In order to make something useful of this generated force, more is required than simply a single, current-carrying conductor in a magnetic field. Figure 3 illustrates a coil carrying a current I in a static magnetic field of flux density B. Assume that the coil is mounted on a rotational axis O - O’. The interaction of the current I in coil segment AB with the magnetic field B will create a force F, shown downward in the drawing.
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| Figure 3 |
The interaction of the current I in coil segment CD in the same magnetic field B will also generate a force F. Since the direction of the current in CD is opposite to the direction of the current in AB, the resulting force is in the opposite direction (upward in Figure 3). Note that the current in coil segment AC is parallel to the magnetic field, and the net force in this coil segment must be zero.
The coil in Figure 3 will rotate about axis O - O’ ninety degrees and stop with the coil oriented vertically (segment AC perpendicular to the magnetic field). If the direction of the current flow is then reversed, the coil will rotate 180ˇ and, again, come to a stop. Thus, the arrangement shown in Figure 3 represents a crude rotary solenoid.
One could, theoretically, continue to switch the direction of current at the appropriate angular position and generate continuous rotational motion. In order to make such a device practical, however, it is necessary to add a means by which current can be conducted to the coil and switched at the proper time to create continuous motion. These functions are performed (in brush-type motors) by the brushes and commutator of the motor.
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