Magnetism and Electromagnetism

• Bar magnet

  Has a North Pole and a South Pole, with a region surrounding it where a force acts on other magnets or magnetic materials
• Unlike poles of magnets

  Attract
• Like poles of magnets


  Repel
• Magnetic field of a bar magnet

  • Strongest at the poles
  • Can be plotted using a compass, showing field lines from North to South
• Induced magnet


  Magnetic field created by a current flowing in a conductor
• Magnetic field around a current-carrying wire

  1. Concentric circles around the wire

  2. Direction can be determined using the right-hand rule
• Solenoid

  Coil of wire that can carry a current, behaves like a bar magnet
• Magnetic field of a solenoid

  • Dense field lines in the middle
  • Field lines curve around the sides
• Motor effect


  Relationship between current, magnetic field, and force on a conductor
• Fleming's left-hand rule


  Used to determine the direction of force on a current-carrying conductor in a magnetic field
• Loudspeaker


  Uses the motor effect to convert electrical signals into sound waves
• Generator effect


  Inducing a potential difference across a conductor moving in a magnetic field, or a changing magnetic field around a conductor
• Factors affecting generator output

  • Stronger magnetic field
  • Higher speed of movement
  • More turns of wire
• Alternator


  Generator that produces alternating current (AC)
• Dynamo


  Generator that produces direct current (DC)
• Transformer


  Device that allows changing the current and potential difference in a circuit
• How a transformer works

  1. Alternating current in primary coil creates alternating magnetic field in iron core

  2. Alternating magnetic field induces potential difference in secondary coil
• Step-up transformer


  Has more turns in secondary coil than primary, increases potential difference
• Step-down transformer

  Has fewer turns in secondary coil than primary, decreases potential difference
• National Grid uses step-up transformers to increase voltage for transmission, then step-down transformers to decrease voltage for household use