(a) Field energy is equal to the coenergy.
(b) Field energy is greater than the coenergy.
(c) Field energy is lesser than the coenergy.
(d) Coenergy is zero.
Answer: (a) Field energy is equal to the coenergy.
Explanation:
CONCEPT: Magnetic Co-energy
- Co-energy is
the dual quantity of energy.
- It
is a non-physical (mathematical) quantity, mainly used
for theoretical analysis.
- Co-energy
is especially useful for calculating:
- Magnetic
force
- Electromagnetic
torque
in electromechanical energy conversion systems. - Co-energy
has the same units as energy (joules).
- For
systems incapable of storing energy, the co-energy is
zero.
Linear Electromagnetic System
- In
a linear magnetic circuit, the permeability remains constant.
- Hence,
the flux linkage (λ) is directly proportional to
the current (i):
- Here, L is
the inductance of the system.
- The λ–i
characteristic of a linear system is a straight line.
Field Energy 
- Magnetic
field energy is defined as the area under the λ–i curve.
- Mathematically:
- Substituting :
Co-energy 
- Magnetic
co-energy is defined as the area under the i–λ curve.
- Mathematically:
- Substituting :
Equality of Energy and Co-energy
- Since
both expressions give:
- Therefore,
in a linear electromagnetic circuit:
- Field
energy = Co-energy
Final Answer (MCQ Conclusion)
- For
a linear electromagnetic system, the correct statement is:
- This
equality holds only for linear systems.
- In non-linear
magnetic systems, field energy and co-energy are not equal.
✔️ Key One-Line Exam Statement
In a linear magnetic circuit, field energy equals
co-energy because flux linkage is directly proportional to current.