Making and Handling Magnets

Methods by which a magnet can be demagnetized can be given as mentioned below: 
$(1)$ Heating a magnet strongly
$(2)$ Hammering the magnet several times or repeated hammering
$(3)$ By passing an alternating current through the magnet
Hence, the correct options $(A)$ and $(B)$.

Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). These include iron, nickel, cobalt, some alloys of rare-earth metals, and some naturally occurring minerals such as lodestone.

A magnet can be easily turned into a permanent magnet by excessive heating.

Magnetite is a naturally occurring iron ore that exhibits strong magnetic properties, historically known as lodestone.

Casting of a magnetic material, pressing and sintering of magnetic material and extruding of magnetic material are the processes used for making permanent magnets.

Iron, cobalt and steel are ferromagnetic materials which can be used to make magnet but ebonite cannot be used to make magnets.

Among the following options, magnetite is the only natural magnet.

The phenomenon of converting a substance into a magnet by flowing an electric current through a coil around it is known as electromagnetism and the magnet so formed is known as electromagnet.

When a magnet is treated above 176℉ (80)℃ magnet will quickly loose their magnetic properties. The magnet will become permanently demagnetized if exposed to those temperatures for a certain length of time.

By passing an alternating current the magnetic property can be removed because the alternating current produces a reverse magnetizing field which opposes the object magnetism.

 

All the three statements are true that the ferromagnetic materials lose their magnetism on heating above the Curie temperature,

By all these actions, all the arranged magnetic domains which exhibit magnetic property due to their specific arrangement get rearranged and that specific formation of magnetic domain which is responsible for its magnetic behavior is destroyed.

A mixture of iron and nickel is magnetized by applying an external magnetic field. This convert the mixture into a temporary magnet for the time when the field has been applied.

Magnetization. There are a variety of methods that can be used to establish a magnetic field in a component for evaluation using magnetic particle inspection. It is common to classify the magnetizing methods as either direct or indirect. With direct magnetization, current is passed directly through the component.Rubbing the nail with the permanent magnet causes a stronger, more lasting magnetism in the nail. For this to work, only one pole of the magnet must stroke the nail from one end to the other in a single direction. ... It usually takes around 20 to 30 strokes before the nail becomes sufficiently magnetized.

A permanent magnet needs to retain its magnetism (high retentivity) and resist demagnetization by external fields (high coercivity).

The coercivity of a substance is a measure of the reverse magnetising field required to destroy the residual magnetism of the substance.