Describe what is meant by "passive shielding" and "active shielding".?

Question

Electromagnetic Fields
Describe what is meant by "passive shielding" and "active shielding".

Answer 1

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Passive Shielding

Magnetic shielding through the use of high permeability material. The iron provides a return path for the stray field lines of magnetic flux and so significantly decreases the flux away from the magnet.
Passive shielding (see also Faraday cage) significantly eases the problems of siting a MR Image in a confined space. Ferromagnetic objects are less prone to being attracted to the magnet, ancillary electronic equipment, credit cards and computer disks can be brought closer to the magnet and the MRI safety limit for pacemaker wearers (the 5 gauss line = 0.5 mT) is reduced from, typically, 10 m to 2 m from the magnet. A passive shield for a whole-body MRI magnet weights many tons. An alternative method of controlling stray field is active shielding.

Active Shielding

Magnetic shielding through the use of secondary shielding coils designed to produce a magnetic field that cancels the field from primary coils in regions where it is not desired. These coils may be inside the magnet cryostat. Active shielding can be applied to the main magnet or to the gradient magnetic fields.

Source(s):

http://www.mr-tip.com/serv1.php?type=db1&dbs=Passive%20Shielding
http://www.mr-tip.com/serv1.php?type=db1&dbs=Active%20Shielding

Answer 2

In engineering, the word "active" usually means that something requires power in order to work.

"Passive" usually means that it does not require power.


A passive shield is probably a conductive plate that is electrically grounded. It should reflect electromagnetic waves and electric fields to some extent.


I assume that an active shield is an electromagnet.