Left Hand Rule

The left-hand rule is a law for judging the relationship between the direction of the ampere force F (or motion), the direction of the magnetic induction intensity B, and the direction of the current I of the energized conductor rod when it is in a magnetic field. The left-hand rule and the right-hand rule are one of the focuses of the electromagnetics section.

The left-hand law is a simplified form of the cross product of two vectors to determine the direction of the force.

The left-hand rule was proposed by British electrical engineer John Ambrose Fleming (JohnAmbroseFleming, 1849~1945). In 1885, Fleming became a professor of electrical engineering at the University of London, because students often got the directions of magnetic fields, currents and forces wrong. So, he wanted to use a simple method to help students remember. The "left-hand rule" was born.

Straighten the index finger, middle finger, and thumb of the left hand so that they are perpendicular to each other in space. The direction of the index finger represents the direction of the magnetic field (from N to S), the middle finger represents the direction of the current (from positive to negative), and the direction of the thumb is the direction of the force. When using it, you can remember that the middle finger, index finger, and thumb refer to "electricity, magnetism, and force".

Main Application:

To Judge The Ampere Force:

The direction of force in a wire in a magnetic field. According to the left-hand rule: stretch out the left hand, make the thumb perpendicular to the other four fingers and in the same plane, let the magnetic field line flow in from the palm, the four fingers point in the direction of the current, and the thumb points in the direction of the ampere force (that is, the direction of the conductor's force). ).

To Judge The Lorentz Force:

Flatten the left palm, let the magnetic field line pass through the palm, the four fingers indicate the direction of the current, and the direction of the thumb perpendicular to the four fingers is the direction of the Lorentz force. Note that the motion charge is positive, and the direction of the thumb is the direction of the Lorentz force. Conversely, if the moving charge is negative, and the four fingers are still used to indicate the direction of charge movement, then the opposite direction of the thumb is the direction of the Lorentz force.

The Relationship Between The Two:

The Ampere force is the resultant force of the Lorentz force on an electric charge moving in a direction in a conductor.

When the current direction is parallel to the magnetic field, the directional movement direction of the charge is also parallel to the magnetic field direction, the Lorentz force is zero, and the resultant Ampere force is also zero.

The Lorentz force does not do work because the direction of the force is perpendicular to the direction of motion of the particle. According to the work formula W=FScosθ, when θ=90°, W=0. The ampere force is perpendicular to the direction of the current in the wire, but not necessarily perpendicular to the direction of movement of the wire. Generally, it is on the same straight line, so the work done by the ampere force is not 0.