Magnetic Shielding

The idea of preventing one component from affecting another through their common electric or magnetic field is called shielding. Examples are the braided copper wire shield around the inner conductor of a coaxial cable, metal shield can that encloses an RF coil, or a shield of magnetic material enclosing a cathode-ray tube.
The problem in shielding is to prevent one component from inducing an effect in the shielded component. The shield material are always metals, but there is a difference between using good conductors with low resistance like copper and aluminum and using good magnetic materials like soft iron.













A good conductor is best for two shielding function. One is to prevent induction of static electric charges. The other is to shield against the induction of a varying magnetic field. For static charges, the shield provides opposite induced charges, which prevent induction inside the shield. For a varying magnetic field, the shield has induced to produce induction inside the shield.
The best shield for a steady magnetic field is a good magnetic material of high permeability. A steady field is produced by a permanent magnetic, a coil with steady direct current, or the earth’s magnetic field. A magnetic shield of high permeability concentrates to magnetic flux. Then there is little flux to induce poles in a component inside shield. The shield can be considered as a short circuit for the lines of magnetic flux. Read More!

Magnetic Flux

The entire group of magnetic field lines, which can be considered to flow outward from the north pole of magnet, is called magnetic flux. Its symbol is the Greek letter (phi). A strong magnetic field has more lines of force and more flux than a week magnetic field.
THE MAXWELL
One Maxwell (Mx) unit equals one magnetic field line. In Fig.13-5, as an example, the flux illustrated is 6 Mx because there are 6 field lines flowing in or out for each pole. A 1-1b magnet can provide a magnetic flux of about 5000 Mx. This unit is named for James Clerk Maxwell (1831-1879), an important Scottish mathematical physicist who contributed much to electrical and field theory. Read More!

Magnetic Field Around an Electric Current

In Fig.15-1, the iron filings aligned in concentric rings around the conductor shoe the magnetic field of the wire. The iron filings are dense next to the conductor, showing that the field is strongest at this point. Furthermore, the field strength decreases inversely as the square of the distance from the conductor. It is important to note the following two factors about the magnetic lines of force:


1) The magnetic lines are circular, as the field is symmetrical with respect to the wire in the center.
2) The magnetic field with circular lines of force is in a place perpendicular to the current in the wire.
From points C to D in the wire, its circular magnetic field is in the horizontal plane because the wire is vertical. Also, the vertical conductor between points EF and AB has the associated magnetic field in the horizontal plane. Where the conductor is horizontal, as from B to C and D to E, the magnetic field is in a vertical plane.
These two requirements of a circular magnetic field in a perpendicular plane apply to any charge in motion. Whether electron flow or a motion of positive charges is considered, the associated magnetic field must be at right angles to the direction of current.
In addition the current need not be in a wire conductor. As an example, the beam of moving electrons in the vacuum of a cathode-ray tube has an associated magnetic field. In all cases, the magnetic field has circular lines of force in a plane perpendicular to the direction of motion of the electric charges. Read More!