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1. Do magnets ever lose their magnetism?

Yes, it is possible for a permanent magnet to lose its magnetism. There are three common ways for this to occur:

1) Via heat: Ferromagnet materials will lose their magnetism if heated above a point known as the Curie temperature. At this point, the energy being put into the magnet from the heat will permanently disrupt the magnetic domain structure of the material, turning it into a paramagnetic material [a similar effect occurs in materials called hard ferrites, which exhibit a form of magnetism called ferrimagnetism; the analogous temperature for these materials is known as the Neel point]. You would have to re-magnetize the magnet again, either in a solenoid or with another permanent magnet, in order to restore the magnetism. If you heat a magnet up a little bit, it will lose some of its magnetism, but on returning to room temperature [depending on how high it was heated, and on the shape of the magnet itself], full magnetism can be restored.

2) Via a demagnetizing magnetic field: permanent magnets exhibit a characteristic called coercivity, which is the ability of a material to withstand being demagnetized by an applied magnetic field. Modern permanent magnet materials such as Sm-Co and Nd-Fe-B have high coercivities; older materials such as Alnico or ceramic [hard ferrite] materials have lower coercivities. With a strong enough magnetic field of opposite polarity, it is therefore possible to demagnetize the magnet [whether this comes from another permanent magnet, or a solenoid]. Interestingly, an opposing magnetic field is sometimes applied to a magnet in order to 'knock it down', or to lower its overall magnetic output, so that it can be used appropriately in an application.

3) Via shock: this really only applies to older materials such as magnetic steels and Alnico materials; the mechanism that creates coercivity means that they are susceptible to being demagnetized if enough energy is transmitted through the material via a shock, such as being dropped or hit with a hammer. Modern materials do not suffer this type of problem.

 

2. Can magnetic lines of force intersect?

No, magnetic lines of force cannot intersect with each other. As the direction of the magnetic field at any point on the magnetic lines of force is the tangent to the line at that point. Hence, if we suppose that two magnetic lines of force intersect with each other at a point then there are two tangents at that point showing two different directions of the magnetic field at the same point that implies there are two magnetic intensities at the point which is not possible. Hence, two magnetic lines of force never intersect with each other.

 

3. What happens to magnetic property of a bar magnet, if it is heated?

If a bar magnet is heated, due to the thermal or heat energy, the tiny or small molecular magnet regains the kinetic energy and orient themselves in any directions. As the temperature further increases to its melting point, the orientation of the molecular magnets are completely changed and they are arranged in random or haphazard fashion resulting in the lost of its magnetic property i.e. ferromagnet materials will lose their magnetism if heated above a point known as the Curie temperature. At this point, the energy being put into the magnet from the heat will permanently disrupt the magnetic domain structure of the material, turning it into a paramagnetic material [a similar effect occurs in materials called hard ferrites, which exhibit a form of magnetism called ferrimagnetism; the analogous temperature for these materials is known as the Neel point]. You would have to re-magnetize the magnet again, either in a solenoid or with another permanent magnet, in order to restore the magnetism. If you heat a magnet up a little bit, it will lose some of its magnetism, but on returning to room temperature [depending on how high it was heated, and on the shape of the magnet itself], full magnetism can be restored.

 

4. What is the reason behind the attraction of a magnet with the magnetic substances?

The magnetic substances contains a large number of magnetic domains which are oriented randomly in all directions so there is no magnetic effect. When the magnet is brought nearer to the magnetic substance then the magnetic field of the magnet causes alignment of domains of magnetic substance along the magnetic field so that the magnetic substance becomes temporary magnet. During this process, the south & north pole of temporary magnet lies close to north & south pole of permanent magnet respectively. As unlike poles attract each other. Hence, this is the reason behind the attraction of a magnet with the magnetic substances.

 

5. Define neutral point. When the north pole of a bar magnet points south where will the neutral point lies?

When two magnetic fields act against each other, there exist certain regions or points where they totally cancel each others effect. When certain magnetic materials are placed at such region or points, they do not feel any force and so they don't have any particular orientation. Such points are called as neutral point. Hence, neutral point is defined as that point in the magnetic field where the field due to a magnet is completely neutralized by the horizontal component of the earth's magnetic field.

When the north pole of a bar magnet points south then the cancellation of magnetic forces can occur only at certain points along the equatorial line where the fields are in opposite directions but equal in magnitude. That means the neutral point lies on the equatorial line of the magnet where the fields are in opposite directions with each other and equal in magnitude.

 

6. Why the magnetic needle cannot show proper direction when kept at neutral point?

At neutral point, the field due to the magnet is completely neutralized by the horizontal component of the earth's magnetic field. Hence, there is no net force to influence a magnetic needle kept at that point. So, the magnetic needle cannot show proper direction when kept at the neutral point.

