electromagnetism

• Electromagnetism is due to the magnetic fields around electric currents
• The fields can cause forces with other nearby magnets which can be used to make motors spin and loudspeakers produce sound
• Magnetic field


  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
• The strength of the magnetic field is greater closer to the wire and if the current is increased
• Solenoid


  A straight coil of wire which can carry an electric current to create a magnetic field
• The field inside a solenoid is strong and uniform
• The small magnetic fields caused by the current in each coil add together to make a stronger overall magnetic field
• Electromagnet


  A magnet made by wrapping a coil of wire around an iron bar and passing an electric current through the coil
• The iron core increases the solenoid's magnetic field strength
• A simple electromagnet is made by coiling wire around an iron nail
• Electromagnets are used in devices such as electric bells, and door locks that can be controlled remotely
• Electromagnet


  A magnet created by an electric current flowing through a coil of wire
• Motor effect


  The effect where a force is exerted on a wire carrying a current in a magnetic field
• Calculating the motor effect force


  Force = magnetic flux density × current × length
• Motor effect force calculation

  • Force = 0.4 N
• Motor effect force calculation

  • Force = 0.0225 N
• Electromagnets


  Magnetic fields around electric currents
• Electromagnets


  • The fields can cause forces with other nearby magnets
  • The fields can be used to make motors spin
  • The fields can be used to make loudspeakers produce sound
• Motor effect


  The effect where a force is exerted on a wire carrying a current in a magnetic field
• Fleming's left hand rule


  1. Forefinger aligned with magnetic field

  2. Second finger aligned with current

  3. Thumb shows direction of motor effect force
• Electric motors


  Use the motor effect to spin
• Loudspeakers and headphones


  Use the motor effect to produce sound
• Current in the wire increases


  Force on the wire increases
• Strength of the magnetic field increases

  Force on the wire increases
• The force is greatest when the direction of the current is 90° to the direction of the magnetic field
• There is no motor effect force if the current and magnetic field are parallel to each other
• Electromagnets


  Magnetic fields around electric currents
• Electromagnets


  • The fields can cause forces with other nearby magnets
  • The fields can be used to make motors spin and loudspeakers produce sound
• Electric motor


  1. Coil of wire carrying a current in a magnetic field

  2. Coil experiences a force that tends to make it rotate
• Fleming's left hand rule


  Can be used to explain why the coil turns
• Explaining an electric motor

  1. Current in the left hand part of the coil causes a downward force

  2. Current in the right hand part of the coil causes an upward force

  3. Coil rotates anticlockwise because of the forces
• Electric motor


  • Momentum of the motor carries it on round a little
  • Split ring commutator changes the current direction every half turn
• When the coil is vertical, it moves parallel to the magnetic field, producing no force
• Once the conducting brushes reconnect with the commutator after a half turn, current flows in the opposite direction through the wire in the coil
• Each side of the coil is now near the opposite magnetic pole, so the motor effect forces continue to cause anticlockwise rotation of the coil
• Electromagnets


  Magnetic fields around electric currents
• Electromagnets


  • The fields can cause forces with other nearby magnets
  • Can be used to make motors spin
  • Can be used to make loudspeakers produce sound
• How loudspeakers work


  1. Variations in electric current cause variations in magnetic field produced by electromagnet

  2. Causes cone to move

  3. Cone vibrations cause pressure variations in air, forming sound waves
• Alternating current supplied to loudspeaker creates sound waves