Field around a current-carrying conductor: Which rule should be used to determine the circular direction of magnetic lines of force surrounding a straight conductor carrying conventional current?

Introduction / Context:
Predicting magnetic field direction is necessary when laying out conductors, diagnosing electromagnetic interference, and understanding actuator behavior. Rules of thumb provide quick directionality without vector calculus.

Given Data / Assumptions:

• Straight conductor with conventional current (positive to negative).
• We are interested in the azimuthal field direction encircling the wire.
• Standard sign conventions apply.

Concept / Approach:
The right-hand rule for a straight conductor states: point the right thumb in the direction of conventional current; the curl of the fingers gives the direction of magnetic field lines around the wire. Lenz’s law predicts the direction of induced currents opposing change in flux, and Faraday’s law relates induced emf to rate of change of flux linkage—neither directly gives the static field direction around a current-carrying wire. Some left-hand rules apply to motors (Fleming’s), not to this field direction case.

Step-by-Step Solution:

Verification / Alternative check:
Biot-Savart and Ampère’s law yield the same azimuthal field direction; the right-hand rule is their mnemonic counterpart.

Why Other Options Are Wrong:

• Lenz’s law / Faraday’s law: Concern induction phenomena, not static field direction.
• Left-hand rule: Used for motor force directions or electron flow conventions, not this case.

Common Pitfalls:
Mixing up which hand/rule applies to motors versus fields, or to induction versus current-generated fields.

Final Answer:
the right-hand rule