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.
When Will a Diver Experience Greater Pressure?
A diver experiences greater pressure at greater depth. Pressure = P₀ + ρgh. Every 10 m depth adds ~1 atm (101 kPa) of pressure in seawater.
Who is the Father of Physics? (Galileo, Newton, and Einstein)
Find out who is known as the father of physics. Learn about the contributions of Galileo Galilei, Isaac Newton, and Albert Einstein to physical sciences.
Why are Sound Waves Called Mechanical Waves?
Learn why sound waves are called mechanical waves in Physics. Understand what a medium is, longitudinal wave nature of sound, and why sound cannot travel in vacuum.
Why Do We Feel Uneasy When a Ferris Wheel Moves Downward?
When a Ferris wheel moves downward, your apparent weight decreases: N = m(g − v²/r). The reduced normal force causes a stomach-drop sensation. Centripetal force explained with examples.
Why We Cannot Hear Echo in a Classroom
Echo is not heard in a classroom because the walls are too close. Minimum distance for echo = 17 m. Most classrooms are 8–10 m — the reflected sound merges with the original.
Turn this guide into revision flashcards, a practice exam, or an AI-generated podcast — free, no signup required.