MadSci Network: Physics

Re: What is Fleming's Left hand and right hand rule, and what do they do

Date: Mon Jan 22 19:47:19 2001
Posted By: Drew Procyk, Staff, Physicist, Beam Technology R&D, FEI Company
Area of science: Physics
ID: 979396870.Ph

Hello Marianne,
Hopefully I can shed some light on the right and left hand rules.  First, 
who _was_ this Fleming guy?  Sir John Ambrose Fleming lived 1849-1945 in 
England and was an electrical engineer.  Most of his work was in electric 
lighting and telegraphs.  He developed some of the first electric ship 
lights, but he is most noted for developing the vacuum diode- a device 
that converts an alternating current radio signal to a direct current that 
can be used in a telephone receiver.  You can learn more about Fleming at 
one of the on-line encyclopedias such as

Now on with the right and left hand rules.  They are handy ways to 
remember the relationship between magnetic field, current, and the 
resulting force on the conductor (or motion if it's allowed to move).  
Finger positions: for the right-hand rule, if you hold your right hand so 
that you point your index finger to something in front of you, have your 
thumb pointing toward the ceiling, and second (middle) finger to the left 
so that each finger is perpendicular to the other two.  The same finger 
finger positions on the left hand are used for the left-hand rule.

An electric motor is very simply a loop or coil of wire that is free to 
rotate in a magnetic field.  When there is a current in the wire, a 
magnetic field is generated which will want to align itself with the 
external magnetic field.  This causes the loop to want to rotate.  For 
this we use the LEFT hand rule to determine the direction of the force on 
the wire and hence which way the motor will turn.  If your fore finger 
points along the magnetic field and the central finger along the direction 
of the current, then the thumb points along the direction of motion of the 

A dynamo or generator is essentially a type of electric motor which has 
permanent magnets in it to provide the magnetic field.  Instead of 
supplying a current to the coil and getting rotational motion of the shaft 
in return, you supply rotational motion and get a current in return.  In 
this case the RIGHT hand rule can be used to determine the direction of 
the current flow in the wire loop.  If your fore finger points along the 
magnetic field and the thumb points along the direction of motion of 
conductor, then the central finger would give us the direction of the 
induced current.

I checked what my fingers represented in the CRC Handbook of Chemistry and 
Physics to make sure I was right (or left?)

Good luck,

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