topic 8

• What happens when two insulating materials are rubbed together?

  Electrons are transferred from one to the other, caused a build up of static due to friction. The one thats looses the electrons has a positive static charge, and vice versa.
• What happens to two like charges

  They repel
• What happens to opposite charges

  They attract
• How can a charged object attract an uncharged object?

  A negatively charge object can attract a neutral object, because as they become closer, the -ve charge will repel the electrons in the neutral object, meaning they move away, leaving the front side of the neutral object with a +ve charge, so the object will attract each other.
• How is static electricity used in paint sprayers?

  The paint droplets are negatively charged as they leave the sprayer, and the object being coated will be positively charged. All the paint is attacted the object, meaning you don't need to move the sprayer as the droplets will cover the back as well.
• What are the benefits of using an electrostatic paint sprayer?

  - droplets repel each other, preventing clumps

  - droplets cover back as well - don't need to move sprayer

  - even coating achieved

  - hardly any paint is wasted
• How does static cause sparks?

  As an electyric charge builds on the object, the p.d. between the earth and the object (previously 0V) increases. When the p.d. gets large enough, electrons can jump across the gap, and this is a spark.
• When can static electricity be dangerous?

  Static can be dangerous when flammable gases / liquids are used which a spark could ignite

  
  

  E.g. petrol stations or operating theatres,

  on airplanes - it can interfere with communication equipment.
• How can we stop static charge building up on earthed objects?

  Connecting the charged object to the ground using a copper wire - earthing the object to provide an easy route for the static charges to go to the ground, therefore preventing a spark. Fuel tankers have to be earthed to prevent a spark which could cause an explosion.
• How do you draw electric fields?

  USe field lines - they always go from +ve to -ve

  They are always at a right angle to the surface

  The closer the lines are together, the stronger the field is.

  The field is strongest closer to the object
• Where do electric fields form?


  around a charged object
• What happens when a charged object is placed in an electric field?

  It feels a force caused by the two fields interacting
• How do electric fields explain sparking?

  The strong electric field around the object causes electrons in the air particles to be removed - ionisation. This means the air (which is normally an insulator) becomes much more conductive, so a current can flow through it. This is the spark.
• Describe magnetic fields

  - field lines go from north to south

  - they show how the field would act on a north pole

  - the field is stronger closer to the magnet.
• what happens when two like poles interact?

  they repel
• what happens when a north and a south pole interact?

  they attract
• What are three main magnetic elements?

  iron, nickel, cobalt

  some alloys and compounds of the elements are also magnetic
• How can you create an induced magnet?

  When you put any magnetic material in a magnetic field, it becomes magnetised and has its own poles. When you remove the material, how quickly it stops being a magnet depends on the material it is made from.
• What is difference between magnetically "soft" and "hard" materials?

  Magnetically soft materials (eg pure iron) lose their magnetism very quickly

  Magnetically hard materials (eg steel) lose their magnetism more slowly. These materials are used to make permanent magnets.
• What are some uses for magnetic materials?

  Fridge doors, cranes, maglev trains, MRI machines and speakers and microphones
• What is magnetic flux density?

  A measure of how many field (flux) lines there are in a region - shows the strength of the magnetic field
• What are the units for magnetic flux density?

  Tesla (T), or N/Am
• What is the formula for force (involving magnetic flux density)?


  F = B x I x l

  Force = Magnetic flux density x current x length
• How do you find which way a force in a magnetic field is acting?

  Use fleming's left-hand rule
• What is fleming's left hand rule to find the direction of the magnetic force?

  First finger points in direction of the magnetic field

  second finger points in direction of current

  thumb will point in the direction of the force
• How does a basic d.c. motor work?

  - forces act on the two side arms of a coil of wire which is carrying a current, acting in opposite directions on each side, meaning the coil will rotate

  - there is a split ring commutator which swaps the contacts every half turn, meaning that the motor keeps rotating in the same direction.
• What is a solenoid?

  A coil of current-carrying wire that produces a magnetic field. It is an example of an electromagnet, it can be turned on and off using an eletric current.
• How can you increase the field strength of solenoid?

  Add a block of iron into the centre of the coil. The iron core will become an induced magnet when the current is flowing. You can also add more coils.
• What is electromagnetic induction?

  The induction of a potential difference (and current if there's a complete circuit) in a wire which is experiencing a change in magnetic field
• What is a dynamo?


  A machine that generates electricity (d.c)
• How do dynamos work?

  generate direct current. they use a split ring commutator as their electrical contact- swaps the connection every half turn to keep current flowing in the same direction- generating dc. Their construction is much like a motor
• What is an alternator?


  A generator of alternating current
• How do alternators work?


  a current is induced in a coil. as the coil spins the current changes direction every half turn.

  use slip rings and brushes so the contacts dont swap every half turn which maintains and alternating pd, generating an ac
• How do microphones work?

  - sound waves hit flexible diaphragm that is attached to a coil of wire wrapped around a magnet

  
  

  - causes coil to move in the magnetic field, generating a current

  
  

  - movement of coil depends on properties of sound wave (louder moves further)

  
  

  - overall microphone has converted pressure variations of a sound wave into variations in a current in an electrical circuit
• How do loudspeakers work?

  ~ a.c. current passes through a coil of wire, which is surrounded by a permanent agent

  ~ a force is exerted on the coil => moves back and forth due to the a.c. current

  ~ these movements make the cone vibrate to amplify sounds
• What do transformers do?

  Change the size of the voltage of an alternating current
• How do transformers work?

  1.An alternating current passes through the primary coil.

  
  

  2.The alternating current produces a magnetic field that continuously changes direction. The soft iron core increases the strength of the magnetic field.

  
  

  3.The secondary coil cuts through the changing magnetic field, inducing an alternating voltage across the ends of the coil.

  
  

  4.An alternating current flows if a circuit is connected to the secondary coil

  
  

  5.It is important to note that there is no electrical connection between the primary and the secondary coils.
• What is the transformer equation to find the output p.d.?


  Vp/Vs = Np/Ns (input p.d./ output p.d. = no of turns on primary coil/ no. of turns on secondary coil)
• What is the transformer equation involving current through each coil?


  Vp x Ip = Vs x Is (primary coil p.d. x primary current = secondary p.d. x secondary current)
• How do power stations generate electricity?

  burning fuel releases energy > energy boils water > steam turns turbines > turbines turn electrical generators