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Magnetism. Physics 114. Concepts . Magnetic field Magnetic force. Skills . Determine the direction of magnetic field created by electric current Determine the direction of magnetic force on electric current Two right hand rules. Magnets. Magnets have magnetic poles – north and south

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magnetism

Magnetism

Physics 114

Lecture VII

concepts
Concepts
  • Magnetic field
  • Magnetic force

Lecture VII

skills
Skills
  • Determine the direction of magnetic field created by electric current
  • Determine the direction of magnetic force on electric current
  • Two right hand rules

Lecture VII

magnets
Magnets
  • Magnets have magnetic poles – north and south
    • Like poles repel
    • Unlike poles attract
  • Similarity to electric interaction

Lecture VII

magnetic field
Magnetic field
  • Introduce magnetic field:
    • Field lines go from north pole to south
    • Mnemonic rule – birds fly from north to south

Lecture VII

magnetic field of the earth
Magnetic field of the Earth
  • Earth has a magnetic field B~5x10-5 T
  • Compass - a small magnet in a form of an arrow - is used to determine the direction of the magnetic field
  • South magnetic pole is located close to the north geographic pole, that is why north end of the compass is pole is pointing there (unlike poles attract)

Lecture VII

magnetic field7
Magnetic field
  • Magnetic field is labeled by B
  • It is measured in Tesla and Gauss

1T=10 kG

1T=1N/A m

Lecture VII

magnets8
Magnets
  • North and south poles do not exist separately!!!
    • two halves of a broken magnet still have south and north pole each
  • Different from electric charges – positive and negative charges can exist separately

Lecture VII

magnetic field created by currents
Magnetic field created by currents
  • Electric currents – moving electric charges - create magnetic field
  • Stationary electric charges do not create magnetic field
  • First right hand rule
    • Thumb along the current
    • Wrap your fingers around the wire
    • fingers show the direction of the magnetic field

Lecture VII

magnetic field created by current
Magnetic field created by current
  • Magnetic field B created by current I at a distance r from the conductoris

m0 = 4p 10-7Tm/A- magneticpermeability of free space

Lecture VII

magnetic field of loop current
Magnetic field of loop current
  • Magnetic field is in ~ same direction inside a current loop
  • Several loops create stronger magnetic field - solenoid

Lecture VII

magnetic field of a solenoid
Magnetic field of a solenoid
  • Magnetic field inside solenoid
    • Is parallel to its axis
    • Depends on the current I
    • Depends on the number of loops per unit length n=N/l:

B=m0nI

    • Does not depend on diameter
    • Does not depend on the total length

Lecture VII

magnetic force on electric currents
Magnetic force on electric currents
  • Magnetic field exert a magnetic force on electric currents – moving electric charges

F=IlBsinq

q- angle between B and I

F is max when B is perpendicular I and zero when B is parallel to I

Lecture VII

direction of the magnetic force
Direction of the magnetic force
  • Second right hand rule:
  • Fingers along the current
  • Bend to show the direction of the magnetic field
  • Thumb shows the direction of the force

Lecture VII

magnetic force on moving charge
Magnetic force on moving charge
  • Magnetic force F is perpendicular to the velocity v of a particle with charge q

F=qvB sin q

  • Charged particles move in circles in magnetic fields

Lecture VII

slide16

Magnetic fields are used to separate matter from antimatter

And measure particle velocity

Bubble chamber picture

Lecture VII

torque
Torque

Lecture VII

current loop in magnetic field
Current loop in magnetic field
  • A=ab – area of the loop
  • Magnetic field exerts a torque on a loop parallel to the magnetic field:

Lecture VII

current loop in magnetic field19
Current loop in magnetic field
  • q – angle between magnetic field and a perpendicular to the loop !!!
  • Magnetic field orients a loop current perpendicular to B.
  • N loops:

Lecture VII

force between two currents
Force between two currents
  • Parallel currents attract
  • Anti-parallel repel

l

Lecture VII

ferromagnetism
Ferromagnetism
  • Electrons inside atoms – little loop currents
  • When placed inside magnetic field microscopic currents orient to enhance the external field
  • This effect happens only in some materials – ferromagnetic, e.g. iron.

domains

Lecture VII

magnetic materials
Magnetic materials
  • Ferromagnetic Km>>1 (~1000)
    • Iron, cobalt, nickel, some aloys

residual magnetism can be destroyed (orientation of the domains randomized) by mechanical shock or high T

  • Paramagnetic Km>1 just a bit (1+10-4,-5)
    • No domains, but molecules have magnetic moment
    • Aluminum, magnesium, oxygen, platinum
  • Diamagnetic Km<1 a really tiny bit (1-10-5,-6)
    • Molecules do not have a magnetic moment
    • Copper, diamond, gold, silicon, nitrogen

Lecture VII