17. Force on a Current-Carrying Conductor in a Magnetic Field
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Just as a current-carrying conductor produces its own magnetic field (as shown by Oersted), the reverse effect also occurs: when a current-carrying conductor is itself placed inside an external magnetic field, it experiences a mechanical force. This force arises from the interaction between the external magnetic field and the magnetic field produced by the current in the conductor. The magnitude of this force depends on the strength of the magnetic field, the magnitude of the current, the length of the conductor within the field, and the angle between the conductor and the direction of the field — the force is maximum when the conductor is placed perpendicular to the magnetic field, and it is zero when the conductor is placed parallel to the field.
Fleming's Left-Hand Rule
The direction of the force on a current-carrying conductor placed in a magnetic field is given by Fleming's Left-Hand Rule, devised by the British engineer John Ambrose Fleming. This rule is used specifically for finding the direction of force in devices where electrical energy is converted into mechanical motion, most notably the electric motor. The rule is applied as follows: stretch the thumb, the forefinger (index finger), and the middle finger of the left hand so that all three are mutually perpendicular to one another (like the three edges of a corner of a box meeting at a point). If the forefinger points in the direction of the magnetic field, and the middle finger points in the direction of the current, then the thumb points in the direction of the force (and hence the motion) experienced by the conductor. A simple memory aid: Fore-finger = Field, Middle finger = Current (or Motion... students commonly remember it as F-B-I for Force, B(field), I(current), but the standard scheme is Field–Current–Thumb-as-Motion). This principle — a current-carrying conductor experiencing a force in a magnetic field — is the fundamental working principle of the electric motor, described later in this chapter.