Introduction / Context:
When a ferromagnetic core has been magnetized by coil current, some magnetization persists even after the current is removed. This remanent magnetization is called residual magnetism (or remanence) and is central to understanding hysteresis, memory effects, and core reset procedures in magnetic devices.
Given Data / Assumptions:
- Electromagnet with a ferromagnetic core.
- After excitation ceases, some flux remains due to domain alignment.
- We are not describing leakage flux or external field geometries.
Concept / Approach:
- Ferromagnetic domains align under applied field; removing the field does not randomize them fully.
- The B–H hysteresis loop shows nonzero flux density (Br) at zero applied field, representing residual magnetism.
- Resetting cores may require reverse magnetizing force to bring B to zero (coercivity Hc).
Step-by-Step Reasoning:
Apply current to coil → magnetizes core via H = N * I / l.Remove current → H returns to zero but B remains at Br due to hysteresis.This leftover magnetization is residual magnetism.
Verification / Alternative check:
Examine hysteresis curve: at H = 0, B = Br. Demagnetization requires applying negative H to reach B = 0 at H = −Hc.
Why Other Options Are Wrong:
- Force of attraction between poles: Describes interaction, not remanence.
- Flux not confined to core: That is leakage flux.
- Circular magnetic field around a current: That is the field from a straight conductor, unrelated to remanence.
- None of the above: Incorrect because residual magnetism is accurately defined in option A.
Common Pitfalls:
- Confusing residual magnetism with leakage flux or stray fields.
- Assuming cores lose all magnetization instantly when current is removed.
Final Answer:
Magnetism that remains in the core of an electromagnet after the current through the coil is turned off
Discussion & Comments