(455) Hand rule for moving charge in magnetic field [IB Physics SL/HL]

Third Hand Rule Introduction

• The Third Hand Rule is used for a moving charge or wire in a magnetic field.

• Common learning methods include the FBI mnemonic, but this teacher prefers a more personal approach.

Personal Anecdote

• The teacher shares a story about dating his wife, Ina, and how they taught each other through their interests, including Star Wars.

• Inspired by a scene in "Return of the Jedi" where the Emperor shoots lightning, the teacher adapts this imagery to explain physics concepts.

Hand Rules Overview

• Left-Hand Rule: Used for negative charges (e.g., electrons).

• Right-Hand Rule: Used for positive charges or current in a wire.

  • Right-hand rule applies to conventional current (direction of positive charge).

Memorable Mnemonic

• The teacher refers to electrons as "Leftons" to remember the left-hand rule.

Drawing the Hand Rules

• The left hand symbolically drawn to represent the negative charge using fingers pointing into the page, thumb to the right, palm indicating the force direction (down).

• Magnetic field lines drawn as X's (pointing into the page).

Example Problems

1. Electron in Magnetic Field

   • Situation: Electron moves to the right in a magnetic field directed into the page.

   • Application: Use left-hand rule (fingers into the page, thumb to the right, palm points down) predicts the electron's path curves downwards.

2. Proton in Magnetic Field

   • Situation: Proton moves to the right in an upward-directed magnetic field.

   • Application: Use right-hand rule (fingers up, thumb to the right, palm points out of the page) predicts the proton curves out of the page.

3. Neutron in Magnetic Field

   • Situation: Neutron moves to the right in a downward magnetic field.

   • Application: No hand rule applies since neutrons are neutral, the path remains undeflected.

Conclusion

• By applying the hand rules, you can determine the behavior of charged particles in a magnetic field easily by remembering to think of the Emperor from Star Wars and using the catchphrase "Use the force."