Magnetism

Magnetism

1. Introduction

   • Magnetic materials are attracted to both poles of a magnet
   • 4 magnetic materials: iron, steel, nickel, cobalt
     • Atoms act like tiny magnets and group to from domains
       • Domains are usually in random directions in a non-magnet
   • Induced Magnetism- domains line up in the presence of a magnet
     • When removed from magnet, iron loses most of its magnetism
       • Hitting/heating iron causes it to lose magnetism as it shakes up domains

2. Magnetic Fields

   • Field lines from North to South
     • Never cross
     • Closer lines mean stronger field
   • Iron filings show shape of field and compass shows direction

3. Solenoid

   • Stronger if:
     • larger current
     • more turns
     • add an iron core
       • Iron loses magnetism easily compared to steel
   • Right hand grip rule
     • Fingers curl in current direction
     • Thumb shows field direction

4. Bell

   • Switch closed
     • Current flows
       • EM on
         • Armature attracted
           • Contacts pulled apart
             • Hammer hits bell
               • Circuit broken
                 • EM off
                   • Armature springs back
                     • Repeats

5. Relay

   • 2 seperate circuits
     • 1 low voltage DC controls high voltage AC
   • Close switch in low voltage
     • EM on
       • Armature attracted
         • High voltage complete

6. Loudspeaker

   • Current flows through coil it becomes EM and permanent magnet exerts force
   • Either pushes coil out of PM or into it, depending on current direction
   • A.C current makes coil vibrate making a sound
   • Higher pitch- higher frequency current Louder sound- higher amplitude current

7. Motor Effect

   • Current in wire produces magnetic field
     • If near permanent magnet the fields interact
       • Producing a force perpendicular to both current and field (of magnet) direction
       • If magnet field and current are parallel, force is zero and are maximum when perpendicular
   • Fleming's Left Hand Rule
     • First- Field (N to S)
     • SeCond- Current
     • ThuMb- Motion/force
   • Larger force if
     • Larger current
     • Stronger magnet
     • More turns
   • F=BIl
     • F- force (N)
     • B- magnetic flux density (T- Tesla)
     • I- Current (A)
     • l= length of wire (m)
     • Assume current and field are perpendicular

8. DC Electric Motor

   • Current around coil, two sides in opposite directions so force is opposite
     • Causes it to rotate but eventually it will stop in vertical position
   • To reverse current every half-turn, split-ring commutator is connected, half to each side of coil
     • Carbon brushes (good conductor, low friction) connect battery to two halves
       • Stops wires getting caught up
   • Energy (J)= Power (W) x Time (s)
     • Power (W)= P.d. (V) x Current (A)