The Left Hand Thumb Rule (also called Maxwell's Left Hand Screw Rule or Left Hand Rule) is a simple rule used to find the direction of the magnetic field around a current-carrying straight conductor.
The magnetic field around a straight conductor was first discovered by Danish physicist Hans Christian Ørsted in 1820, when he noticed that a compass needle deflected when placed near a current-carrying wire — proving that electricity and magnetism are linked.
'If we hold a current-carrying straight conductor in the left hand such that the thumb points in the direction of the conventional current (from + to -), then the fingers wrapped around the conductor indicate the direction of the magnetic field lines around it.'
The magnetic field lines form concentric circles around the wire, and the left hand thumb rule tells us whether they go clockwise or anticlockwise when viewed from a specific direction.
There is often confusion between the two rules:
| Rule | Used For |
|---|---|
| Left Hand Thumb Rule | Direction of magnetic field around a current-carrying conductor |
| Right Hand Thumb Rule (same concept) | Used when dealing with conventional current |
| Fleming's Left Hand Rule | Direction of force on a current-carrying conductor in a magnetic field (used in motors) |
| Fleming's Right Hand Rule | Direction of induced current in a conductor moving in a magnetic field (used in generators) |
Note: Some textbooks attribute the thumb rule to the right hand for conventional current. The key is consistency with which hand your textbook uses.
The **Left Hand Thumb Rule** determines the direction of the **magnetic field** around a straight current-carrying wire. **Fleming's Left Hand Rule** determines the direction of the **mechanical force (motion)** on a current-carrying conductor placed inside an external magnetic field.
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