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!

Faraday's Law of Induced Voltage

The voltage induced by magnetic flux cutting the turns, of a coil depends upon the number of turns and how fast the flux moves across the conductor. Either the flux or the conductor can move. Specifically, the amount of induced voltage is determined by the following three factors.
1) Amount of flux. The more magnetic lines of force that cut across the conductor, the higher the amount of induced voltage.
2) Number of turns. The more turns in a coil, the higher the induced voltage. The V/ind is the sum of all individual voltages generated in each turn in series.
3) Time rate of cutting. The faster the flux cuts a conductor, the higher the induced voltage. Then more lines of force cut the conductor within a specific period of time.
These factors are fop fundamental importance in many applications. Any conductor with current will have voltage induced in it by a change in current and its associated magnetic flux.
The amount of induced voltage can be calculated by faraday’s law:

 V/ind =N d (Weber’s)
dt (seconds)
Where N is the number of turns and d/dt specifies hoe fast the flux cuts across the conductor. With d/dt in Weber’s per second, the induced voltage is in volts.
As an example, suppose that magnetic flux cuts across 300 turns at the rate of 2 Wb/s.
To calculate the induced voltage,
 V/ind = N d
Dt
=300 (2)
V/ind = 600 V
It is assumed that all the flux links all the turns, which is true with an iron core Read More!