Magnetism & Electromagnetic Induction

• Magnetic field lines


  Run from the North pole to the South pole

  
• Magnetic forces


  • Due to interactions between magnetic fields
• Magnetic poles


  • Opposite poles attract, like poles repel
• Bar magnet


  • Field lines around the magnet

  
• Magnetic field strength


  Stronger where field lines are closer together
• Identifying magnetic field shape


  Sprinkle iron filings on a card placed on the magnet and gently tap the card
• Identifying magnetic field direction


  Use a plotting compass

  
• Compass


  • Aligns with the field, North pole points towards the South pole
• Earth's magnetic field


  • North pole is actually a magnetic South pole
• Uniform magnetic field


  • Between two slab magnets

  
• Magnetic materials


  • Iron, steel, cobalt, nickel
• Induced magnetism


  Poles created in material to cause attraction, strengthening the overall field

  
• Making a permanent magnet


  1. Stroke steel with one end of a magnet in one direction

  2. Put steel in a coil carrying a direct current

  3. Hammer steel in a magnetic field
• Demagnetising a magnet


  1. Hammer it

  2. Heat it up

  3. Put it in a coil carrying alternating current
• Magnetically soft materials


  Easy to magnetise but lose magnetism when removed from field
• Magnetically hard materials


  Difficult to magnetise but retain magnetisation when made permanent magnets
• Magnetic field around current-carrying situations


  • Vertical wire passing through card
  • Coil of wire

  
• Magnetic field strength


  Decreases as distance from current-carrying wire increases
• Magnetic field direction


  Points away from North, towards South

  
• Increasing current


  Increases magnitude of magnetic field
• Reversing current direction


  Reverses direction of magnetic field
• Solenoid


  Long straight coil of wire that creates a strong, uniform magnetic field inside

  
• Electromagnet


  Coil of wire carrying current, often with soft iron core to increase field strength
• Electromagnet


  • Can be switched on/off, field strength increased by increasing current or turns
• Motor effect


  Wire carrying current in magnetic field experiences a force perpendicular to both field and current

  
• Fleming's Left Hand Rule


  Thumb = force, first finger = magnetic field, second finger = current
• Equal and opposite force on magnet when wire experiences force
• Turning effect on coil in magnetic field

  Opposite currents on different sides experience opposite forces
• DC motor


  • Includes split ring commutator
• Commutator


  Reverses direction of current to allow motor to continue rotating
• Electromagnetic induction


  Voltage induced in wire/coil in changing magnetic field or moving through field
• Factors affecting induced voltage


  • Number of turns, magnetic field strength, rate of change of field, speed of motion through field
• Direction of induced EMF


  Opposes the change causing it
• Induced current in circuit


  Direction found using Fleming's Right Hand Rule
• AC generator


  Also called an alternator
• AC generator


  • Rotating-coil design with slip rings
• Doubling rotation rate


  Doubles peak voltage and frequency
• DC generator


  Uses split-ring commutator instead of slip rings
• Moving coil microphone


  • Pressure variations in sound wave cause diaphragm vibration, coil movement in magnetic field induces voltage

  
• Reverse effect used in loudspeakers and headphones