Magnetization and Demagnetization

Demagnetization is the process of causing a magnet to lose its magnetic properties. This happens when the alignment of the magnetic domains is disturbed or randomized, reducing or eliminating the magnetic field.

i. Induction method: In this method, a magnetic substance (such as a steel bar) is placed near or in contact with the pole of a strong magnet. The magnetic field from the magnet causes the domains in the bar to align temporarily. When the bar is removed, some of the alignment remains, and the bar becomes a magnet. Example: Placing a steel nail next to a bar magnet will make the nail temporarily magnetic.

ii. Stroking method: This involves rubbing a magnetic material (like steel) with one or two poles of a permanent magnet in one direction repeatedly. This aligns the magnetic domains in the direction of the stroking. After several strokes, the material becomes magnetized. Example: Stroking a needle with a bar magnet repeatedly in the same direction.

iii. Electrical method: A magnetic material can be magnetized by placing it inside a coil of wire and passing an electric current through the coil. The current creates a magnetic field around the coil, which aligns the domains in the material. This is the principle used in creating electromagnets. Example: Placing an iron rod inside a solenoid (coil) and turning on the current.

In this case an iron nail placed near a bar magnet will be induced with magnetism.

This is a method used to magnetize a steel bar by rubbing it with one or two permanent magnets. The process aligns the magnetic domains inside the steel bar, turning it into a magnet.

Types of stroking methods

Single touch method:

1. This involves using one bar magnet to stroke a steel bar.
2. The magnet is stroked repeatedly in one direction only, for example, from end A to B.
3. The North Pole of the bar magnet is usually used for stroking.
4. After each stroke, the magnet is lifted at point B and returned to point A without dragging it back.
5. After several strokes, the steel bar becomes magnetized.
6. The end where stroking begins (A) becomes the North Pole, and the end where stroking stops (B) becomes the South Pole.

ii. Double touch method:

1. This involves using two bar magnets simultaneously.
2. The steel bar is placed flat, and the two bar magnets are stroked from the center of the bar to each end at the same time using both hands.
3. One magnet's North Pole strokes toward one end, and the other magnet's South Pole strokes toward the opposite end.
4. After each stroke, the magnets are lifted and returned to the center for the next stroke.
5. Repeating this several times magnetizes the bar.
6. The two ends become opposite poles, and the center becomes a neutral point.

This method uses electric current to magnetize a magnetic material such as a steel bar. It is based on the principle that an electric current flowing through a wire produces a magnetic field.

Procedure:

1. A cylindrical coil (solenoid) is made by winding many turns of insulated copper wire.
2. The ends of the wire are connected to a battery in series to form a closed circuit.
3. A steel bar is placed inside the solenoid.
4. When the current is switched on, the solenoid produces a magnetic field along its axis.
5. This magnetic field aligns the magnetic domains in the steel bar.
6. After switching off the current, the steel bar is removed and tested — it is found to be magnetized.

Explanation:

1. The magnetic field generated by the current in the solenoid is uniform and strong inside the coil.
2. This field causes the internal magnetic domains of the steel bar to align in the same direction.
3. The steel bar retains this alignment, especially because steel is a hard magnetic material, making it a permanent magnet after the process.

Electrical method of demagnetization

This method involves the use of alternating current (a.c.) to remove magnetism from a magnet.

Procedure:

1. A magnet is placed inside a solenoid (a coil of insulated copper wire).
2. An alternating current (a.c.) is passed through the solenoid.
3. The magnet is then slowly withdrawn from the solenoid while the current is still flowing.
4. The magnet is held in the West-East (W-E) direction during this process.

Explanation:

1. The alternating magnetic field created by the a.c. randomizes the magnetic domains inside the magnet.
2. As the magnet is removed from the coil, the strength of the magnetic field decreases gradually, helping to completely destroy the alignment of domains.
3. When held in the W-E direction, the Earth's magnetic field does not remagnetize it, because the movement avoids alignment with Earth's natural magnetic field.

Other methods of demagnetization

i. Heating the magnet

• Heating a magnet to a high temperature causes the magnetic domains to become disordered due to increased molecular motion.
• When the material is cooled in a non-magnetic environment, it no longer remains magnetized.

ii. Hammering while pointing in the E-W direction

• Physically striking a magnet with a hammer while it is aligned East-West disturbs the alignment of its domains.
• This randomization leads to loss of magnetism.
• The E-W direction ensures minimal influence from the Earth's magnetic field during the process.

Methods of storing magnets

To prevent magnets from losing their magnetism over time, proper storage is essential.

Storage technique:

1. Magnets are stored in pairs with unlike poles together (i.e., North facing South).
2. A magnetic keeper (a soft iron bar) is placed across the poles of each magnet.
3. The keepers maintain the alignment of domains and help preserve the magnetic strength.