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In mathematics and physics, the right-hand rule is a common mnemonic for understanding notation conventions for vectors in 3 dimensions. It was invented for use in electromagnetism by British physicist John Ambrose Fleming in the late 19th century.
When choosing three vectors that must be at right angles to each other, there are two distinct solutions. This can be seen by holding your hands together, palm up, with the fingers curled. If the curl of your fingers represents a rotation from the first axis to the second, then the third axis can point either along your right thumb or your left thumb.
There are variations on the mnemonic depending on context, but all variations are related to the one idea of choosing a convention.
The right-hand rule in relation to a three dimensional coordinate axis system is described as follows. The thumb is orientated along the Z-Axis, the first or index finger is orientated to the X-Axis, and the second finger is orientated to the Y-Axis. The finger-tips of the three digits denote the positive direction of a linear translation or displacement. A positive rotation of the Z-Axis, anti-clockwise, is indicated by the third and fourth fingers. Positive rotations of the X and Y-Axis are also anti-clockwise.
The Cartesian axis system may be assigned X, Y, and Z; this will often be used to describe a global coordinate system. A local coordinate system may be described as: X’, Y’ and Z’ or 1, 2, and 3.
The cross product of two vectors is often encountered in physics and engineering. For example, in statics and dynamics, torque is the cross product of lever length and force, and angular momentum is the cross product of linear momentum and distance from an origin. In electricity and magnetism, the force exerted on a moving charged particle when moving in a magnetic field B is given by:
The direction of the cross product may be found by application of the right hand rule as follows: Using your right hand,
For example, for a positively-charged particle moving to the North, in a region where the magnetic field points West, the resultant force will point up.
A different form of the right-hand rule, sometimes called the right-hand grip rule or the corkscrew-rule, is used either when a vector (such as the Euler vector) must be defined to represent the rotation of a body, a magnetic field or a fluid, or vice versa when it is necessary to decode the rotation vector, to understand how the corresponding rotation occurs.
This version of the rule is used in two complementary applications of Ampère's circuital law:
The rule is also used to determine the direction of the torque vector. If you grip the imaginary axis of rotation of the rotational force so that your fingers point in the direction of the force, then the extended thumb points in the direction of the torque vector.
The right-hand rule is just a convention. When applying the rule to current in a straight wire for example, the direction of the magnetic field (counterclockwise instead of clockwise when viewed from the tip of the thumb) is a result of this convention and not an underlying physical phenomenon.
The first form of the rule is used to determine the direction of the cross product of two vectors. This leads to widespread use in physics, wherever the cross product occurs. A list of physical quantities whose directions are related by the right-hand rule is given below. (Some of these are related only indirectly to cross products, and use the second form.)
|Axis or vector||Right-hand||Right-hand (alternative)|
|X, 1, or A||First or index||Thumb|
|Y, 2, or B||Second finger or palm||First or index|
|Z, 3, or C||Thumb||Second finger or palm|
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