Hysteresis Loop
In ferromagnetic materials, magnetization does not depend only on the applied magnetic field (H). It also depends on the past magnetic state of the material. The hysteresis curve of a ferromagnetic material shows how magnetization (M) changes when the magnetic field (H) is increased, decreased, or reversed.
- Starting from an unmagnetized state, the magnetization increases rapidly when the magnetic field is applied and follows the path OA until the material reaches saturation.
- When the field is reduced back to zero, the curve does not return along the same path—some magnetization remains in the material. This magnetization is called retentivity.
- To reduce the magnetization to zero, a magnetic field must be applied in the opposite direction, and the amount of reverse field needed to make M = 0 is called the coercive force.
- As the reverse field increases further, the material becomes magnetized in the opposite direction and reaches negative saturation. When the field is brought back to zero and then increased again in the original direction, the magnetization follows a different path and eventually returns to point A.
- This entire closed curve formed by the paths ACDEFGA is known as the hysteresis loop, which reflects the magnetic “memory” of the material—showing that magnetization depends not only on the current field but also on the material’s previous magnetic history
Figure1. Hysteresis loop
MCQs on Hysteresis Loop
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The hysteresis loop represents the relationship between:
- a) Electric field and current
- b) Magnetization (M) and magnetic field (H)
- c) Charge and voltage
- d) Force and displacement
Answer
b) Magnetization (M) and magnetic field (H)
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The value of magnetization retained even after removing the external magnetic field is called:
- a) Permeability
- b) Retentivity
- c) Coercivity
- d) Susceptibility
Answer
b) Retentivity
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The magnetic field required to reduce magnetization to zero after saturation is called:
- a) Magnetic intensity
- b) Saturation field
- c) Coercive field
- d) Demagnetizing constant
Answer
c) Coercive field
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A material with a large hysteresis loop area is generally used for:
- a) Transformer cores
- b) Permanent magnets
- c) Electromagnets
- d) Low-energy applications
Answer
b) Permanent magnets
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Energy loss in a magnetic material during one cycle of magnetization is equal to:
- a) Total area under the B-H curve
- b) Area of the hysteresis loop
- c) Height of coercivity
- d) Width of retentivity
Answer
b) Area of the hysteresis loop