CHAPTER 7 FORCES ON A CONDUCTOR

7.1       CURRENT-CARRYING CONDUCTOR IN A MAGNETIC FIELD

Figure 7.1 is similar to Figure 6.1 in that it shows a conductor in a magnetic field, but in this case there is a current from an external source being passed through that conductor.

FIGURE 7.1

FORCES ON A CONDUCTOR

The reaction between the current and the magnetic field through which it is passing causes a mechanical sideways force on the conductor.  If the conductor is free to move, it will move sideways in the direction of the force.  This is the basis of operation of all electric motors.

There is for motors a ‘Fleming’s Left-hand Rule’ corresponding to his Right-hand Rule for generators.  If the left hand is held with the thumb, forefinger and centre finger extended mutually at right angles, then, with the magnetic field in the direction (North to South) pointed by the forefinger and the direction of current in the direction indicated by the centre finger, the thumb will point in the direction of the mechanical force on the conductor (or of its motion if it is free to move).

The magnitude of the force on the conductor depends on the strength of the magnetic field and the strength of the current, and on nothing else.

Use is made of these laws and rules when considering the Principle of Operation of Motors described in the manual ‘Electric Motors’.