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Fleming's left-hand rule

Fleming's left-hand rule

A graphic illustrating Fleming's left-hand rule. Arrows indicating the direction of the field, the current and the resulting motion and a hand indicating the Fleming's left-hand rule are shown.

When a wire carrying an electric current is moved in a magnetic field of a magnet the magnetic field induced by the wire reacts with the magnetic field of the magnet causing the wire to move outwards. Fleming's left hand rule helps you to predict the movement.

 

Fleming's left-hand rule (for motors), and Fleming's right-hand rule (for generators) are a pair of visual mnemonics. They were originated by John Ambrose Fleming, in the late 19th century, as a simple way of working out the direction of motion in an electric motor, or the direction of electric current in an electric generator.

When current flows in a wire, and an external magnetic field is applied across that flow, the wire experiences a force perpendicular both to that field and to the direction of the current flow. A left hand can be held, as shown in the illustration, so as to represent three mutually orthogonal axes on the thumb, first finger and middle finger. Each finger is then assigned to a quantity (mechanical force, magnetic field and electric current). The right and left hand are used for generators and motors respectively.

Distinction between the right-hand and left-hand rule

Fleming's right-hand rule

Fleming's left-hand rule is used for electric motors, while Fleming's right-hand rule is used for electric generators.

Different hands need to be used for motors and generators because of the differences between cause and effect.

In an electric motor, the electric current and magnetic field exist (which are the causes), and they lead to the force that creates the motion (which is the effect), and so the left hand rule is used. In an electric generator, the motion and magnetic field exist (causes), and they lead to the creation of the electric current (effect), and so the right hand rule is used.

To illustrate why, consider that many types of electric motors can also be used as electric generators. A vehicle powered by such a motor can be accelerated up to high speed by connecting the motor to a fully charged battery. If the motor is then disconnected from the fully charged battery, and connected instead to a completely flat battery, the vehicle will decelerate. The motor will act as a generator and convert the vehicle's kinetic energy back to electrical energy, which is then stored in the battery. Since neither the direction of motion nor the direction of the magnetic field (inside the motor/generator) has changed, the direction of the electric current in the motor/generator has reversed. This follows from the second law of thermodynamics(the generator current must oppose the motor current, and the stronger current outweighs the other to allow the energy to flow from the more energetic source to the less energetic source).

The rule for motors can be recalled by remembering that "motors drive on the left in Britain". The rule for generators can be recalled by remembering that either the letters "g" and "r" is common to both "right" and "generator", or the phrase "Jenny is always right" ("Genny" being a common shortened version of Generator).

 

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