Download App

Physics 2 Marks Questions

MAGNETISM AND MATTER · Rajasthan Board (RBSE) STD 12 Science (Rajasthan - English Medium). 16 questions.

Questions

2 Marks Questions

Take a timed test

16 questions · self-marked practice — reveal the answer and mark yourself.

Question 12 Marks
Explain Gauss's law for magnetism
Answer
Image
→As shown in the Fig., consider a surface ' S ' in a uniform magnetic field.
→To calculate the magnetic flux associated with the surface, imagine the surface ' S ' to be divided into many small area elements.
→Consider a small vector area element $\overrightarrow{\Delta S}$ from all such area elements.
→Magnetic flux passing through this area element.
$\Delta \phi_{ B }=\overrightarrow{ B } \cdot \overrightarrow{\Delta S }$
→Total magnetic flux associated with the surface S ,
Image
[That is because, for any enclosed surface, number of magnetic field lines leaving the surface is same as the number of field lines entering the surface. This means that the total positive flux is same as total negative fiux and hence the net magnetic flux is zero.]
In the eq. (1) 'all' stands for 'all area elements $\overrightarrow{\Delta S}$ '. This can be compared with the Gauss's law of electrostatics.
$\sum \overrightarrow{ E } \cdot \overrightarrow{\Delta S }=\frac{q}{\varepsilon_0}$
→From eq. (1), the Gauss's law for magnetism can be written as follows :
"The net magnetic flux through any closed surface is zero."
→Magnetic flux is a scalar quantity. SI unit of magnetic flux is :
$W b \text { (weber) }= T m^2$
View full question & answer→
Question 22 Marks
Derive the equation of magnetic potential energy of a magnetic dipole placed in a uniform magnetic field and discuss its special cases.
Answer
→Torque acting on a magnetic needle (magnetic dipole) in a uniform magnetic field, $\vec{\tau}=\vec{m} \times \overrightarrow{ B }$
Where, $\vec{m}$ - Magnetic dipole moment
$\vec{B}$ - External Magnetic field
$\theta$ - Angle between $\vec{m}$ and $\overrightarrow{ B }$
→Work required to be done to displace the magnetic needle from this condition by a very small angle $d \theta$,
$\begin{array}{rlrl}
d W=\tau d \theta \\
\rightarrow \therefore \text { Total Work } W  =\int \tau d \theta \\
\therefore W=\int m B \sin \theta d \theta \\
\therefore W =m B \int \sin \theta d \theta \\
\therefore W  =m B(-\cos \theta) \\
\therefore W  =-m B \cos \theta
\end{array}$
→This work is stored in the form of potential energy.
$\begin{array}{ll}
\therefore & U =-m B \cos \theta \\
\therefore & U =-\vec{m} \cdot \overrightarrow{ B }
\end{array}$
Special Cases :
(i) When the magnetic needle is parallel to the field, $\theta=0$
$\therefore$ Potential energy (P.E.)
$\therefore U =-m B \cos \theta=-m B \cos 0$
$\therefore U =-m B$ (minimum)
→Which shows the maximum steady (stable) condition of the needle.
(ii) When the magnetic needle is anti parallel to the magnetic field,
$\theta=\pi\left(180^{\circ}\right)$
$\therefore$ P.E. $U =-m B \cos \pi$
$\therefore \quad U =m B$ (maximum)
→Which shows the most unstable position of the needle.
(iii) When the magnetic needle is perpendicular to the magnetic field,
$\theta=90^{\circ}\left(\frac{\pi}{2} rad \right)$
$\therefore$ P.E. $U =-m B \cos 90^{\circ}$
$\therefore \quad U=0$
View full question & answer→
Question 32 Marks
Explain paramagnetism and paramagnetic substances.
Answer
→Paramagnetic substances are those which get weakly magnetised when placed in external magnetic field.
→Atoms (or ions or molecules) of paramagnetic materials possess a permanent magnetic dipole moment, but due to continuous (ceaseless) random thermal motion of atoms, net magnetisation is zero. So in normal condition, such substances do not behave as a magnet.
→ When such substances are placed in sufficiently strong external magnetic field $\left(\overrightarrow{B_0}\right)$ at low temperature, the atomic dipole moments of individual atoms are aligned in the direction of magnetic field ( $\overrightarrow{ B _0}$ ), and they get weakly magnetised.
Image
→Therefore, as shown in Fig., magnetic field inside a paramagnetic substance is enhanced, and the field lines get concentrated inside the material. This enhancement is slight, generally one part in $10^5$.
→When placed in a non-uniform magnetic field, they tend to move from weak field to strong, i.e. get weakly attracted to a magnet.
→This effect (property) is known as paramagnetism and such materials are known as paramagnetic materials.
→Some examples of paramagnetic materials are : aluminium, sodium, calcium, oxygen (at STP) and copper chloride.
→For a paramagnetic material both $\chi$ and $\mu_r$. depend not only on the material, but also on the sample temperature. As the field is increased or the temperature is lowerd the magnetisation increases until it reaches the saturation value at which point all the dipoles are perfectly aligned with the field.
View full question & answer→
Question 42 Marks
Explain diamagnetism and diamagnetic material.
Answer
→Simple explanation of dia-magnetism :
→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 the 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.
→This happens due to induced current in accordance with Lenz's law.
→Thus, the substance develops a net magnetic moment in direction opposite to that of applied field and hence repulsion.
This is a simple explanation of diamagnetism.

