PHYSICS UNIT 8: MAGNETISM

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PHYSICS UNIT 8 MAGNETISM
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MAGNETISM

• Magnetic Fields are always dipolar N and S poles
• law of magnetism like poles repel, unlike poles
  attract
• magnetic force is directly proportional to the
  magnets strength, inversely proportional to the
  square of the distance between the magnets
• only certain materials can be magnetized
  ferromagnetic materials (Fe,Co,Ni ores/alloys)

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MAGNETISM

• Magnets are always dipoles north-seeking N pole,
  south-seeking S pole

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MAGNETISM

• magnetic field region of magnetic force
• detect using a compass (needle points in field
  direction)

• field runs N to S outside magnet, S to N inside
  magnet

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MAGNETISM

• magnetic field strength, B strength of the
  magnetic field, in Tesla, T
• earths magnetic field at the surface 0.5 x
  10-4 T
• field directions
• ? left right ?
• up ? down ?
• out toward you in away from you

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MAGNETISM

• Earth is a magnet
• north geographic pole is an S pole
• field like a bar magnet

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MAGNETISM

• Source of Magnetism
• electrons are magnetic due to spin
• paired electrons opposite spin, cancel out
• occurs in most materials
• unpaired electrons ? magnetic atoms
• occurs in ferromagnetic materials

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MAGNETISM

• magnetic domains regions of aligned magnetic
  atoms
• magnetic object aligned magnetic domains

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ELECTROMAGNETISM

• Field Around A Wire a current (moving charge)
  produces a magnetic field encircling the wire

• right hand rule grasp conductor in right hand,
  with thumb pointing in I direction fingers will
  circle wire in B direction

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ELECTROMAGNETISM

• magnetic field strength around a wire depends on
  amount of current in wire, distance from wire
• Bm0I/2pr
• B magnetic field strength (T)
• I current in wire (A)
• r distance from wire (m)
• m04p10-7 Tm/A

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ELECTROMAGNETISM

• Field Around A Solenoid (coil)

• like bar magnet when carrying current
• add iron core electromagnet (much stronger
  field)
• solenoid field strength, Bm0nI n
  loops/length of coil

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ELECTROMAGNETISM

• right hand rule for solenoids grasp coil in
  right hand, with fingers circling coil in I
  direction thumb will point to N pole
• core is pulled into solenoid when turned on
  (striker, switch)

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ELECTROMAGNETISM

• parallel wires exert forces on each other due to
  their magnetic fields
• currents in same direction wires attract

• currents in opposite direction wires repel

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PHYSICS

• UNIT 8 MAGNETISM

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MAGNETIC FORCES

• cathode ray tube (TV monitor picture tube)

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MAGNETIC FORCES

• cathode ray tube (TV monitor picture tube)

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MAGNETIC FORCES

• Magnetic fields exert force on moving charges
• magnetic force on a moving charge,
• F qv x B (vector cross product)
• F force (N)
• B field strength (T)
• q particle charge (C)
• v speed (m/s)

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MAGNETIC FORCES

• F is perpendicular to v and B
• can only make the particle turn
• F is a centripetal force
• Bqv mv2/r
• mass spectrometer shoot particles into magnetic
  field to find their massm Bqr/v

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MAGNETIC FORCES

• right hand rule for particles, left hand rule
  for particles point thumb in v direction,
  index finger in B direction (N to S) other
  fingers will bend in F direction

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MAGNETIC FORCES

• Magnetic fields exert force on current-bearing
  wires
• magnetic force on a wire, F BIL
• B magnetic field strength (T)
• I current (A)
• L length of wire in field (m)
• for a coil, L length of wire in field of
  loops

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MAGNETIC FORCES

• right hand rule for wires point thumb in I
  direction, index finger in B direction other
  fingers will bend in F direction

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PHYSICS

• UNIT 8 MAGNETISM

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ELECTROMAGNETIC INDUCTION

• Electromagnetic Induction a voltage (and a
  current) is induced in a wire when it moves in a
  magnetic field
• induced voltage V BLv
• V potential difference between ends of wire (V)
• L wire length inside field (m)
• v wire speed (m/s)
• induced current I V/R

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ELECTROMAGNETIC INDUCTION

• either the wire or the field can move
• wire must cut across field lines to induce a
  voltage

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ELECTROMAGNETIC INDUCTION

• right hand rule for electromagnetic induction
  point thumb in v direction, index finger in B
  direction other fingers will bend to point I
  direction

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ELECTROMAGNETIC INDUCTION

• Generators spinning a coil in a B field induces
  current in the coil

Green wire voltage
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ELECTROMAGNETIC INDUCTION

• generator current is alternating current (AC)

• frequency current cycles/sec (Hz) coil
  rotation rate
• DC generator coil connects to commutator, which
  switches external connection when current
  switches

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ELECTROMAGNETIC INDUCTION

• Lenzs Law an induced current generates a
  magnetic field which opposes the change that
  induced it
• mechanical energy must go in to get electrical
  energy out
• ex The Drop Zone - magnets on cars move past
  metal fins current is induced in fins which
  takes energy from the cars, slowing them down

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PHYSICS

• UNIT 8 MAGNETISM

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UNIT 8 REVIEW

• m0 4p10-7 Tm/A
• B m0I/2pr
• B m0nI
• n loops/length of coil
• F Bqv
• m Bqr/v
• F BIL
• V BLv V IR
• e /- 1.60x10-19 C

• right hand rule for field around a wire
• right hand rule for field around a solenoid
• right (or left) hand rule for force on particles
• right hand rule for force on wires
• right hand rule for induced voltage