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Thank you for the answer. However, I don't understand the physical explanation of bound current. In the picture, current of magnetic dipoles are canceled in the interior of the object and the current of each dipoles in the edge contributes to the current flowing around the boundary. I don't know why this current flowing around the boundary is considered as a constant quantity, what is the physical reason behind?Charles Link said:Bound magnetic surface current per unit length ## \vec{K}_m=\vec{M} \times \hat{n} ##. In this case bound magnetic current ## I_m=K_m \, t ##. ## \\ ## (Defining ## M ## such that ## B=\mu_o(H+M) ##, as opposed to ## B=\mu_oH+M ##). ## \\ ## For uniform ## M ##, you compute ## K_m=M \times \hat{n} ## at all surfaces. If ## M ## is parallel to ## \hat{n} ##, then ## K_m=0 ## at that point. This concept of magnetic surface currents becomes very useful in explaining how the magnetic field arises from a cylindrical permanent magnet that has uniform magnetization ## \vec{M} ## along the axis. For the magnetized cylinder, the surface currents are like the current that flows on a solenoid. They are geometrically equivalent. ## \\ ## In general, bound magnetic current density is ## J_m=\nabla \times \vec{M } ##. With Stokes' theorem at a boundary where ## \vec{M} ## is discontinuous, this becomes surface current per unit length ## K_m=\vec{M} \times \hat{n} ##. ## \\ ## See also: https://www.physicsforums.com/insights/permanent-magnets-ferromagnetism-magnetic-surface-currents/ This Insights uses cgs units, but hopefully it is still readable.
The bound current of a magnetized object refers to the flow of electric current within the material of the object itself. This current is induced by the magnetic field produced by the object's magnetization. It is different from free currents, which are caused by the movement of electric charges.
The main difference between bound current and free current is the source of the electric current. Bound currents are caused by the magnetization of a material, while free currents are caused by the movement of electric charges. Additionally, bound currents are confined within the material of the object, while free currents can flow through external circuits.
The magnitude of bound current is affected by the strength of the magnetic field, the permeability of the material, and the size and shape of the magnetized object. The direction of the bound current is also dependent on the direction of the magnetic field and the orientation of the object.
Bound current can be indirectly measured by measuring the magnetic field produced by the magnetized object. By using Ampere's law, which relates the magnetic field to the current, the bound current can be calculated. However, directly measuring the bound current is difficult and often requires specialized equipment.
Bound current has various applications in electromagnetism, such as in the design and function of magnetic materials and devices. It is also essential in understanding the behavior of electromagnetic waves and their interaction with materials. Additionally, bound current plays a crucial role in the study of magnetic fields in astrophysics and geophysics.