Magnetism and Electromagnetism

• what is the region around the magnet called

  magnetic field
• what are permanent magnets

  they produce their own magnetic field
• what are induced magnets

  become a magnet when placed in a magnetic field
• what way do the arrows on field lines run?

  north to south
• what is the flux density?

  the density of the field lines - it indicates the strength of the field at that point (the closer the lines the higher the flux density = stronger the field and greater the force)
• what does the compass needle align with, and what direction does it point?

  it aligns with the Earth's magnetic field and it points to the magnetic north
• how can a magnetic compass be used to plot the field around a bar magnet?

  1. place the bar magnet on a piece of paper
  2. place the compass at one end of the magnet
  3. mark on the paper where the point of the compass needle is
  4. move the compass so the tail of the needle is at the point that has just been marked
  5. mark on the paper a new point where the needle is
  6. repeat and connect the marks until the full field is plotted
• what happens when a current flows in a conducting wire?


  a magnetic field is produced around the wire
• what does the direction of the field lines depend on?

  the direction of the current and can be found with the right hand grip method
• demonstrate the right hand grip method

  grip the wire in your right hand, with the thumb pointing in the direction of the current

  
  

  the fingers curled around the wire will point in the direction that the field lines should be drawn
• what does the strength of the field depend on?

  the size of the current and the distance from the wire
• how is a solenoid formed?

  when a wire is looped into a cylindrical coil
• how can you increase the strength of the magnetic field?

  • shape the wire into a solenoid - will increase the strength because it concentrates the field, making it more stronger and more useful - like in electromagnets, motors and transformers.

  OR

  • add an iron core - boosts the field even more by becoming magnetised, acting as a temporary magnet.
• how can the north pole of a solenoid be found with the right hand grip method


  hold the solenoid in your right hand with you fingers following the direction the current flows

  your thumb will point to the north pole of the solenoid
• how does the electromagnetic device of an electric bell work

  when the with is pushed, the electromagnetic is magnetised

  
  

  the electromagnet attracts the armature

  
  

  the hammer strikes the gong and breaks the circuit

  
  

  the armature springs back, completing the circuit again and demagnetising the electromagnet

  
  

  the cycle repeats for as long as the button remains pushed

  
• what is the motor effect


  when a current carrying conductor is placed in a magnetic field it experiences a forced (the motor effect)
• what is the motor effect caused by

  the field created by the current interacting with the magnetic field
• how can the force be increased


  by increasing either the size of the current, the length of the conductor or the flux density
• what is the equation for force


  force = magnetic flux density (T) x current x length of the wire within the field
• what happens if the current or magnetic field is reversed

  the direction of the force is reversed
• how can the direction of the force be found using Fleming's left hand rule

  hold the left hand so that the thumb, first finger and second finger are all at right angles to one another

  
  

  align the first finer, so that it points in the direction of the magnetic field from north to south

  
  

  rotate the wrist so that the second finer points along the wire in the direction that the current is flowing

  
  

  the thumb will be pointing in the direction of the force - the direction the conductor would move
• what are the conditions for the left hand rule to work

  can be used to find the direction of the field / current / movement as long as the other two are known
• a current carrying coil in a magnetic field will...

  rotate
• why?

  the current going up one side of the coil I in the opposite direction to the current coming back down the other side, so one side moves up and the other move down
• basis of an electric moron - continuously rotating coil

  
• how do loudspeakers and headphones produce sound waves

  they use the variations in an alternating current to produce sound waves
• loudspeakers

  as the current travels through the coil, it experiences a force due to the magnetic field, this makes the speaker cone move

  
  

  because the current is alternating, the direction of the force alternates and speaker cone oscillates

  
  

  increasing the electrical power will produce a larger force - the cone will oscillate with a higher amplitude and produce louder sound waves

  
  

  the frequency of the sound produced matches the frequency of the alternating current

  
  

  the size of the speaker affects how quickly or slowly it can oscillate and how big these oscillations will be

  
  

  a large cone is better for low pitched sounds and a small cone is better for high pitched sounds
• what is the generator effect

  when conductors and magnetic fields interact, a potential difference is induced (the generator effect)
• how can this potential difference be induced

  an electrical conductor moving relative to a magnetic field

  
  

  a change in magnetic field around a conductor
• what will increasing the speed of the movement or magnetic field increase the size of


  the induced potential difference
• what will reversing the direction of the movement or magnetic field do


  reverse the direction of any induced current
• what does an induced current generate

  a magnetic field that opposes the original change or the change in magnetic field
• wha is the generator effect used in

  alternators to generate an alternating current

  
  

  dynamos to generate a direct current
• what happens as the magnet rotates

  the direction of the field through which the coil passes alternates - this induces an alternating potential and an alternating current
• what happens in a dynamo


  the coil rotates and the magnet is fixed. as the coil rotates it generates a potential difference in one direction
• what does the use of the split-ring commutator mean


  once the coil has passed the upright position, the connections are reversed - as a result the direction of the current output is always in the same direction
• in both cases, the induced potential difference is greatest when the magnet field and the coil are parallel, so the magnetic field is being 'cut' by the rotating coil at the fastest rate

  in both cases, the induced potential difference is greatest when the magnet field and the coil are parallel, so the magnetic field is being 'cut' by the rotating coil at the fastest rate
• for alternators, what is the frequency of the alternating current

  the speed of rotation
• a moving coil microphone uses the generator effect to convert sound waves into electrical signals


  the sound waves hit the microphone

  
  

  the changes in air pressure related to the sound wave cause the microphone diaphragm to oscillate

  
  

  the microphone diaphragm and coil vibrate at the same frequented as the incoming sound wave - the bigger the amplitude of the sound, the bigger the amplitude of the vibration

  
  

  this induces a potential difference on the amplitude of the incoming wave
• what does a basic transformer consist of


  a primary and secondary coil wrapped around a soft iron core