In order to understand how DC generators produce electricity, it is crucial to understand how the relative motion of a conductor moving through a magnetic field ( B ) induces forces that put the conductor’s electrons into motion; however, prior to engaging in a discussion about electromagnetic induction, we must briefly re-examine how the magnetic field of a current-carrying wire interacts with the B-fields of permanent magnets. Always remember that electric motors use pre-existing currents to create rotational motion within a rotor while generators produce current as a consequence of mechanical forces being imparted upon a system. Let’s first consider the orientation of the B-fields of current-carrying wires whose currents, from our point of view, travel into and out of the screen or page we are viewing:
The orientation of the B-fields above will either be additive or in opposition to the north-to-south orientation of a permanent magnet’s B-field through which they travel. Consider the B-field of the permanent magnet below:
It is clear that the B-field of a current-carrying conductor heading into our screen or page will have a cancellation effect upon the magnet’s magnetic lines of flux ( ɸ ); thus, the flux density immediately above the conductor will be lessened. This in turn will cause the wire at hand to move upward within the magnet’s magnetic field:
The converse effect is observed when the direction of current flow is reversed:
What if, however, a currentless conductor is moved upward or downward within the magnetic field at hand??? There must certainly be some sort of symmetry with observations that are made regarding wires that carry currents, correct??? Well yes, this is indeed the case. Beforehand, current gave rise to forces upon a conductor within the magnetic field. When dealing with generators, a conductor is moved through the B-field by an outside agent. This motion in turn gives rise to forces upon the wires electrons, thus creating a current:
The aforementioned current’s B-fields have the same orientation as those predicted by the Left-Hand rule of wires that carry a pre-existing current. Let’s now imagine what would happen if a path was created through which current travels from the leftmost wire to the one on the right. If such a connection is made, a twisting motion can be used to establish a coordinated flow of direct current through the conducting system that has been established:
It is important to note that this twisting loop of wire is attached to a split-ring commutator whose function will be reviewed shortly:
If the wire loop rotated without interruption within the magnet’s B-field, there would come a time when the loop’s right side would be moving upward while the left side of the loop switches direction and moves downward; thus, the direction in which current travels would reverse. This is not what is desired within a DC apparatus. The introduction of a split ring to the system prevents such a reversal of current from occurring:
The magnitude of the current produced is directly proportional to the rate of the revolving system; however, this magnitude shifts in proportion to the density of magnetic lines of flux being traversed at any moment. If we begin with the area of the loop oriented perpendicular to the magnetic flux, the induced current will be at a maximum when each arm of the loop is positioned midway within the field, as is the case in the diagram above. As each arm of the loop begins to move out of the field, the magnitude of the current will decrease proportionally. The current will reach a minimum when, once again, the loop’s area is perpendicular to the magnet’s B-field. At this point, the splits within the commutator will prevent the aforementioned current reversal from occurring.
As we will see later, all that is needed to convert the DC generator above into a DC motor is a pre-existing current and a mechanical push that puts the system’s rotor into motion.
RENAISSANCE PHYSICS 