Edexcel IGCSE Physics: Double Science 复习笔记:6.2.2 Generators & Dynamos

Edexcel IGCSE Physics: Double Science 复习笔记:6.2.2 Generators & Dynamos

Generators & Dynamos

 

 

  • The generator effect can be used to:
    • Generate a.c in an generator
    • Generate d.c in a dynamo

     

 

 

 

Alternator

  • A simple alternator is a type of generator that converts mechanical energy to electrical energy in the form of alternating current

 

An alternator is a rotating coil in a magnetic field with commutator rings

 

  • A rectangular coil that is forced to spin in a uniform magnetic field
  • The coil is connected to a centre-reading meter by metal brushes that press on two metal slip rings (or commutator rings)
    • The slip rings and brushes provide a continuous connection between the coil and the meter

     

  • When the coil turns in one direction:
    • The pointer defects first one way, then the opposite way, and then back again
    • This is because the coil cuts through the magnetic field lines and a potential difference, and therefore current, is induced in the coil

     

 

 

  • The pointer deflects in both directions because the current in the circuit repeatedly changes direction as the coil spins
    • This is because the induced potential difference in the coil repeatedly changes its direction
    • This continues on as long as the coil keeps turning in the same direction

     

  • The induced potential difference and the current alternate because they repeatedly change direction

ac-alternator-output

A.c output from an alternator - the current is both in the positive and negative region of the graph

 

 

Dynamos

  • A dynamo is a direct-current generator
  • A simple dynamo is the same as an alternator except that the dynamo has a split-ring commutator instead of two separate slip rings

 

The-electric-motor

A dynamo is a rotating coil in a magnetic field connected to a split ring commutator

  • As the coil rotates, it cuts through the field lines
    • This induces a potential difference between the end of the coil

     

  • The split ring commutator changes the connections between the coil and the brushes every half turn in order to keep the current leaving the dynamo in the same direction
    • This happens each time the coil is perpendicular to the magnetic field lines

     

 

 

 

  • Therefore, the induced potential difference does not reverse its direction as it does in the alternator
  • Instead, it varies from zero to a maximum value twice each cycle of rotation, and never changes polarity (positive to negative)
    • This means the current is always positive (or always negative)

     

 

dc-dynamo-output 

D.C output from a dynamo - the current is only in the positive region of the graph

 

 

 

Bicycle Dynamo

  • A bicycle dynamo is used to supply electricity to bicycle lights whilst in motion
  • It consists of a rotating magnet placed inside (or next to) a coil
  • The magnet is rotated by its connection to the bicycle inside the coil
    • This is sometimes called the friction wheel and the axle / spindle

     

  • The magnetic field lines cut through the sides of the coil
    • This induces a potential difference in the coil

     

  • Since the magnetic field is constantly changing direction as it rotates, so does the output potential difference
    • This means the output current is also changing direction

     

 

 

 

  • Therefore, a bicycle dynamo, unlike a normal dynamo, produces alternating current (a.c)

A bicycle dynamo consists of a magnet rotating in a coil due to the motion of the wheels

 

Exam Tip

Motors and generators look very similar (as do microphones and loudspeakers), but they do very different things.When tackling a question on either of them, make sure you are writing about the right one!You might be expected to give the above explanations - make sure that you understand their subtle differences!

 

 

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