topic 12 - magnetism and motors

• iron, cobalt and nickel are the only common magnetic material
• a magnetic material is affected by a magnetic field
• magnets

  • is an object which produces a magnetic field
  • each magnet has two ends called poles - a north-facing pole and a south-facing pole
  • like poles (north-north or south-south) repel
  • opposite pole like north-south will attract
• induced magnet

  • only becomes magnetic when it is placed in a magnetic field
  • they lose magnetism if they are removed from the field
  • eg paper clips , nails
• permanent magnet

  • is always magnetic
  • eg a bar magnet
  • permanent magnets are made by placing a magnetic material in a very strong magnetic field
• uses of magnets

  • maglev trains
  • speakers
  • microphones
  • electric motors
  • reed switch
  • MRI scans
  • compass
• magnetic fields

  • are lines with arrows which are used to represent the magnetic field around a bar magnet
  • theses lines are called FLUX
• drawing magnetic field lines

  • line go north to south
  • there is a greater density of field line at the poles
  • all field lines have arrow heads
  • lines do not cross
• magnetic field strength

  • the magnetic field strength around a magnet is strongest at the poles
  • this is where the greatest density of flux (field line )
  • the closer the field line are together the stronger the magnetic field
  • iron fillings can be used to show this , they collect at the strongest part of the magnetic field
• uniform magnetic fields

  • produced between parallel north-facing and south facing poles
  • ( when magnets attract each other )
  • uniform magnetic fields have the same strength at all points ( the lines are equidistant )
• plotting the shape of a magnetic field

  1. place a magnet on a large piece of paper . draw around it in pencil
  2. place compass so it touches one end of the magnet and mark the position and direction of the needle
  3. place the compass in a new adjacent position and mark the location and position of needle
  4. repeat process, then join up dots to create field line
• earth's magnetic field

  • magnetic field is caused by molten iron core
  • field is strongest at the poles and weakest at the equator
  • the north facing pole of a compass points north because the earth's geographical north pole is the magnetic south pole
• detecting earth's magnetic field

  • using bar magnet on a string
  • it will rotate to point to the north
  • and will tilt relative to the horizontal , depending on where the earth's magnetic field it is in
• electromagnetism

  • when a current flows through a wire ( or any straight conductor )
  • a magnetic field is created , this field is circular
  • can be visualised by using iron filling
  • when current is switched on the fillings line up with the direction of the field
  • plotting compass can also be used to find the shape of the field
• direction of magnetic field - wire

  • the direction of the magnetic field of a wire depends on the direction of the current is flowing
  • reversing the current will reverse the direction of the field
• the strength of magnetic field - wire

  • the field is the the strongest near the wire ( highest flux density ) field line closer together
  • gets weaker as distance from the wire increase
  • the strength of the field can be increased by increasing the current
• direction of magnetic field for wires - right hand rule

  • use right hand
  • point thumb in direction of current ( towards the negative side of the wire )
  • the way you finger curls gives the direction of the magnetic field
  • in diagram the current is represented as flowing into the page as (x) or out of the page (.)
• solenoids

  • curling into a coil creates a solenoid
  • when a current flows , the magnetic fields around each part of the wire forms a overall magnetic field
  • the magnetic field is the strongest ( highest flux density ) inside the coil
  • field is also nearly uniform within the coil ( flux is almost equidistant and parallel )
• right hand rule - solenoids

  • to determine the direction of the magnetic field in a solenoid
  • your finger follow the direction of the current
  • your thumb will point at the north pole of the magnetic field outside the solenoid
• electromagnetism

  • the strength of the magnetic field around a electromagnet can be increased on three way:
  • adding an iron core ( iron is a magnetic material ) , therefore becomes a magnet when current flows through the wire around it strengthening the magnetic field
  • increasing current flowing through the wire
  • adding more turns of wire to the coil
• magnetic fields

  • current carrying wire can interact with magnetic field of a magnet
  • this causes a force and moves the wire
  • the wire experiences most force when its is perpendicular ( at right angle ) to the magnetic field
  • it experience no force when it is parallel to the magnetic field
  • F = BIL
  • F = force ( N, newtons)
  • B = magnetic field strength ( T , teslas )
  • I = current ( A , amps )
  • L = length of wire ( metres , m )

  A) force experienced by wire

  B) the magnetic field

  C) current

  
• electric motors

  • current flows through a coil
  • causing a magnetic field around the wire of coil
  • this interaction of the magnetic field and permanent magnet , causes a force on the wire
  • the left hand rule states that due to changing direction of current (A.C) in the permanent magnetic field , one side of the coil will always be forced up and one side will always be forced down
  • this causes the coil to rotate
• improving and altering electric motors

  • adding more coils of wire ( each wire will experience a force , increasing total force of motor )
  • increasing the current
  • increasing magnetic field of permanent magnet
  • moving magnet closer together
  • decreasing resistance of wire
• dynamos

  • split ring commutator ensures that force on the coil always turns in the same direction
  • if the current in one section of the wire is moving away from the commutator half a turn later the force on it would still be upwards and this would have the effect of trying to turn the coil in the opposite direction.
  • The commutator ensures that the current is always flowing in the same direction every time the coil flips
  • the coil has the same force on each side every time it flips so the coil can keep rotating