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P3.3.1 The motor effect
a) When a current flows through a wire a magnetic field is produced
around the wire.
b) This magnetism will interact with any other magnetism nearby. This
could cause attraction or repulsion but can also cause MOVEMENT.
c) The ELECTROMAGNET is a device in which magnetism can be
SWITCHED ON or OFF and its STRENGTH can be CONTROLLED.
d) It is designed to have AN IRON CORE, with a COIL of INSULATED
WIRE around it: CURRENT is passed through the coil.
The strength depends on the CURRENT and the size and number of the
TURNS of wire in the coil (as well as the core material)
P3.3.1 The motor effect
a) In some applications, it is important for an electromagnet to switch
on and off quickly even when it is supplied by a DC supply such as a
battery.
e.g. the electric bell
P3.3.1 The motor effect
The motor effect relies on the fact that a conductor carrying a
CURRENT in a magnetic field will experience a force provided it is not
parallel to the magnetic field.
The effect is greatest if the conductor and field are AT RIGHT ANGLES.
The directions of the CURRENT, FIELD and FORCE are governed by
FLEMING’S LEFT HAND RULE.
The conventions are that
a) Current flows from
positive to negative
b) Field flows from North
to South
P3.3.1 The motor effect
a) If the direction of the current is reversed the wire will move
down.
b) If the field direction is reversed, the wire will reverse its
movement
P3.3.1 The motor effect
a) If the power supply is an ALTERNATING ONE, the wire will
vibrate up and down. This is the basis of the MOVING COIL
LOUDSPEAKER
b) The alternating voltage signal could come from an audio
amplifier
c) The coil is attached to a cardboard cone which vibrates and
sends out SOUND waves
P3.3.1 The motor effect
a) To produce actual rotation, the wire inside the magnetic field
must be wound into a coil which can rotate.
b) The sides of the coil will experience forces in OPPOSITE
DIRECTIONS because the current in the two ‘arms’ will flow in
opposite directions.
In which way will this coil
rotate at the time shown in
the drawing?
P3.3.1 The motor effect
a) The sides of the coil will experience forces in OPPOSITE
DIRECTIONS because the current in the two ‘arms’ will flow in
opposite directions.
P3.3.1 The motor effect
a) This movement would stop with the coil in the VERTICAL
POSITION unless the current direction was REVERSED. This is
achieved by a COMMUTATOR which has sliding contacts
P3.3.1 The motor effect
In this simple design, the coil rotates until each side reaches a gap
in the commutator. Its momentum carries it until each side
connects with the other carbon brush so rotation is achieved.
P3.3.1 The motor effect
In this simple design, the coil rotates until each side reaches a gap
in the commutator. Its momentum carries it until each side
connects with the other carbon brush so rotation is achieved.
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