Download 1. Current carrying wires in external magnetic fields

The Motor Effect and the Generator Effect
1. Current carrying wires in external magnetic fields
• When a current carrying conductor is in the presence of
a magnetic field then a magnetic force will be induced
onto the conductor. This is the principle behind an
electric motor.

Moving charges experience a force upon
entering a magnetic field. Wires contain
electrons (charged particles). If these charges
are in motion (current flowing) then the wire
experiences a force.

The size of the force is dependent upon the
size of the fixed magnetic field and the
amount of current in the wire.
Fm  I B
Hey! We’ve seen this formula before. The
only difference is that a single length of wire
is replaced with a loop of wire to create a
motor.

In the diagram
below, a 5.0 cm
wire experiences a
0.023 N force up
out of the page in a
1.25 T magnetic
field. What is the
magnitude and
direction of the
current in the
conductor?


-the magnetic force on each side of the
loop will rotate it onto the vertical position
The loop will remain in the vertical position
SO: A major problem
must be solved:
 Maintaining the same
direction of rotation
for the loop of wire

To solve this
problem, consider
the diagram on the
following slide. A
loop of wire (called
an armature) is
placed within an
external magnetic
field.
•An electric current sent through the armature will produce a
force on each side of the armature as shown in the diagram.
• The forces result in rotation of the armature. If the armature is
allowed to rotate far enough (180°) sides A and B will have
switched places.
The result is that the armature flops back and forth,
eventually coming to a stop in a position where the
forces balance. This would not be a very useful motor.
To maintain continuous rotation in the same
direction, the electron flow must be reversed in the
loop every half turn

For the loop to rotate continuously, a split
ring commutator is used which reverses the
electron flow every 180 degrees.
NOTE: The input in a motor is
electrical energy and the output is
mechanical energy.
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.html#c1
http://www.walter-fendt.de/ph14e/electricmotor.htm
http://www.edumedia-sciences.com/en/a182-dc-motor


Electromagnetic induction – the generation or
production of a current in a conductor moved
through a magnetic field
Discovered by English scientist Michael
Faraday in the 1800’s (also by an American
Joseph Henry)


When a wire passes through changing lines of
magnetic field strength, a force is exerted
onto the valence electron within the
conductor
The result is the production of an induced
voltage as electrons move from one end to
the other in a conductor, thus acting as a
power source.


We must have a changing magnetic field
achieved by varying the distance of the
conductor from the source of the magnetic
field ( moving the wire or the magnets
creating the field)
The input is mechanical energy and the
output is electrical energy which is the
opposite of the motor effect.




The potential difference is created as a result of the 3rd
LHR.
Conductor motion: thumb
•Magnetic field: fingers
•Potential difference: palm (the direction the force is
acting on the electrons)
Induced current
in the wire is
directed out of
the plane of the
page.



When a conductor is moved through B, a
force acts onto the electrons moving them to
one end of the wire
when electrons are forced to one end of the
wire, a potential difference is produced
Work done to the electrons increases their
electric potential energy