The work done on a unit N pole in moving once around any single closed path in a magnetic field is equal to ___________. 

Concept:

In magnetic field theory, the work done on a unit North (N) pole in moving once around any closed path in a magnetic field is known as the magnetomotive force (MMF). According to Ampere’s Circuital Law, this MMF is equal to the total Ampere-turns enclosed by the path.

Ampere’s Law:

The line integral of magnetic field intensity \( H \) around a closed path is equal to the total current enclosed by the path:

\( \oint H \cdot dl = NI \)

Where \( NI \) is the total ampere-turns linked with the path.

Evaluation of Options:

Option 1: EMF in respective path –  Incorrect
EMF relates to electric circuits, not magnetic work done on poles.

Option 2: number of turns in respective path –  Incorrect
Turns alone don’t represent the work; current is also required.

Option 3: Ampere turns linked with the path –  Correct
This represents the correct physical quantity corresponding to the work done on a magnetic pole.

Option 4: current in respective path –  Incorrect
Current alone doesn't define the total magnetic work; number of turns must be considered too.