7. Magnetism and Electromagnetism

• Magnetic force


  Non-contact force that can act at a distance, caused by magnetic fields
• Magnet


  Has two poles, north and south, where magnetic forces are strongest
• Like poles


  Repel each other
• Unlike poles


  Attract each other
• Magnetic materials


  • Iron
  • Steel
  • Nickel
  • Cobalt
• Types of magnets


  • Permanent magnets
  • Induced magnets
• Inducing magnetism


  Bringing a magnetic material within the magnetic field of a permanent magnet causes it to temporarily produce its own magnetic field and turn into an induced magnet
• Magnetic field


  Region of space where a magnet, magnetic material, or current-carrying wire experiences a force
• Magnetic field lines


  Invisible lines with arrows that point away from north pole and towards south pole, the closer the lines the stronger the field
• Observing a magnetic field


  Place a bar magnet on paper, use a compass to draw dots in the direction the compass points, join the dots to reveal field lines
• Right hand grip rule


  Used to determine direction of magnetic field around a current-carrying wire
• Solenoid


  Coiled wire that produces a magnetic field similar to a bar magnet on the outside, with a uniform field inside
• Electromagnet


  Magnet created by placing an iron core inside a solenoid, can be turned on and off by controlling the current
• Motor effect


  Force created when a current-carrying conductor is placed in a magnetic field
• Fleming's left hand rule

  Used to determine direction of force in the motor effect
• Motor effect


  The force created when a current-carrying wire is placed in a magnetic field
• Magnitude of the force


  • Increased by increasing the magnetic flux density of the field (the strength)
  • Increased by increasing the current
• F


  Force measured in N
• B


  Magnetic flux density measured in Tesla (T), which tells you the strength of the magnetic field
• I


  Current measured in A
• l


  Length of the wire in the field measured in m
• Fleming's Left Hand Rule

  1. Used to work out the direction of the force created by the motor effect

  2. Hold left hand with thumb, first finger, and second finger perpendicular to each other

  3. First finger points in direction of magnetic field

  4. Second finger points in direction of current

  5. Thumb points in direction of force
• Electric motors


  • Devices that rotate when an electrical current passes through them
  • Use the motor effect to explain how they work
  • Consist of a coil of wire that is free to rotate in a magnetic field between two magnets
  • When current flows through the wire, a magnetic field is created around it
  • The magnetic field of the wire interacts with the magnetic field of the magnets, causing a force (the motor effect)
  • As the wire is in the shape of a coil, the current either side of the coil flows in opposite directions, causing the forces to act in opposite directions
  • This begins a rotating motion in the coil until it is in the vertical position
  • At this point, the split ring commutator swaps the direction of the current, swapping the direction of the forces, allowing the motor to continue rotating
  • The split ring commutator swaps the direction of the current every half turn to keep the motor rotating