Download Section 6 - Movement from Electricity

USING ELECTRICITY
SUMMARY NOTES
Section 6 — Movement from Electricity
Electrodynamics
(a)
A magnetic field exists round a wire which is carrying an
electric current.
(b)
An electromagnet is made up of a coil of wire wound on an
iron core. It works as a magnet only when there is an electric
current in the coil.
(c)
Electric bells and relays make use of the principle of the
electromagnet.
Current-carrying Conductor in a Magnetic Field
When a wire carrying a current is placed in a magnetic field, a
force acts on it. The direction of this force depends on the
direction of the current and on the direction of
the magnetic field. If either is reversed, the
direction of the force is reversed. When the field
and the current are at right angles to each other,
the force is at right angles to both.
The Electric Motor
When a rectangular loop of wire carrying a current is placed in a
magnetic field, forces act on the two sides of the loop. They act in
opposite directions and tend to make the loop turn.
If the loop is free to turn - (i), it will move until it reaches the
position shown in (ii). At this position, the forces no longer have a
turning effect. In practice, the loop will be carried slightly past this
position due to its inertia.
If the current through the loop is now reversed - (iii), the two
forces will again have a turning effect causing the loop to make
another half turn in the same direction.
By reversing the current every half turn, the coil will keep rotating
in the same direction. To do this automatically, a split ring
commutator is used with sliding contacts, called brushes, touching
it.
By using a coil of wire instead of a single loop, a much greater
turning force is produced.
Commercial Motors
Field Coils
In a 'real' motor, the magnetic field is provided by fixed coils,
usually called field coils, which carry the current.
Advantage:
This is a more compact, lighter and cheaper way of providing a
magnetic field than using a permanent magnet.
Brushes
The brushes in commercial motors are made of carbon (graphite)
Advantages: because carbon is a good conductor and it is soft
enough to wear away as it slides over the commutator, without
wear occurring to the commutator itself. The brushes are held
against the commutators by springs so that they always make good
contact with it. If the brushes
wear away completely, they can
be replaced.
Rotating Coil and Commutator
The commutator is made up of several segments with a separate
coil connected to each pair of segments directly opposite each
other.
Advantage: The greatest turning force exists when a coil is at right
angles to the direction of the magnetic field; so, by having several
coils mounted at various angles to one another, one of the coils is
always moving at approximately right angles to the magnetic field.
This makes the rotation of the coil much smoother.