2023 ELECTRICITY AND MAGNETISM

Magnetic Domain

 A magnet is made up of many magnetic domains.

Magnetic domain is a group of atomic magnets pointing in the same direction.Atomic magnet is represented by an arrow with arrowhead as a North pole and tail as a South Pole.

 

In a magnetized bar, the magnetic domains are all pointed in the same direction as shown above. The North and South poles of adjacent domains cancel each other out except the poles at the both ends, and hence producing North and South poles at the ends.  

Note that every magnet has a maximum strength. We say it is magnetically saturated and cannot be any stronger. This occurs when all the magnetic domains pointed in one direction.

However, in an unmagnetized bar, the magnetic domains are pointed in the random directions such that the magnetic effects of the atomic magnets are cancelled out. Thus, the bar is not magnetized.

                                                      

Magnetic Induction

Magnetic induction is a process where a magnetic material becomes an induced magnet when it is brought near to or in contact with a magnet.

Process

The magnetic material (i.e iron bar) becomes an induced magnet with the end nearer the magnet having opposite pole to that of the magnet. Thus, it is attracted to the magnet as unlike poles attract.

                                   

Note

• Magnetic induction has to occur before the attraction of magnetic material by a magnet. 

Induced magnetism is a temporary process. If the permanent magnet is removed, the magnetic material will usually lose its induced magnetism.

Types of Magnetic Material

The following diagram shows the difference in the magnetic properties of Soft Magnetic Material (Iron) and Hard Magnetic Material (Steel). 

                   

Example

The diagram shows an aquarium magnetic cleaner which would clean both sides of the fish tank at the same time.  When the magnet A is moved over the outside surface, magnet B would follow it by moving over the inside surface. 

                        

(a) Explain why B follows the movement of A.
(b) Explain why there is a practical limit to the thickness of the fish tank glass for the device to work.
(c) When A and B are placed on opposite sides of a vertical sheet of iron, B remains where it is even when A is removed.  Suggest a reason for this.

Answer:
(a) Magnet A and B have opposite poles facing each other. Thus, B follows the movement of A as unlike poles attract each other.
(b) Magnetic field strength becomes weaker when the window is too thick, hence the magnets may no longer attract each other strongly and be held in place.
(c) The iron sheet is induced with poles opposite to that of the magnetic pole next to it. Hence attraction still occurs.