Fleming's left-hand rule for motors

Lore & Background

Fleming's left-hand rule is one of a pair of visual mnemonics, with the right-hand rule used for generators. The rule was created by John Ambrose Fleming in the late 19th century as a simple way to work out the direction of motion in an electric motor. When current flows through a conducting wire and an external magnetic field is applied, the wire experiences a force perpendicular to both the field and the current direction. The left hand is held with thumb, forefinger, and middle finger mutually orthogonal, each assigned to a quantity: the thumb represents the direction of motion (or force), the forefinger represents the magnetic field, and the middle finger represents the current.

Several variants exist. One variant uses the FBI mnemonic, where F (thumb) stands for force, B (forefinger) for magnetic flux density, and I (middle finger) for current. Another variant, called the 'Fire the field, feel the force and kill the current' approach, uses a pretend gun gesture: index finger as barrel (field), thumb as hammer (force), and middle finger displayed to 'kill the current'. The article notes that if the mnemonic is taught with a different arrangement of parameters to fingers, it could reverse the roles of the two hands.

Reader's Guide

Fleming's left-hand rule is significant because it provides a simple, memorable method for determining the direction of force (motion) in an electric motor, given the directions of current and magnetic field. The rule is used for motors because in an electric motor, electric current and magnetic field exist as causes, leading to force that creates motion as the effect. This contrasts with the right-hand rule for generators, where motion and magnetic field are causes that create electric current. The distinction between hands is necessary due to the differences between cause and effect, as illustrated by the fact that many electric motors can also act as generators: if a vehicle's motor is disconnected from a charged battery and connected to a flat battery, the vehicle decelerates as the motor becomes a generator, reversing the current direction while motion and magnetic field remain unchanged. The physical basis involves the interaction of magnetic fields: when electrons flow in a conductor, they generate a cylindrical magnetic field (as discovered by Hans Christian Ørsted). An external magnetic field interacts with this induced field, causing magnetic lines of force to be deflected, creating tension and a force that expels the conductor from the external field, producing torque in an electric motor.

Did You Know?

• Fleming's left-hand rule was originated by John Ambrose Fleming in the late 19th century.
• The rule uses the left hand, with the thumb for motion, forefinger for magnetic field, and middle finger for current.
• One variant of the rule is the FBI mnemonic, where F stands for force, B for magnetic flux density, and I for electric current.
• The rule is used for motors because current and magnetic field are causes that produce motion as the effect.