magnetic field patterns

• Magnetic field line patterns are all slightly different around:

  Straight wires

  Flat circular coils

  Solenoids
• STRAIGHT WIRE

  • shape & direction of magnetic field cn be investigated w plotting compasses
  • magnetic field is made up of concentric circles
  • A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles
  • distance from wire increase = circles further apart (field strongest closest to wire, weaker as distance increase)
  • right-hand thumb rule: direction of magnetic field
  • Increasing amount of current flowing thru wire increases strength of magnetic field: field lines closer tgt

  A) 90

  B) further apart

  C) magnetic field

  
• FLAT CIRCULAR COIL

  • When a wire is looped into a coil, the magnetic field lines circle around each part of the coil, passing through the centre of it
  • To increase the strength of the magnetic field around the wire it should be coiled to form a solenoid
  • The magnetic field around the solenoid is similar to that of a bar magnet

  
• label

  A) north

  B) south

  
• THE MAGNETIC FIELD IN A SOLENOID IS UNIFORM WHEN THE COIL IS LONG ENOUGH FOR THE FIELDS FROM ONE END TO OVERLAP WITH THE OTHER END
• THE MAGNETIC FIELD IN A SOLENOID IS STRONGER THAN THAT OF THE FLAT COIL BECAUSE IT HAS MORE TURNS AND THEY ARE ALL ALIGNED TOGETHER
• SOLENOID (electromagnet)

  • field in solenoid strong & uniform
  • fields from individual coils:

  1. Add tgt -> v strong, almost uniform field along centre of solenoid
  2. Cancel to give a weaker field outside solenoid

  • each end behaves like north & south pole of a magnet
  • To work out the polarity of each end of the solenoid it needs to be viewed from the end:

  1. If current travelling around clockwise: south
  2. anticlockwise: north
  3. If current change direction then north & south r reversed
  4. If no current flowing thru the wire: no field produced around or thru solenoid

  
• • The strength of the magnetic field produced around a solenoid can be increased by:

  1. Increasing the size of the current which is flowing through the wire
  2. Increasing the number of turns to the coil
  3. Adding an iron core through the centre of the coils

  • The iron core will become an induced magnet when current is flowing through the coils
  • The magnetic field produced from the solenoid and the iron core will create a much stronger magnet overall
• • ‘uniform field’ means a field which has the same strength and direction at all points. This is represented by parallel field lines
  • When discussing the strength of an electromagnet, avoid saying “add more coils”: The coil describes the overall object – the individual loops of wire should be referred to as turns.
  • The correct phrase to use is “add more turns to the coil”.