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P6E
Magnetic fields and wires
A wire carrying an electric current
produces a magnetic field. The shape of
this field depends upon the shape of the
wire.
A straight wire
The magnetic field around a straight wire
consists of ‘concentric circles’ (circles
around the same centre). These are at
right angles to the direction in which the
electric current flows. Study the
animation below to make sure you
understand this.
Note that the direction of the magnetic
field is reversed if the direction of the
electric current is reversed.
A single turn in a coil of wire
The magnetic field around a coil of wire
with a single turn also consists of
concentric circles, but there are two sets
of them acting in opposite directions.
A solenoid
A solenoid consists of a long piece of wire
made into several coils. Its magnetic field
is the same as the magnetic field
produced by a simple bar magnet.
Current upwards
Current downwards
No current
Uses of electric motors
Electric motors transfer electrical energy
in two main ways - to the load (as useful
work) and to the surroundings (mostly as
waste heat energy).
Motors are found in a variety of everyday
applications. For example, they are used
in:
Current away
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fans
electric drills
food processors
washing machines
CD and DVD players
car windscreen wipers
You should be able to recall uses like
these.
DC electric motors
A force can act on a wire carrying an
electric current when it is in a magnetic
field.
This does not happen if the wire is
parallel to the magnetic field, but it
does happen when the wire is at right
angles to the magnetic field.
The direction the wire moves depends on
the direction of the current and the
direction of the magnetic field.
The direction of the wire is reversed if:
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the direction of the current is
reversed
the direction of the magnetic field is
reversed
Current torwards
A turning force is produced if the wire is
made into a coil. This effect is used in
simple DC (direct current) electric motors.
The turning force in a motor increases
when the number of turns on the coil is
increased. It also increases when:

the electric current is increased

the strength of the magnetic field is
increased
DC (direct current) electric motors
Fleming’s left-hand rule
Fleming’s left hand rule is a simple way to
predict the relative directions of the
movement of the wire, the magnetic field
and the electric current.
You need to point your thumb, forefinger
(index finger) and second finger at right
angles to each other for this to work.
Then, as you can see in the diagram below,
your thumb shows the movement of the
wire, your forefinger that of the field and
your second finger that of the current.
More about motors
In a simple DC (direct current) electric
motor, the direction of the electric
current must be changed every half-turn.
This means that the force on the coil is
maintained as the coil turns. The coil’s
momentum carries it on at the start of
each half-turn.
The use of a split-ring commutator allows
the coil to make electric contact with the
DC supply, while ensuring that the current
changes direction every half-turn.
Practical electric motors use curved
magnets rather than flat ones. These
produce a radial magnetic field - which
means that the electric current stays at
right angles to the magnetic field for
longer.