7- Magnetism

• Permemnant magnet: prodcues its own magnetic field
• Bar magnets have two ends- they are called poles

  • Magnetic forces are strongest at the poles 
  • Two like poles repel each other: force of repulsion (forcs them appart)
  • Two unlike poles attract eachother 
  • Attraction & repulsion are non-contact forces
• Induced magnet: becomes a magnet when placed in a magnetic field 

  • Always causes a force if attraction. 
  • If you take away the permanent magnet then induced magnet loses most of all of their magnetism quickly
• Four types of magnetic material: iron, steel (alloy of iron), cobalt and nickel: can be made into permanent of induced magnet 
• Magnetic field: a region around a magnet where a force acts on another magnet or on magnetic material 

  • Whn you place magnetic material into the magnetic field it becomes an induced magnet (its attracted to the permanent magnet)
  • The strength of the magnetic field depends on the distance from the magnet (closer= stronger, further = weaker)
• Direction of magnetic fiel can be found using a compass

  • Magnetic compass contains small bar magnet

  1. Place compass at north pole of magnet, draw a cross at the north pole of the compass 
  2.  Move compass so south pole of compass is at the cross
  3. Draw cross at the north pole again, continue until you’ve complete a full magnetic field line 
  4. Join the crosses together, draw direction of field (north to south)
  5. Repeat starting at different points around north pole

  
• Magnetic field diagram: where lines are closer together shows where the field is strongest (at the poles of the magnets)

  
• Earth magnetic field: 

  • Without any magnets near compass- it always pointsi n the north direction this tells us the earth has its own magnetic field (due to the earths core)
• When current flows through conduction wire a magnetic field is produced around the wire 

  • Prove it using a compass- it’ll start facing north (earths direction of magnetic field but once current is passing through the needle (of compass) deflects away from current 
• Strength of magnetric field depends on current: large current = strong magnetic field 

  • Magnetic field is strong closer to the wire (further away = magnetic field decreases)
  • If you change direction of the current, you change the direction of the magnetic field 
• One way to increase strength of magnetic field is to coild the wire: a solenoid

  • When you turn on the current with a solenoid you get a strong and uniform magnetic field inside it

  
• shows you the direction of conventional current

  
• 3 things that increase the strength of the magnetic filed produced by a solenoid: 

  1. Increase size of current = increase strength of magnetic field 
  2. Strength increases if you increase number of coils 
  3. Place piece of iron in solenoid = increase in strength (this is called iron core)
• Electromagnetic are useful as you can change the strength of the magnetic field by changing size of the current. 

  • Can also turn it on and off
• motor effect: A wire carrying a current creates a magnetic field. This can interact with another magnetic field, causing a force that pushes the wire at right angles

  • Magnetic field around wire interacts with magnetic field between magnets and the wire experiences a force in the upwards direction, causing the wire to move upwards (this is the motor effect)

  If the conductor is parallel to the magnetic field it will not experience a force

  
• Formula: (given in exams)

  • Only applies to a wire which is at right angles to magnetic field
  • Magnetic flux density measures stength of magnetic field

  
• To determine direction of the force, use Flemings left hand rule

  
• The electric motorA coil of wire carrying a current in a magnetic field experiences a force that tends to make it rotate.

  • current in the left-hand side of the coil causes a downward force, and current in the right hand side of the coil causes an upward force;
  • the coil rotates anticlockwise because of the forces are in opposite directions;
  • each side of the coil is now near the opposite magnetic pole.

  
• The electric motor

  The direction of rotation of the coil can be reversed by:

  reversing the direction of the current OR

  reversing the direction of the magnetic field (changing over the north and south poles).
• electric motor

  The speed of rotation of the coil can be increased by:

  • increasing the size of the current;
  • using a stronger magnet;
  • increasing the number of turns of wire in the coil;
  • reducing friction between the coil and the axel it rotates on