Electronic Properties of Materials: Magnetic Properties






























































  • When a medium is placed inside a solenoid carrying current, the atoms in the medium respond to the magnetic field produced in the solenoid. As a response, each atom acquires a net magnetic moment in the direction of the applied magnetic field. At the atomic level, each atom produces a current loop around it. The neighboring atoms acquire current loops in opposite directions as shown in the figure below, which cancel their net effect. Thus, there is no net current flow inside the bulk of the medium. But the surface current loops don't cancel, and thus produce a net flow of current in the direction of the applied magnetic field on the surface of the medium. The total magnetic field is the sum of the applied magnetic field due to the external currents and the field produced due to the surface currents. Thus, the magnetic field will increase in the presence of a medium (say an iron bar) in a solenoid due to the additional current produced as a result of the magnetization of the medium.

    Mathematically,
    B={mu}_{circ}(I+I_m)={mu}_{circ}(I+M) (since I_m=M)
    doubleright B={{mu}_{circ}}I+{{mu}_{circ}}M
    doubleright B=B_{circ}+{{mu}_{circ}}M(since B_{circ}={{mu}_{circ}}I)

    Figure: A medium showing the current loops inside the bulk and the surface currents. The current loops inside the medium don't contribute to the total magnetic field as they cancel each other. The surface currents contribute to the total magnetic field.

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