6. Magnetism and Electromagnetism

• Magnets


  Repel and attract other magnets and attract magnetic materials
• Non-magnetic materials


  • Not attracted to magnets and cannot be magnetised (e.g. glass, plastic)
• Magnetic materials


  • Attracted to magnets and can be magnetised (e.g. iron, steel, cobalt, nickel)
• Magnetism can be induced

  By placing magnetic materials in a magnetic field
• Magnetically hard materials


  Magnetic materials that can be permanently magnetised (e.g. steel)
• Magnetically soft materials


  Magnetic materials that are only temporarily magnetised (e.g. soft iron)
• Magnetic field lines


  Represent the magnetic force on a north pole at a given point
• Direction of magnetic field lines

  • Shows the direction of the force
• Spacing of magnetic field lines

  • Shows the magnitude of the force
• Magnetic field lines from magnets point from north to south
• There is a uniform magnetic field between the opposite poles of two magnets placed close together
• Electric current passing through a conductor


  Produces a magnetic field around it
• Electromagnets


  • Consist of a coil of wire wrapped around a magnetically soft core and can be turned on and off
• Right-hand grip rule


  Determines the direction of the magnetic field produced by a current carrying wire
• The magnetic field created by a current carrying solenoid is like the field produced by a bar magnet
• The magnetic field created by a current carrying flat circular coil
• Motor effect


  A force acts on a current-carrying conductor in a magnetic field
• Fleming's left-hand rule


  Shows the relative directions of the force, field, and current
• If the current is reversed or the magnetic field is reversed


  The force will be reversed
• If the magnitude of the current or of the magnetic field is increased


  The magnitude of the force will increase
• DC motors


  • Consist of a coil of wire in between two permanent magnets, experience a turning effect due to the forces exerted on it in the magnetic field
• Split ring commutator


  • Used to ensure the direction that the current flows in the coil reverses every half turn
• Loudspeakers


  • Consist of a coil attached to a cone in a magnetic field, when alternating current flows through the coil, the cone is continuously pushed away and pulled back, making a sound
• Frequency of the sound

  Can be altered by changing the frequency of the alternating current used
• Generator effect


  When there is relative movement between a conducting wire and a magnetic field, a voltage will be induced
• Induced voltage


  Can be increased by moving the wire more quickly, using a stronger magnetic field, or increasing the length of the wire inside the magnetic field
• A voltage is also induced in a coil with a changing magnetic field through it
• More quickly the magnetic field changes

  The greater the induced voltage
• Electricity generation


  • Can be generated by rotating a magnet within a coil or by rotating a coil in a magnetic field
• Voltage can be increased

  By increasing the length of wire inside the magnetic field, using a stronger magnet, or increasing the speed of rotation
• Transformer


  • Consists of two coils wrapped around a soft iron core and is used to change the size of a voltage
• Step-up transformer


  Has more turns on the secondary than the primary, which means the voltage of the secondary is greater than that of the primary
• Step-down transformer


  Has fewer turns on the secondary than the primary, which means the voltage of the secondary is less than that of the primary
• For a 100% efficient transformer, because the power used is constant, Ip*Vp = Is*Vs