ModerateAssistant Engineer (Electrical), Class-2, Road and Building Department2025✓ Editorially verified

A physical system of electromechanical energy conversion consists of a stationary part creating a magnetic field with electric energy input, and a moving part giving mechanical energy output. If the movable part is kept fixed, the entire electrical energy input will be _______

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Answer & Explanation

Correct AnswerA. Stored in the magnetic field

Quick Explanation

In conventional electromagnetic energy-conversion devices, electrical input establishes magnetic flux in the coupling field, so energy is stored primarily in the magnetic field before being converted into mechanical work and/or returned electrically.

Formula / Key Relation

Electrical input = mechanical output + losses + ΔW_field

Magnetic field energy:

W_m = ∫ B²/(2μ) dV

Detailed Explanation

Concept & Reasoning

Motors, generators, relays and actuators use a magnetic coupling field between stationary and moving parts. The energy balance separates electrical input, mechanical output and change in stored field energy. For magnetically coupled devices, the relevant storage term is magnetic-field energy (or co-energy for convenient force/torque derivation).

How to Solve It in the Exam

Electromechanical machines → magnetic coupling field.

Important Exam Point

Co-energy is a mathematical convenience for force/torque; it is not a different physical storage field.

Related Revision Path

This question sits in the revision path Electrical Machines → Electromechanical Energy Conversion → Field Energy .

Energy Stored in Magnetic Field explanatory diagram

Quick Trick

Electromechanical machines → magnetic coupling field.

Common Mistake

Selecting electric-field energy as the dominant coupling mechanism of ordinary rotating electrical machines.

Exam Tip

Co-energy is a mathematical convenience for force/torque; it is not a different physical storage field.

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