The correct option is Fleming’s left-hand rule.
Explanation
When a current-carrying conductor is placed in an external magnetic field, it experiences a mechanical force due to the interaction between the magnetic field produced by the current and the external magnetic field. This is often referred to as the motor effect. The direction of this force is perpendicular to both the direction of the current and the direction of the magnetic field.
Option Analysis
- Fleming’s left-hand rule Fleming’s left-hand rule: Correct. This rule is specifically used to determine the direction of force (or motion) acting on a current-carrying conductor placed in a magnetic field. According to this rule, if the thumb, forefinger (index finger), and middle finger of the left hand are stretched mutually perpendicular to each other:
- The Forefinger points in the direction of the Magnetic Field.
- The Middle finger points in the direction of the Current.
- The Thumb indicates the direction of the Force (or Motion).
- Right-hand thumb rule Right-hand thumb rule: Incorrect. This rule (also known as Maxwell’s Corkscrew Rule) is used to find the direction of magnetic field lines around a straight current-carrying conductor, not the force on the conductor itself.
- Lenz’s law Lenz’s law: Incorrect. This law relates to electromagnetic induction. It states that the direction of induced current is such that it opposes the change in magnetic flux that produced it. It does not determine the force on a current-carrying conductor in a static field.
- Coulomb’s law Coulomb’s law: Incorrect. This law quantifies the electrostatic force of attraction or repulsion between two stationary point charges. It is unrelated to magnetic forces on current-carrying conductors.
Key Takeaway
Fleming’s Left-Hand Rule is used for electric motors (to find force/motion), whereas Fleming’s Right-Hand Rule is used for electric generators (to find the direction of induced current).