electromagnetism

• Poles of a magnet


  Places where the magnetic forces are strongest
• When two magnets are brought close together


  They exert a force on each other
• Two like poles


  Repel each other
• Two unlike poles


  Attract each other
• Contact or non-contact force

  Non-contact
• Permanent magnet


  Produces its own magnetic field
• Induced magnet


  A material that becomes a magnet when placed in a magnetic field. It always causes a force of attraction. When removed from the field it loses most/all of its magnetism quickly.
• Unmagnetised iron


  Will be attracted to a magnet
• Unmagnetised iron


  Will not be repelled by a magnet
• Magnetic field


  The region around a magnet where a force acts on another magnet or on a magnetic material
• Around a magnet, the field is strongest at the poles
• Magnetic materials


  • iron
  • steel
  • nickel
  • cobalt
• Magnetic field line


  A line showing the direction of magnetic force. Many field lines can be drawn around a magnet. The direction of lines tells us the direction of the magnetic field and the spacing of the lines tells us the strength of the field.
• Magnetic compass


  Contains a small bar magnet that is free to rotate
• Magnetic field lines point from north to south
• The Earth has a magnetic field
• A compass needle points in the direction of the Earth's magnetic field
• The geographic North pole has a magnetic south pole
• Investigating the magnetic field around a bar magnet

  Use a plotting compass. Place the magnet on a piece of paper. Place the compass next to the magnet. Draw a dot at the red end of the compass needle. Move the compass so that the white end of the needle is next to the dot. Place a new dot at the red end of the compass needle. Repeat to follow the field line dot-to-dot. Start from a different place to find a different field line.
• The magnetic field around a bar magnet can be sketched
• Factors that set the strength of the magnetic field around a current carrying conducting wire

  • The size of the current in the wire
  • The distance from the wire
• Solenoid


  A wire wrapped into a spiral (like a spring)
• The magnetic field around a solenoid can be sketched
• Magnetic field inside a solenoid


  Strong and uniform (the same strength everywhere)
• Adding an iron core increases the strength of the magnetic field of a solenoid
• An electromagnet is a solenoid with an iron core
• Demonstrating the magnetic effect of a current

  Place a compass beneath a conducting wire. Align the wire so that compass needle is parallel to the wire. Pass a current through the wire. The compass needle will move to be perpendicular to the wire when the current flows.
• Sketch the magnetic field around a wire
• When a wire carrying a current is placed in a magnetic field


  It may feel a force
• Motor effect


  The force felt by a current carrying wire in a magnetic field
• Devices that rely on the motor effect

  • electric motors
  • loudspeakers
• Fleming's Left Hand Rule


  A rule to work out the direction of the force on a wire in a magnetic field
• Thumb in Fleming's Left Hand Rule

  Thrust or force
• First finger in Fleming's Left Hand Rule

  Magnetic Field
• Second finger in Fleming's Left Hand Rule


  Current
• Three ways to make the force on a wire in a magnetic field larger
• F = B I l

  An equation to calculate the force on a wire in a magnetic field, where F is force, B is magnetic flux density, I is current, and l is length of wire
• Magnetic flux density


  The strength of the magnetic field, measured in Tesla (T)
• The equation F = B I l can be applied when the current is at right angles to the magnetic field
• DC Electric Motor


  A device that converts electrical energy into mechanical energy using the motor effect