 

7. Classify ferro, dia and paramagnetic material on the basis of susceptibility.

The property of the magnetic substance due to which magnetization of the magnetic substance occurs to different extents is called susceptibility which is defined as the ratio of the intensity of magnetization to the strength of the magnetising field. It is denoted by and given as

                      

1. The magnetic materials which have large positive susceptibility are called ferromagnetic material.

2. Paramagnetic materials have small positive susceptibility.

3. Dia magnetic materials have small negative susceptibility.

 

8. When does a ferromagnetic substance becomes paramagnetic and why?

A ferromagnetic materials are those materials that are strongly attracted by magnets and magnetic field. Its susceptibility is positive and large and it has very high permeability. Due to the fall in susceptibility above curie temperature all its atomic magnets are randomly oriented so, these materials becomes paramagnetic materials above curie's temperature.

 

9. Does diamagnetism depends upon the temperature or not?

No, diamagnetism is independent of temperature. In diamagnetic material, the induced magnetic moments of the atomic magnets are  always opposite to the applied magnetizing field. So, the magnetic moments are independent of the temperature. Hence, the diamagnetism is independent of the temperature.

 

10. On which principle does the deflection magnetometer works?

The deflection magnetometer works on the principle of tangent law which states that "if a freely suspended magnet is in equilibrium under the action of two mutually perpendicular magnetic fields F & H (say) making angle f with H then the relation between F & H is given as F = H tan f ".

 

11. While performing an oscillation magnetometer experiment, we use a brass bar to remove the torsion in the thread. Can we replace it by a similar iron bar? Explain.

While performing an oscillation magnetometer experiment, we use a brass bar to remove the torsion in the thread. It is because brass is as alloy, non-magnetic substance & does not align in any particular direction. No, we can't replace it by a similar iron bar as the iron bar is magnetic material & if it is used then it is feebly magnetized due to the earth's magnetic field. So, iron aligns itself along the direction of earth's field. Hence, we cannot replace a brass bar by a similar iron bar to remove torsion in the thread.

 

12. Can the earth's magnetic field be vertical at a place? What will happen to a freely suspended magnet at such place?

Yes, the earth's magnetic field can be vertical at a place i.e. at poles. At poles, the value of angle of dip is 900. Using tangent law, the magnet placed at poles will make angle of 900 i. e. the magnet will be vertical at such place.

 

13. The Horizontal and Vertical components of the earth's magnetic field are equal at a place. What is the value of angle of dip?

The Horizontal component H and Vertical component V of the earth's magnetic field are equal at a place i.e. V = H. The angle of dip at a place is given by the relation:

         

Hence, the angle of dip where the horizontal & vertical components of the earth's magnetic field are equal is 450.

 

14. What is meant by the statement that the declination at a place is 130E ?

The statement that the declination at a place is 130 E means that the magnetic meridian is 130 east of the geographical meridian & the compass needle points 130 east of the north as the angle between the magnetic meridian & the geographic meridian is called the declination.

 

15. Why does an iron nail experience a force of attraction when brought near a magnet?

The magnetic substances like iron contains a large number of magnetic domains which are oriented randomly in all directions so there is no magnetic effect. When the magnet is brought nearer to an iron then the magnetic field of the magnet causes alignment of domains of iron along the magnetic field so that the iron becomes temporary magnet whose North pole or South pole are close with South pole or North pole of a permanent magnet . As unlike poles attract each other and there develops a Coulomb's force of attraction between them. Hence, an iron nail experience a force of attraction when brought near a magnet.

 

16. Why do both poles of a magnet attract an unmagnetized piece of iron?

An unmagnetized piece of iron contains many domains arranged randomly whose effect of each of these domains cancel one another. So, the iron piece is not magnet and does not  follow the property of a magnet. There is no repulsion but attraction between iron and each pole of a magnet. As we know that the middle part attraction is poor but attractive property of magnet at poles is maximum. So, both poles of a magnet attract an unmagnetized piece of iron.

 

17. What happens if a bar magnet is cut into two pieces perpendicular to its length and along its length?

If a bar magnet is cut into two pieces perpendicular to its length and along its length then a single bar magnet is transfigured into two bar magnets having magnetic moment less than the original bar magnet.

 

18. Why is repulsion considered as sure test of magnetism?

There is always attraction between the unlike poles but there is repulsion between like poles. Hence, the attraction occurs between unlike poles & between magnet and magnetic substances and there is cent percent for the repulsion of like poles. Hence, repulsion is considered as sure test of magnetism.

 

19. Is the magnetic field near a magnet uniform or non-uniform?

The magnetic field near a magnet is non-uniform because uniform magnetic field is the space in which the lines of force are parallel to each other but in magnet, lines of force are supposed to emerge from North pole & enter into magnet at south pole which are not parallel. Hence, the magnetic field near a magnet is non-uniform.

 

20. Does a field of a bar magnet exert a torque on the magnet itself?

No, a field of a bar magnet does not exert a torque on the magnet itself. A magnet when place in the external uniform magnetic field experience a torque but a field of a bar magnet is non-uniform. So, a field of a bar magnet does not exert a torque on the magnet itself.

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