Magnetomotive force is equal to__________________

Question

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Option A: $Current 	imes Number \ of \ turns$

Explanation & Concepts:
Magnetomotive force (MMF) is the physical quantity that drives magnetic flux through a magnetic circuit, analogous to electromotive force (EMF) in an electric circuit. It is defined as the product of the number of turns in a coil and the current flowing through it.

Working Principle: When a current $I$ flows through a coil of $N$ turns, a magnetic field is produced due to the alignment of magnetic domains within the core material. The intensity of this field is directly proportional to the total ampere-turns, which represents the source potential for the magnetic flux $\Phi$.

Key Formulas: $MMF = N \cdot I$ — Fundamental definition of MMF where $N$ is turns and $I$ is current.; $MMF = \Phi \cdot \mathcal{R}$ — Hopkinson's law (Magnetic Ohm's Law) where $\Phi$ is flux and $\mathcal{R}$ is reluctance.

Key Points: MMF is measured in Ampere-turns (AT).; It is a scalar quantity.; Higher MMF increases the magnetic flux density in a circuit with constant reluctance.; It acts as the source potential in magnetic circuits.

Advantages: Simple linear relationship between current and field intensity.; Easily controllable by varying the current source.

Disadvantages: Saturates quickly due to core material limitations.; Produces non-linear flux response in ferromagnetic materials.

Applications: Electromagnets; Electric motors and generators; Transformers

Additional Info: The SI unit for MMF is the Ampere (A), as turns are dimensionless.. Option D refers to Magnetic Field Intensity ($H = \frac{NI}{l}$), which is MMF per unit length.

Answer: A is correct — Magnetomotive force is mathematically expressed as the product of current in Amperes and the number of turns in the coil ($F = NI$).

Exam Tip: Always distinguish between MMF ($NI$) and Magnetic Field Intensity ($H = \frac{NI}{l}$); remember that $H$ includes the length of the magnetic path.

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Option B: $Current / Number \ of \ turns$

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Option C: $Current / Number \ of \ turns \ per \ unit \ length$

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Option D: $Current 	imes Number \ of \ turns \ per \ unit \ length$

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