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Some commonly known facts about magnetism:

1. Every magnet regardless of its size and shape has two poles called north pole and south pole.

2. Isolated magnetic monopoles do not exist. If a magnet is broken into two or more pieces then each piece behaves like an independent magnet with some what weaker magnetic field.

3. Like magnetic poles repel each other, whereas unlike poles attract each other.

4. When a bar magnet/ magnetic needle is suspended freely or is pivoted, it aligns itself in geographically north south direction.

Methods of magnetizing a magnetic material are : 

1. Single-touch method – A magnet is rubbed along the magnetic substance from one end to another. 

2. Double-touch method – Opposite poles of two magnets are placed next to each other at the centre of the magnetic substance and rubbed in opposite direction along it. 

3. Magnetizing by induction – It is a method to make temporary magnets under the influence of a magnetizing force. The magnetic properties disappear when the influence of the magnetizing force is removed. (A permanent magnet is brought close to the magnetic substance but not touched.) 

4. Electric current – A magnet made by using electric current is called an electromagnet. It is a temporary magnet and stops behaving like a magnet when the current is stopped. Electromagnets can become permanent magnets if certain materials like steel and special alloys are placed in a coil and very strong current is passed through the coil.

 (a) cannot intersect at all

Correct answer is (d) 90°

Correct answer is (d) 30°

tan δ = \(\frac{B_V}{B_H}\) = \(\frac{1}{\sqrt{3}}\)  ⇒ δ = tan^-1 \((\frac{1}{\sqrt{3}})\)= 30°

Paramagnetic substances are those which get weakly magnetised when placed in an external magnetic field. They have tendency to move from a region of weak magnetic field to strong magnetic field, i.e., they get weakly attracted to a magnet. 

Ferromagnetic substances are those which gets strongly magnetised when placed in an external magnetic field. They have strong tendency to move from a region of weak magnetic field to strong magnetic field, i.e., they get strongly attracted to a magnet.  

Gauss’s law in magnetism: It states that the surface integral of the magnetic field (\(\vec B\)) over a closed surface S is equal zero.
\(\oint \vec{B} \cdot \overrightarrow{d S}=0\)
Gauss’s law indicates that there are no sources or sinks of magnetic field inside a closed surface.

Electrons in an atom orbiting around nucleus possess orbital angular momentum. These orbiting electrons are equivalent to current-carrying loop and thus possess orbital magnetic moment. Diamagnetic substances are the ones in which resultant magnetic moment in an atom is zero. When magnetic field is applied, those electrons having orbital magnetic moment in the same direction slow down and those in the opposite direction speed up. Thus, the substance develops a net magnetic moment in direction opposite to that of the applied field and hence repulsion of magnetic field lines takes place. 

Example : Some diamagnetic materials are bismuth, copper, lead, silicon, nitrogen (at STP), water and sodium chloride. Diamagnetism is present in all the substances. However, the effect is so weak in most cases that it gets shifted by other effects like paramagnetism, ferromagnetism, etc. The most exotic diamagnetic materials are superconductors. These are metals, cooled to very low temperatures which exhibits both perfect conductivity and perfect diamagnetism. Here the field lines are completely expelled! χ = (μ_r - 1) = 0. A superconductor repels a magnet and diamagnetism in superconductors is called the Meissner effect, after the name of its discoverer. Superconducting magnets can be gainfully exploited in variety of situations, for example, for running magnetically levitated superfast trains. 

The magnetic hysteresis loop is the closed B – H curve for cycle of magnetisation of ferromagnetic material.

Hysteresis Loop

The hysteresis loop indicates the relationship between the intensity of magnetization and the magnetizing field. This loop is generated by measuring the magnetic flux that is coming out from the ferromagnetic substance while changing the external magnetizing field.

Advantages of Hysteresis Loop

1. A smaller region of the hysteresis loop is indicative of less loss of hysteresis. 

2. Hysteresis loop provides a substance with the importance of retentivity and coercivity. Therefore the way to select the right material to make a permanent magnet is made simpler by the heart of machines.

