Applications of the Generator Effect (AQA GCSE Physics)

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

Alternators

• An alternator, or a.c. generator, is a device which converts energy from motion into an electrical output
• An alternating potential difference (p.d.) is generated which causes an alternating current to flow
• A simple alternator consists of 
  • a rotating coil of wire between the poles of a permanent magnet
  • slip rings and brushes connected to an external circuit

Structure of a simple alternator

A simple alternator consists of a rotating coil in a magnetic field connected to an external circuit via slip rings and carbon brushes

• The functions of each component are shown in the table:

Table of components of a simple alternator

Component	Function
permanent magnet	to provide a uniform magnetic field
rotating coil	to cut the magnetic field as it rotates and allow an induced current to flow
slip rings	to allow the alternating current to flow between the coil and the external circuit
carbon brushes	to provide a good electrical connection between the coil and the external circuit

 

Operation of an alternator

• A rectangular coil rotates in a uniform magnetic field
• The coil is connected to an external circuit via slip rings and brushes
  • The induced p.d. in the coil can be measured by adding a galvanometer (centre-zero meter) to the external circuit

• A p.d. is induced in the coil as it cuts the magnetic field
  • The pointer defects first one way, then the opposite way, and then back again
  • This indicates the size and direction of the p.d. is constantly changing

• As a result of the alternating p.d., an alternating current is also produced as the coil rotates
  
  • This continues as long as the coil keeps turning in the same direction

Motion of an alternator

The size and direction of the induced p.d. (and current) depend on the orientation of the coil with the field

• A maximum p.d. is induced when
  • the position of the coil is horizontal
  • the motion of the coil is perpendicular to the field

• This is because the greatest number of lines are cut when the coil is moving perpendicular to the field

• No p.d. is induced when
  • the position of the coil is vertical
  • the motion of the coil is parallel to the field

• This is because no lines are cut when the coil is moving parallel to the field

Dynamos

• A dynamo, or d.c. motor, is a device which converts an electrical input into motion
• A simple dynamo contains the same components as an alternator except instead of slip rings, it uses a split-ring commutator 

Structure of a simple dynamo

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

• The functions of each component are shown in the table:

Table of components of a simple dynamo

Component	Function
permanent magnet	to provide a uniform magnetic field
rotating coil	to provide the rotation as the current flows through it
split ring commutator	to allow the connection between the coil and the external circuit to change every half turn
carbon brushes	to provide a good electrical connection between the coil and the external circuit

 

Operator of a dynamo

• 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)

The 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)

Structure of a bicycle dynamo

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