Magnetomotive force (mmf): For a 75-turn coil carrying 4 A, what is the mmf developed by the winding?

Introduction / Context:
Magnetomotive force (mmf) is the driving “cause” of magnetic flux in a magnetic circuit, analogous to electromotive force in electric circuits. Correctly computing mmf is essential when sizing coils and predicting magnetic field strength in cores and air gaps.

Given Data / Assumptions:

• Number of turns N = 75.
• Coil current I = 4 A.
• We neglect leakage and assume steady DC current.

Concept / Approach:
The definition of mmf is F = N * I, with units ampere-turns (At). This simple relation underlies the calculation of field strength H = F / l (where l is magnetic path length) and subsequent flux density B = μ * H.

Step-by-Step Solution:

Verification / Alternative check:
If the mean magnetic path length were known, we could find H = F / l and then B = μ * H to corroborate physical magnitudes. The linear proportionality in F confirms that doubling turns or current doubles mmf.

Why Other Options Are Wrong:

• 18.75 At or 30 At: These result from mistakenly multiplying or dividing by partial factors (e.g., using 0.25 A or 7.5 turns).
• 187 At: A rounding or arithmetic slip; 75 * 4 is exactly 300.

Common Pitfalls:

• Forgetting the “turns” factor; mmf is not just current.
• Unit confusion: mmf is At, not volts or amperes alone.

Final Answer:
300 At