Electromagnets

• Solenoid

  a coil of wire producing a magnetic field
• You can increase the strength of the magnetic field produced by a wire by wrapping it into a solenoid
• Solenoids have stronger magnetic fields because the magnetic field lines around each loop of wire line up with each other, resulting in lots of field lines pointing in the same direction that are very close to each other
• The magnetic field inside a solenoid is strong and uniform - it has the same strength and direction at every point in that region
• The magnetic field outside of a solenoid is like that of a bar magnet
• You can increase the field strength of a solenoid even more by putting a block of iron in the centre of the coil
• The iron core in a solenoid becomes an induced magnet whenever current is flowing - if you stop the current the magnetic field disappears
• A solenoid with an iron core is called an electromagnet
• Electromagnet

  a magnet whose magnetic field can be turned on and off with an electric current
• Electromagnets are usually used because they're quick to turn on and off or because they create a varying force (like in loudspeakers)
• Electromagnets in cranes:

  • used in places like scrap yards
  • used to attract and pick up things made from magnetic materials
  • used because they can be switched on to pick something up and switched off to drop it
• Electromagnets in other circuits:

  • used to act as switches (e.g. in electric starters of motors)
  • when the switch in circuit 1 is closed, it turns on the electromagnet, which attracts the iron contact on the rocker
  • the rocker pivots and closes the contacts, completing circuit 2, and turning on the motor