Mind map

Magnetism & Electromagnetism

Chloie Galdo English

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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
Magnetic induction
- process whereby an object of a material becomes a magnet when it is near or in contact with a magnet
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.
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
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
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
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
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.