Magnetism and Electromagnetism

• Bar magnet


  A magnet with two ends called poles (North Pole and South Pole)
• Magnets


  • Magnetic forces are strongest at the poles
• When two magnets are brought close together

  They exert a force on each other
• Magnetic forces between poles

  • Like poles (North-North or South-South) repel
  • Unlike poles (North-South) attract
• Non-contact forces


  Attraction and repulsion between magnetic poles occur without the magnets touching
• Permanent magnet


  Produces its own magnetic field
• Induced magnet


  Becomes magnetic when placed in a magnetic field
• Induced magnetism


  Always causes a force of attraction
• When the permanent magnet is removed, induced magnets lose most or all of their magnetism quickly
• Magnetic field


  A region around a magnet where a force acts on another magnet or on a magnetic material
• Plotting the magnetic field pattern of a magnet

  1. Place compass near North Pole of magnet

  2. Draw cross at North Pole of compass

  3. Move compass so South Pole is on cross, draw cross at North Pole

  4. Repeat, connecting dots to show field lines

  5. Show direction from North to South Pole
• The strength of the magnetic field depends on the distance from the magnet
• The magnetic field is strongest at the poles of the magnet
• A compass contains a small bar magnet
• Compass placed near a magnet


  Needle points in North-South direction
• The Earth has its own magnetic field
• The Earth's magnetic field is due to the Earth's core
• Conventional current


  The direction of current flow as defined by the movement of positive charges
• How a current flowing through a wire generates a magnetic field

  1. Current flows through wire

  2. Magnetic field produced around wire

  3. Strength of magnetic field depends on current size

  4. Magnetic field strongest closer to wire

  5. Changing current direction changes magnetic field direction
• Magnetic field around wire

  • Can be detected using a compass
  • Deflects compass needle
• Increasing current


  Increases strength of magnetic field
• Moving further from wire

  Decreases strength of magnetic field
• Right hand grip rule

  Used to determine direction of magnetic field produced by a wire carrying current
• Solenoid


  • Coiled wire
  • Produces strong and uniform magnetic field inside
• Solenoid


  Magnetic field shape similar to bar magnet
• Right hand rule for solenoid

  Curl fingers in direction of conventional current, thumb points to north pole
• Ways to increase strength of magnetic field in a solenoid

  • Increase current size
  • Increase number of turns in coil
  • Place iron core inside
• Electromagnet


  Solenoid containing an iron core
• Electromagnets are useful as the magnetic field strength can be changed by changing the current, and they can be turned on and off
• Relay


  A device that contains two separate circuits - a low-voltage circuit with an electromagnet, and a high-voltage circuit with two metal contacts
• How a relay works

  1. Low-voltage circuit turned off - no current, no magnetic field, high-voltage circuit off

  2. Low-voltage circuit turned on - current flows, magnetic field produced, attracts iron block, closes high-voltage contacts, high-voltage circuit on

  3. Low-voltage circuit turned off - no magnetic field, contacts spring apart, high-voltage circuit off
• Electric doorbell


  A device that uses an electromagnet to make a clapper hit a bell when the doorbell is pressed
• How an electric doorbell works

  1. Switch closed when doorbell pressed

  2. Current flows, magnetic field produced by electromagnet

  3. Iron contact attracted to magnetic field, clapper hits bell

  4. Circuit broken, no current, no magnetic field, iron contact springs back
• Circuits using high-voltage switches can be dangerous, so relays are used to safely turn them on and off