Applications of the Generator Effect
Higher Tier Only
- The generator effect can be used to:
- Generate a.c in an alternator
- 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
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
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)
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
Examiner 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!