Fleming's left hand rule
Encyclopedia
Mnemonic
A mnemonic , or mnemonic device, is any learning technique that aids memory. To improve long term memory, mnemonic systems are used to make memorization easier. Commonly encountered mnemonics are often verbal, such as a very short poem or a special word used to help a person remember something,...
s that is used for working out the direction of motion in an electric motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
, or the direction of electric current in an electric generator. They were coined by John Ambrose Fleming
John Ambrose Fleming
Sir John Ambrose Fleming was an English electrical engineer and physicist. He is known for inventing the first thermionic valve or vacuum tube, the diode, then called the kenotron in 1904. He is also famous for the left hand rule...
in the late 19th century.
When an electric 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 (electric current, magnetic field and mechanical force). The right and left hand are used for generators and motors respectively.
Conventions
- The direction of the mechanical force is simply its literal direction
- 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.
Mnemonics
Several memory aids have been used in order to remember the quantity each finger represents.First variant
- The Fore finger represents the direction of the magnetic Field
- The Centre finger represents the direction of the Current
- The Thumb represents the direction of the Thrust.
Second variant
- The First finger represents the direction of the magnetic Field
- The Second finger represents the direction of the Current
- The Thumb represents the direction of the resultant Motion.
Third variant
Van de Graaff'sRobert J. Van de Graaff
Robert Jemison Van de Graaff, was an American physicist, noted for his design and construction of high voltage generators, who taught at Princeton University and MIT.-Biography:...
translation of Fleming's rules is the FBI rule, easily remembered because it is the acronym for the Federal Bureau of Investigation
Federal Bureau of Investigation
The Federal Bureau of Investigation is an agency of the United States Department of Justice that serves as both a federal criminal investigative body and an internal intelligence agency . The FBI has investigative jurisdiction over violations of more than 200 categories of federal crime...
.
This uses the conventional symbolic parameters of F (for Lorentz force
Lorentz force
In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...
), B (for magnetic flux density) and I (for electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
), 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
FBI mnemonics
The various FBI mnemonics show the direction of the force on a conductor carrying a current in a magnetic field.Other mnemonics exist that use a right hand rule for predicting resulting motion from a preexisting current and field....
page.
Distinction between the right-hand and left-hand rule
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
s, while Fleming's right-hand rule is used for electric generators.
Separate 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 magnet field exist, and they lead to the force that creates the motion; in an electric generator, the motion and magnetic field exist, and they lead to the creation of the electric current.
Many types of electric motor can also be used as an electric generator. A vehicle
Vehicle
A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....
that is powered by such a motor can be accelerated up to high speed by connecting the motor to a fully charged battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
. However, if the motor is then disconnected from the fully charged battery, and connected instead to a completely flat battery, the vehicle will decelerate, while converting the kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
back to electrical energy, and storing it in the battery. It follows, therefore, that while neither the direction of motion nor the direction of the magnetic field (inside the motor/generator) have changed, the direction of the electric current in the motor/generator has reversed.
This follows from the second law of thermodynamics
Second law of thermodynamics
The second law of thermodynamics is an expression of the tendency that over time, differences in temperature, pressure, and chemical potential equilibrate in an isolated physical system. From the state of thermodynamic equilibrium, the law deduced the principle of the increase of entropy and...
. The generator current must oppose the motor current, and the stronger one 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 letter "g" or "r" is common to both "right" and "generator".
Physical basis for the rules
Charged particle
In physics, a charged particle is a particle with an electric charge. It may be either a subatomic particle or an ion. A collection of charged particles, or even a gas containing a proportion of charged particles, is called a plasma, which is called the fourth state of matter because its...
s, flow in the same direction (for example, as an electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
in an electrical conductor
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
, such as a metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
wire
Wire
A wire is a single, usually cylindrical, flexible strand or rod of metal. Wires are used to bear mechanical loads and to carry electricity and telecommunications signals. Wire is commonly formed by drawing the metal through a hole in a die or draw plate. Standard sizes are determined by various...
) they generate a cylindrical magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
that wraps round the conductor (as discovered by Hans Christian Ørsted
Hans Christian Ørsted
Hans Christian Ørsted was a Danish physicist and chemist who discovered that electric currents create magnetic fields, an important aspect of electromagnetism...
).
The direction of the induced magnetic field is sometimes remembered by the corkscrew rule. That is, if the conventional current is flowing away from the viewer, the magnetic field runs clockwise round the conductor, in the same direction that a corkscrew
Corkscrew
A corkscrew is a kitchen tool for drawing stopping corks from wine bottles. Generally, a corkscrew consists of a pointed metallic helix attached to a handle. The user grips the handle and screws the metal point into the cork, until the helix is firmly embedded, then a vertical pull on the...
would have to turn in order to move away from the viewer. The direction of the induced magnetic field is also sometimes remembered by the right-hand grip rule, as depicted in the illustration, with the thumb showing the direction of the conventional current, and the fingers showing the direction of the magnetic field. The existence of this magnetic field can be confirmed by placing magnetic compasses at various points round the periphery of an electrical conductor that is carrying a relatively large electric current.
If an external magnetic field is applied horizontally, so that it crosses the flow of electrons (in the wire conductor, or in the electron beam), the two magnetic fields will interact. Michael Faraday
Michael Faraday
Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....
introduced an analogy for visualising this, in the form of imaginary magnetic lines of force: those in the conductor form concentric circles round the conductor; those in the externally applied magnetic field run in parallel lines above and below the conductor. If those above the conductor are running (from the north to south magnetic pole) in the opposite direction to those surrounding the conductor, they will be deflected so that they pass underneath the conductor (because magnetic lines of force cannot cross or run contrary to each other). Consequently, there will be a large number of magnetic field lines in a small space under the conductor, and a dearth of them above the conductor. Since the magnetic field lines of force are no longer straight lines, but curved to run under the electrical conductor, they are under tension (like stretched elastic bands), with energy stored up in the magnetic field. There is therefore a force that is being applied to the only moveable object in the system (the electrical conductor) to expel it up, and out of the externally applied magnetic field. This is the reason for torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
in an electric motor. (The mechanism of the electric motor is then constructed so that the expulsion of the conductor out of the magnetic field causes it be placed inside the next magnetic field, and for this switching to be continued indefinitely.)