracts

–RepelsRepels

physics d--chapter 21 magnet forces and and magnetic fields

Documents