P7: Magnetism and Electromagnetism

• Magnet


  Any material or object that produces a magnetic field
• Magnets


  • Have two poles: north and south
  • Surrounded by a magnetic field
• Representing magnetic fields


  1. Draw field lines from north pole to south pole

  2. Draw straight lines into south pole and out of north pole

  3. Draw curved lines into south pole and out of north pole

  4. Draw lines looping from north to south
• More dense field lines indicate stronger magnetic field
• Compass


  • Needle is a tiny bar magnet that aligns with magnetic field lines
  • Points towards south pole of magnet
• Finding magnetic poles and field lines using a compass

  1. Place compass near magnet

  2. Draw arrow in direction of compass needle

  3. Arrow pointing towards magnet is south pole

  4. Arrow pointing away from magnet is north pole

  5. Repeat in different places to map field lines
• North poles of two magnets pushed together

  Magnets repel
• Opposite poles of two magnets brought together

  Magnets attract
• Interaction of magnetic fields creates forces of attraction and repulsion
• Magnetic material


  Any object that can be influenced by a magnetic field and has the potential to become a magnet
• Magnet


  Any object that produces a magnetic field
• Common magnetic elements

  • Nickel
  • Cobalt
  • Iron,Steel
• Alloys of nickel, cobalt,steel and iron also count as magnetic materials
• The first letters of nickel, cobalt, iron, steel spell 'ncis'
• Permanent magnet


  Produces its own magnetic field all the time
• Induced/temporary magnet


  Only has a magnetic field temporarily when placed in the field of a permanent magnet
• When a piece of magnetic material is put into the field of a permanent magnet


  It develops its own magnetic field with a north and south pole
• The force between a permanent and induced magnet will always be attractive
• When an induced magnet is removed from the magnetic field

  It will lose its magnetism
• Magnetically soft materials


  Lose their magnetism quickly, like nickel and iron
• Magnetically hard materials


  Lose their magnetism more slowly, like steel
• Electromagnetism


  The phenomenon whereby electric currents produce their own magnetic fields
• How electromagnetism works in wires, coils, solenoids and electromagnets

  1. Imagine a wire with current flowing upwards

  2. Current produces concentric circular magnetic field lines around the wire

  3. Use right hand rule to determine direction of magnetic field

  4. If current flows in opposite direction, magnetic field reverses

  5. Joining two wires into a circular coil stretches magnetic field lines into ellipses

  6. Adding more turns to coil creates a solenoid with strong uniform magnetic field inside

  7. Solenoid acts like a bar magnet with north and south poles

  8. Electromagnet - magnetic field only exists when current is flowing

  9. Can reverse direction of magnetic field by reversing current direction
• Electromagnets


  • Can be turned on and off by controlling current
  • Magnetic field disappears when current is turned off
  • Direction of magnetic field can be reversed by reversing current direction
• Ways to increase electromagnet strength

  • Increase current flowing through solenoid
  • Increase number of turns in coil
  • Decrease length of solenoid coil
  • Add iron core inside solenoid
• Motor effect


  The idea that a current carrying wire in the presence of a magnetic field will experience a force
• Finding the direction and strength of the force on a current carrying wire in a magnetic field

  1. Determine direction of magnetic field

  2. Determine direction of current in wire

  3. Use Fleming's left-hand rule to find direction of force

  4. Calculate force using equation F=BIL
• Fleming's left-hand rule


  • Point thumb up for direction of force
  • Point first finger in direction of magnetic field
  • Point second finger in direction of current
• Current carrying wire at 90 degrees to magnetic field


  Experiences maximum force
• Current carrying wire at angle to magnetic field


  Experiences less force
• Current carrying wire parallel to magnetic field


  Experiences no force
• Motor effect scenario


  • Current flowing through metal rails between poles of horseshoe magnet
• Magnetic flux density


  Magnetic field strength, measured in Teslas
• Equation F=BIL is provided in the exam
• Motor effect


  The idea that a current carrying wire in the presence of a magnetic field will experience a force
• Finding the direction and strength of the force on a current carrying wire in a magnetic field

  1. Determine direction of magnetic field

  2. Determine direction of current in wire

  3. Use Fleming's left-hand rule to find direction of force

  4. Calculate force using equation F = BIL
• Fleming's left-hand rule


  • Point thumb up for direction of force
  • Point first finger in direction of magnetic field
  • Point second finger in direction of current
• Current carrying wire at 90 degrees to magnetic field


  Experiences maximum force
• Current carrying wire at angle to magnetic field


  Experiences less force
• Current carrying wire parallel to magnetic field


  Experiences no force