State the difference between Fleming’s Left-Hand Rule and
Fleming’s Right-Hand Rule. What is the nature of the induced current
produced in the conductor ?
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Answers
Fleming's left-hand rule for electric motors is one of a pair of visual mnemonics, the other being Fleming's right-hand rule[1] (for generators). 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.
Fleming's left-hand rule
When current flows through a conducting wire, and an external magnetic field is applied across that flow, the conducting wire experiences a force perpendicular both to that field and to the direction of the current flow (i.e they are mutually perpendicular). A left hand can be held, as shown in the illustration, so as to represent three mutually orthogonal axes on the thumb, fore 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.
Conventions
The direction of the mechanical force is the literal one.
The direction of the magnetic field is from north to south.
The direction of the electric current is that of conventional current: from positive to negative.
First variant
The Thumb represents the direction of the Thrust on the conductor / Motion of the Conductor
The Fore finger represents the direction of the magnetic Field
The Centre finger represents the direction of the Current.
Second variant
The Thumb represents the direction of Motion resulting from the force on the conductor
The First finger represents the direction of the magnetic Field
The Second finger represents the direction of the Current.
Third variant
Van de Graaff's translation of Fleming's rules is the FBI rule, easily remembered because these are the initials of the Federal Bureau of Investigation.
Fourth variant (FBI)
The F (Thumb) represents the direction of Force of the conductor
The B (Forefinger) represents the direction of the Magnetic field
The I (Centre finger) represents the direction of the Current.
This uses the conventional symbolic parameters of F (for Lorentz force), B (for magnetic flux density) and I (for electric current), and attributing them in that order (FBI) respectively to the thumb, first finger and second finger.
The thumb is the force, F
The first finger is the magnetic flux density, B
The second finger is the electric current, I.
Of course, if the mnemonic is taught (and remembered) with a different arrangement of the parameters to the fingers, it could end up as a mnemonic that also reverses the roles of the two hands (instead of the standard left hand for motors, right hand for generators). These variants are catalogued more fully on the FBI mnemonics page.
Fifth variant (Fire the field, feel the force and kill the current)
This approach to remembering which finger represents which quantity uses some actions. First of all you need to point your fingers like a pretend gun, with the index finger acting as the barrel of the gun and the thumb acting as the hammer. Then go through the following actions:
"Fire the field" out through your index finger
"Feel the force" of the gun recoil up through your thumb
Finally you display your middle finger as you "kill the current"
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.
Fleming's left-hand rule is used for electric motors, while Fleming's right-hand rule is used for electric generators. ... 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.