3. Residual magnetism can be calculated from the B-H graph and it is, therefore, simple to choose material for electromagnets. 

Diamagnetic substances are those which have tendency to move from stronger to the weaker part of the external magnetic field. In other words, unlike the way a magnet attracts metals like iron, it would repel a diamagnetic substance. 

(d) energy loss in magnetising a substance
Energy loss in magnetising a substance.

Since bismuth is diamagnetic, the field in the core coil be sightly less than that when core is empty.

Area represents energy dissipated or heat produced. 

Properties : 

(1) Diamagnetic substance is feebly repelled by a strong magnet. 

(2) When a diamagnetic substance is placed in magnetic field, the magnetic lines of force prefer to pass through the surrounding air rather than through the substance. This is because the induced magnetic field in the diamagnetic substance opposes the external field. 

(3) When a rod of diamagnetic substance , is suspended in a uniform magnetic field the rod comes to rest with its longest axis at right angles to the direction of the field. 

(4) When placed in a non-uniform magnetic field, a diamagnetic substance moves from stronger to weaker parts of the field. 

(5) The relative permeability (μ_r) of a diamagnetic substance is always less than 1. 

(6) The magnetic susceptibility (χ) of a diamagnetic substance has a small negative value. 

(7) The magnetic susceptibility (χ) of a diamagnetic substance does not change with temperature

Substances which at room temperature retain their ferromagnetic property for a long period of time are called permanent magnets. 

The answer is (B) Soft iron

Materials having high retentivity and low coercivity are required for electromagnets. 

Ex: Iron. 

(i). Magnetic field lines are continuous closed loops. 

(ii). Tangent to the field line represents the direction of net field B . 

(iii). The larger the number of field lines crossing unit area normally, the stronger is the magnitude of the magnetic field B. 

(iv). Magnetic field lines do not intersect. 

Substances which at room temperature retain their ferromagnetic property for a long period of time are called permanent magnets. 

Permanent magnets can be made in a variety of ways. 

(1) One can hold an iron rod in the north-south direction and hammer it repeatedly. 

(2) One can also hold a steel rod and stroke it with one end of a bar magnet a large number of times, always in the same sense to make a permanent magnet. 

(3) An efficient way to make a permanent magnet is to place a ferromagnetic rod in a solenoid and pass a current. The magnetic field of the solenoid magnetises the rod. 

(4) The hysteresis curve allows us to select suitable materials for permanent magnets. The material should have high retentivity so that the magnet is strong and high coercivity so that the magnetisation is not erased by stray magnetic fields, temperature fluctuations or minor mechanical damage. Further, the material should have a high permeability. Steel is one-favoured choice. It has a slightly smaller retentivity than soft iron but this is outweighed by the much smaller coercivity of soft iron. Other suitable materials for permanent magnets are alnico, cobalt steel and ticonal. 

(5) Core of electromagnets are made of ferromagnetic materials which have high permeability and low retentivity. Soft iron is a suitable material for electromagnets. On placing a soft iron rod in a solenoid and passing a current, we increase the magnetism of the solenoid by a thousand fold. When we switch off the solenoid current, the magnetism is effectively switched off since the soft iron core has a low retentivity. 

(6) Electromagnets are used in electric bells, loudspeakers and telephone diaphragms. Giant electromagnets are used in cranes to lift machinery, and bulk quantities of iron and steel. 

Iron nail, nickel, cobalt

The intensity at the poles is greater than that at other places.

The magnet is suspended using a thread so that it can oscillate freely in a horizontal plane. When it comes to rest the end pointing south can be marked as south pole.

a. Soft iron 

b. Steel

• the number of turns of coiled con ductor
• the strength of current
• the area of cross section of the soft iron placed inside the coil.

Electric bells, electric motor, telephones, loudspeakers and generators are few common appliances that use electromagnets.

Electromagnets are made of soft iron because soft iron has high susceptibility and low retentivity.