as it rotates, the sum of the torques on all three rotors stays nearly constant. This
combination of current-reversing commutators with multiple rotors enables the motor to
provide a steady source of torque.
Changes direction of current
Reverses magnetic momentRotors with multiple arms
More arms = smoother torque
28.20 - The magnetic field around a wire
Currents produce magnetic fields. In this section, we analyze the orientation of the
magnetic field created by a current flowing through a wire. In the Concept 1 and 2
diagrams, the electrons that make up the current are moving to the right. Conventional
current flow is indicated (as always) with an arrow pointing in the opposite direction.
The Concept 1 diagram uses two compasses to show the direction of the magnetic field
due to the current at two different points. Each needle points in the direction of the
magnetic field at its location.
As the diagram shows, the magnetic field wraps around the current. The arrows indicate
its direction. The field is strongest near the wires. As with electric field diagrams, the
density of lines reflects the strength of the field. This means the lines are drawn close
together near the wire.
How do you determine the direction of the magnetic field? You use the right-hand rule
for currents. Wrap your hand around the wire so that your thumb points in the direction
of the (conventional) current. Your fingers wrap around, just like the concentric circles in
the diagram, and they point in the direction of the magnetic field. You see this shown in
Concept 2. If the direction of the current were to reverse, your thumb would point in the
opposite direction, and your fingers would wrap in the other direction. When the
direction of the current changes, so does the direction of the magnetic field.
Magnetic fields and wires
Currents generate magnetic fields
·Magnetic field “circles” around wireRight-hand rule for currents
Right-hand rule tells direction of field
·Point thumb in direction of conventional
current
·Curled fingers show direction of
magnetic field