P4

• Magnet

  • Any material or object that produces a magnetic field
  • Two poles ( N and S)
  • Magnetic material- Iron, nickel, cobalt
• Magnetic field

  • shows size and direction
  • point from north to south
  • closer the lines, stronger the field
• What is magnetic flux density

  • Strength of magnetic field
  • Measured in Teslas
• Uniform field

  • Value of field strength is same everywhere
  • North and South poles of two permanent magnets near each other
• Compass to see magnetic field

  1. put compass on piece of paper next to magnet
  2. Compass needle will point in direction of field lines
  3. Mark directions
  4. Repeat and join up
• Why is using a compass better than iron fillings

  • Always points north - evidence that Earth has a magnetic north and south so have a magnetic core
  • Drawings of field will still be there after magnet is removed
  • Iron fillings are messy- hard to take off magnets
• Magnets affect magnetic materials and magnets

  • Like repel, opposites attract
  • When magnet is brought to magnetic material, the material acts as a magnet
  • Magnetism induced by original magnet
  • Closer- the stronger the induced magnet
• Permanent magnet

  • never loses magnetism
  • Induced other objects
• Induced magnet

  • lose their magnetism once magnet has been removed
• Moving charges (Current) create a magnetic effect

  larger the current, stronger the magnetic attraction
• Magnetic field around a straight wire

  > Made up of concentric circles with the wire in the centre

  > Use a compass to show magnetic field

  > Further away from wire, the weaker the magnetic field

  > graph is non-linear as strength decreases quickly and slows down with the distance
• Magnetic field around flat circular coil

  > Similar to a bar magnet

  > Concentric elipses (stretched out circles) around the coil
• Flemmings left hand rule

  Thumb- movement

  Forefinger- field

  Second- current
• How to increase the size of the force on the wire

  > Increase strength of magnetic field

  > increase size of current

  > Increase length of wire
• Magnetic field around a solenoid


  > Magnetic field inside a current-carrying solenoid is strong and uniform

  > Outside is like a bar magnet

  Ends of solenoid acts as poles

  > Electromagnetic
• How to increase magnetic effect at end of solenoid

  • Current in wire increase
  • Number of turns increase
  • Length us decreased (number of turns stay the same)
  • soft Iron core is added
• Current in magnetic field

  > Experiences a force

  > Wire between two poles, the two magnetic fields create a force on the wire

  > Wire has to be at 90° to magnetic field

  > Force acts in same direction- changing direction of current or the ,acetic field will change direction of force
• Calculate force 


  Force on current carrying wire (N) =

  Magnetic field strength (T) x Current in wire (A)

  x length of wire (m)
• Motors


  > d.c motor - current carrying coil between two opposite poles of a magnet

  > each side experience force in opposite direction- due to current flowing in different direction and coil is perpendicular to magnetic field `

  > Coil is on spindle and forces act in opposite directions It rotates

  > Split-commutator swaps current at every half turn
• How to reverse motor

  • reversing the current
  • swapping magnet poles (reversing magnetic field)
• Split ring commutator

  • changes current direction every half term as the loop rotates
  • force on left hand of the loop is up
  • force on right side of the loop is down
  • So it rotates in same direction
• How to increase current

  > Move wire/magnet faste

  > Use a stronger magnet

  > More turns on the coil means greater voltage
• Generators

  • opposite of motors
  • Use relative motion of a conductor and magnetic field to induce a p.d and a current
  • If direction of rotation is reversed, the direction of the induced p.d are reversed too
  • current induced in alternator or dynamo will be greater if there are more turns on the coil
  • Magnetic flux density increases if speed of rotation increases
  • Alternators and dynamos are types of generators
• Alternators

  • generate alternating currents
  • Some alternators rotate a magnet in a coil of wire
  • P.d is induced as magnet spins across ends of coils
  • p.d changes direction every half turn because direction of field changes as magnet rotates
  • Produces an alternating current if circuit is complete
  • Slip rings with brushes connected to the rest of the circuit so the contacts doesn't swap every half term to produce a.c
• Dynamos

  • generate d.c
  • rotates coil in a magnetic field
  • Output p.d and current change direction every half rotation of the coil to produce a.c
  • split ring commutator swaps connection every half turn to keep the current flowing in same direction so changes a.c to d.c
• Loud speaker


  > Converts sound waves into electrical signals

  > motor effect to convert electrical signals into vibrations with generate sound waves

  > Coil and cone slide back and fourth along magnet
• Microphone


  > Detects vibrations caused by sound waves and use generator effect to convert into electrical signals

  > Involves a.c
• How loud speakers work

  1. a.c flows through the wire and creates a magnetic field in the coil
  2. Magnetic field of the coil interacts with magnetic field of permanent magnet
  3. Motor effect causes it to move back and fourth along permanent magnet
  4. Causes cone to change a shape
  5. As the current alternates it causes vibrations
  6. Causes pressure variations in the air= sound waves
• How do microphones work

  1. sound waves hit diaphragm
  2. Causes diaphragm and coil of wire to move
  3. Generates a current (electromagnetic induction)
  4. Frequency of p.d is frequency of sound waves
• Electromagnetic induction


  > moving a wire (conductor) through a magnetic field

  > Creates a p.d in the wire

  > p.d will increase current measure with ammeter
• Transformers


  > Change size of p.d of an a.c to reduce heating effect

  > two coils joined by an iron core

  > iron core is a magnetic material that becomes magnetised

  > Coil produces alternating magnetic field so magnetism in core alternates

  Changing magnetic field induced p.d on second coil
• Step down

  • decreases voltage
  • More turns in primary than secondary
• Step up

  • increases voltage
  • More coils in secondary than primary