FBI mnemonics
The various FBI mnemonics (for electric motors) show the direction of the force on a conductor carrying a current in a magnetic field as predicted by Fleming's left hand rule for motors and Faraday's law of induction.
Other mnemonics exist that use a right hand rule for predicting resulting motion from a preexisting current and field.
Configuration, using the left hand
'FBI'
The left hand is held with the thumb, first finger and second finger mutually at right angles.
- The Thumb represents F - Thrust (or resulting Motion).
- The First finger represents B - Field.
- The Second finger represents I or V - Current (in the classical direction, from positive to negative).
Configuration, using the right hand
Other sources use the right hand to predict force, though the fingers used vary:
'FIB'
- Thumb = F ("thrust")
- Index finger = I or V
- Middle finger ("Birdie") = B
'IBF'
In this alternative, some versions recommend not extending the middle finger, but instead imagining the force coming from the palm of the hand.
- Thumb = I or V
- Index finger = B
- Middle finger = F
Other frequently confused rules
Prediction of direction of field ('B')
The rules above are usually used to predict 'F' based upon 'B' and 'I' - the force on a moving charge when moving through a field, whether or not the charge is carried in a wire. However, this rule should not be confused with a different right hand grip rule for the prediction of the direction of a field ('B') produced by current ('I') traveling through a wire.
Symmetry
| Vector | Left-Motor or Right-Generator | Left-Motor or Right-Generator | Left-Motor or Right-Generator | Right-Motor or Left-Generator | Right-Motor or Left-Generator | Right-Motor or Left-Generator |
|---|---|---|---|---|---|---|
| B or Field | First or Index | Thumb | Fingers or Palm | First or Index | Thumb | Fingers or Palm |
| I or Current (+ to - flow) | Fingers or Palm | First or Index | Thumb | Thumb | Fingers or Palm | First or Index |
| Force, Motion, or Thrust | Thumb | Fingers or Palm | First or Index | Fingers or Palm | First or Index | Thumb |
See also
External links
- Overview at ac.wwu.edu (Right Hand - 'FIB')
- Diagrams at magnet.fsu.edu. See figure #9. (Left Hand - 'FBI')
- Description at starprof.com (Right Hand - 'IBF' - palm alternative)
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