Magnetism & Electromagnetism

Magnetism & Electromagnetism

1. Magnets

   • magnetic materials
     • materials can be attracted to a magnet
       • Steel, Iron, Cobalt & Nickel
   • non-magnetic materials
     • materials that cannot be attracted to a magnet
       • Copper, Wood, Plastic & Brass
   • Properties
     • Magnets attract magnetic materials
     • Magnets have two poles
       • magnetic forces are the strongest at the poles
     • A freely suspended magnet comes to rest in the north-south direction
       • this property allows magnets to be used as compasses
     • Like poles repel, unlike poles attract
   • identification
     • attraction
       • move the other end o the object to the near to the same pole of the magnet to see if it repels
         • repulsion
           • magnet
         • attraction
           • unmagnetised magnetic material
     • Repulsion
       • magnet
       • only occurs between 2 magnets
     • nothing happens
       • non-magnetic material

2. Magnetic induction

   • process whereby an object of a material becomes a magnet when it is near or in contact with a magnet

3. Magnetisation & Demagnetisation

   • theory of magnetism
     • each resulting piece of the cut bar magnet is itself a magnet
   • Magnetic domains
     • consists of a group of atomic magnets pointing in the same direction
   • what happen during magnetism
     • magnetic domains are randomly aligned in an unmagnetised material
     • magnetic domains become aligned during magnetisation
   • to magnetise
     • stroking
       • unmagnetised steel bar is stroked several times from one end to the other, in one direction
     • Electrical method
       • large direct current flows through coils of wire( a solenoid), a strong magnetic field is produced. the magnetic field aligns the magnetic domains in the steel bar. After period of time, the steel bar will be magnitised
   • to demagnetise
     • heating
     • hammering
     • Electrical method using an alternating current
       • Place a magnet inside a solenoid in the east-west direction. Connect the solenoid to an alternating current(a.c.) supply. Withdraw magnet with the alternating current still flowing in the solenoid, until it is some distance away.

4. magnetic fields

   • region surrounding a magnet, in which a body of magnetic material experiences a magnetic force
   • magnetic field lines between magnets
     • two unlike poles
     • two like poles
   • magnetic shielding
     • used to prevent surrounding magnetic fields from reaching the sensitive areas of equipment
       • thin sheets of soft magnetic materials like iron or MuMetal(nickel-iron alloy) are used to divert the magnetic fields that are present

5. Temporary and permanent magnets

   • soft and hard magnetic materials
     • soft
       • properties: easily magnetised and demagnetised
       • used to make temporary magnets
         • uses
           • moving coil loudspeakers
           • moving-coil ammeters
           • electomagnets
       • Iron
         • easily magnetised to become a strong induced magnet
         • loses its magnetism easily
     • hard
       • difficult to magnetise and demagnetise
       • used to make permanent magnets
         • uses
           • moving-coil loudspeakers
           • moving-coil ammeters
           • magnetic door catches
       • Steel, Alnico & Neodynium
         • Steel: difficult to magnetise thus becomes a weak induced magnet
           • does not lose its magnetism easily

6. Magnetic Effect of Current

   • Oersted's experiment
     • a current-carrying conductor produces a magnetic field around it
   • magnetic field patterns
     • Magnetic field pattern around a straight wire
       • (b) magnitude of current increased
         • the strength of the magnetic field of a current-carrying wire increases when the current is increased
       • (a) direction of current is reversed
         • the direction of the magnetic field of a current-carrying wire is reversed when the direction of the current reserved
     • Magnetic field pattern around a flat coil
       • the strength of the magnetic field of a flat coil can be increased by increasing the current flowing through the coil.
     • Magnetic field pattern of a solenoid
       • magnetic field strength can increased by increasing the current flowing through the solenoid, increasing the number of turns per unit of the solenoid and placing a soft iron core within the solenoid

7. The Motor Effect - Force due to Magnetic Fields

   • Force on a current-carrying conductor
     • When a current-carrying conductor is placed in a magnetic field, the conductor experiences a force. This effect on the conductor is called the motor effect
     • the direction of the force on a current-carrying conductor is reversed when we reverse the direction of the current or magnetic
     • the force, current and magnetic field are the right angles to one another
     • Fleming's left-hand rule

8. Turning effect on a Current-carrying coil in a Magnetic Field

   • A stiff wire coil is placed between the ples of a string magnet. A current is passed through th coil. the coil experiences a turning effect about the axis.

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