P7

• Magnet


  An object capable of exerting a magnetic force
• North pole


  In a magnet, the end that is attracted to the Earth's magnetic north pole
• South pole


  In a magnet, the end that is attracted to the Earth's magnetic south pole
• The magnetic force is strongest near the magnet's poles
• Magnetic forces


  • Non-contact forces - this means that magnets affect each other without touching
• Like poles (N-N or S-S)


  Repel
• Unlike poles (N-S or S-N)

  Attract
• Magnetic fields


  Fields around magnets that can exert forces on other magnets and magnetic materials
• Magnetic materials


  • Iron, steel, nickel and cobalt
  • Can be magnetised or attracted to a magnet
• Permanent magnet


  Magnet made from a magnetic material, its magnetism cannot be turned on or off
• Permanent magnet


  • Produces its own magnetic field
  • Magnetic field cannot be turned on and off
• Permanent magnets


  • Bar magnets
  • Horseshoe magnets
• Induced magnet


  • Temporary magnet, made from a magnetic material placed in a magnetic field
  • Loses magnetism when removed from the magnetic field
• Induced magnets are only attracted by other magnets, they are not repelled
• Induced magnets lose most or all of their magnetism when they are removed from the magnetic field
• Testing for magnetism


  • Permanent magnet can attract or repel another permanent magnet
  • Permanent magnet can attract a magnetic material but not repel it
• You can only show that an object is a permanent magnet by checking if it repels another magnet
• Magnetic field


  The region around a magnet where a force acts on another magnet or on a magnetic material
• Detecting magnetic fields


  • Magnetic field is invisible, but can be detected using a magnetic compass
  • Compass contains a small bar magnet on a pivot that can rotate
  • Compass needle points in the direction of the Earth's magnetic field or the magnetic field of a magnet
• Mapping magnetic fields using plotting compasses


  1. Place the plotting compass near the magnet on a piece of paper

  2. Mark the direction the compass needle points

  3. Move the plotting compass to many different positions in the magnetic field, marking the needle direction each time

  4. Join the points to show the field lines
• The needle of a plotting compass points to the south pole of the magnet
• The Earth has a magnetic field produced by its iron and nickel core
• Magnetic field lines


  • Never cross each other
  • The closer the lines, the stronger the magnetic field
  • The lines have arrowheads to show the direction of the force exerted by a magnetic north pole
  • The arrowheads point from the north pole of the magnet to its south pole
• Electromagnet


  A magnet made by wrapping a coil of wire around an iron bar and passing an electric current through the coil
• Electromagnet


  • The iron core increases the solenoid's magnetic field strength
• Simple electromagnet


  • Coiling wire around an iron nail
• Solenoid


  A straight coil of wire which can carry an electric current to create a magnetic field
• Solenoid


  • The small magnetic fields caused by the current in each coil add together to make a stronger overall magnetic field
• Magnetic field


  The area surrounding a magnet that can exert a force on magnetic materials
• When a current flows in a wire

  It creates a circular magnetic field close around the wire
• Magnetic field strength


  Is greater closer to the wire and if the current is increased
• The magnetic field inside a solenoid is strong and uniform
• Deflect


  To cause something to change direction
• The magnetic field around a wire can deflect the needle of a magnetic compass
• Electromagnets are used in devices such as electric bells, and door locks that can be controlled remotely
• Electromagnet


  A wire carrying a current creates a magnetic field close to it
• Motor effect


  The effect where a force is exerted on a wire carrying a current in a magnetic field
• The force is at right angles to the current and the magnetic field
• Calculating the motor effect force


  Force = magnetic flux density × current × length
• Calculating the motor effect force

  • A 50 cm wire carries 2 A in a 0.4 T magnetic field, the force is 0.4 N