topic p7- magnetism and electromagnetism

• Magnetic field= a region where other magnets or magnetic materials experience a force (a non contact force)
• 2 poles- north and south

  -same-repel

  -opposite-attract
• magnetic field lines

  the closer together the lines are, the stronger the magnetic field; magnetic field strength decreases with distance from magnet
• permanent magnet

  produce their own magnetic field
• induced magnets

  a magnetic material that turns into a magnet when it’s put in a magnetic field
• electromagnetism

  -when a current flows through a wire, a magnetic field is created around the wire

  -changing the direction of the current changes the direction of the magnetic field- right hand thumb rule

  -the larger the current/ closer to the wire, the stronger the field is
• solenoid- increases the strength of the magnetic field that a wire produces by wrapping the wire into a coil

  the magnetic field is strong and uniform, loops of wire line up with each other
• electromagnet

  -a solenoid with an iron core, it increases the magnetic field strength

  -it can be turned on and off
• uses of electromagnets

  -scrap yards to pick up large objects (crane)

  -within other circuits to act as switches

  -fire alarm bells
• the motor effect

  -when a magnet and a current-carrying conductor exert a force on each other

  -the force always acts at right angles to the magnetic field of the magnets and the direction of the current in the wire
• ways to increase the force on the motor effect

  -increase current

  -increase magnetic field strength

  -increase length of the conductor
• the motor effect info

  • the magnitude of the force increases with the strength of magnetic field
  • the force increases with the current passing through a conductor
• Flemings left-hand rule- find the direction of the force

  • thumb- motion
  • 1st finger- field
  • 2nd finger- current
• electric motors

  1. direct current is passed through a wire
  2. each side of the coil experiences opposite forces
  3. coil rotates

  • direction of the motor can be reversed by reversing the current or swapping the magnetic poles over (reversing the field)
• the generator effect

  when a potential difference is induced in a wire which is moving relative to a magnetic field, or experiencing a change in magnetic field
• two ways to induce a potential difference

  • move the magnet
  • move the wire
• to swap the direction of the potential difference

  • move the magnet in the opposite direction
  • move the wire in the opposite direction
• to increase the size of the induced potential difference

  • increase the speed of the movement or
  • increase the magnetic field strength
• the generator effect info

  • an induced current generates it’s own magnetic field
  • an induced current generates a magnetic field that always acts against the change that made it
• generators and microphones

  • alternators generate an alternating current
  • generators rotate a coil in a magnetic field
  • the current changes direction every half turn
  • they produce an alternating potential difference
• dynamos generate direct current

  • the connection is swapped every half turn to keep the current flowing in the same direction
• • oscilloscopes show the pd generated in the coil changes over time
• microphones (loudspeakers in reverse)

  1. sound waves hit diaphragm
  2. diaphragm moves back + forth
  3. coil of wire moves back + forth
  4. alternating current is generated
• transformers

  • change the pd, only for alternating current
  • have 2 coils of wire, primary + secondary joined with an iron core
  • when an alternating pd is applied across the primary coil, the iron core magnetises and demagnetises quickly; his induces an alternating pd in secondary coil
  • if the second coil is part of a complete circuit, the current is induced
• ratio between primary and secondary pd=

  ratio between number of turns on primary and secondary coils
• if transformer is 100% efficient:

  input power= output power
• step up transformers= increase the pd

  have more turns on the secondary coil than the primary coil
• step-down transformers= decrease the pd

  have more turns on the primary coil than the secondary