Image
→Fig. shows a bar of diamagnetic material placed in an external magnetic field. The field lines in it are repelled and field inside the material is reduced.
→In most cases, this reduction is slight, being one part in $10^5$.
→When placed in a non-uniform magnetic field, a diamagnetic substance experiences net force from stronger to weaker field and tends to move from high to low field. Which means they experience repulsion.
→Some diamagnetic materials are bismuth, copper, lead, silicon, nitrogen (at STP), water and sodium chloride.
→Value of $\chi$ (magnetic susceptibility) is negative $(-1 \leq \chi<0)$ for diamagnetic materials.
View full question & answer→
Question 52 Marks
What is magnetisation ? Derive the relationship between magnetisation and magnetic intensity.
Answer
$\rightarrow$ Magnetisation : "The net magnetic dipole moment per unit volume in a substance is called magnetisation."
$\rightarrow$ Magnetisation $\overrightarrow{ M }=\frac{\vec{m}_{\text {nes }}}{V}$
$\rightarrow$ Magnetisation is a vector quantity and its direction is taken in the direction of magnetic dipole moment.
$\rightarrow$ Its unit is $\frac{ A }{m}$ (or A.m ${ }^{-1}$ ) and dimensional formula is $L ^{--^m} A^1$.
$\rightarrow$ Consider a long solenoid of $n$ turns per unit length and carrying current I.
$\rightarrow$ The magnetic field in the interior of the solenoid,
$B _0=\mu_0 n I$
$\rightarrow$ If the interior of the solenoid is filled with a material having non$-$zero magnetisation, magnetic field $\left( B _m\right)$ is generated due to this core material inside the solenoid.
Therefore, the net field in the interior of the solenoid is equal to the vector addition of both the magnetic fields.
$\therefore \vec{B}=\overrightarrow{B_0}+\overrightarrow{B_m}$
Where $\overrightarrow{ B }_m$ is the field contributed by magnetic core.
$\rightarrow$ This additional field $\overrightarrow{ B _m}$ is proportional to the magnetisation $( \overrightarrow{ M } )$ of the material.
$\therefore \overrightarrow{ B _m} \propto \overrightarrow{ M }$
$\therefore \overrightarrow{ B _m}=\mu_0 \overrightarrow{ M }$
$\rightarrow$ Substituting the value of $\overrightarrow{ B _m}$ from eq. $(3)$ into eq. $(2),$
$\therefore \vec{B}=\vec{B}_0+\mu_0 \overrightarrow{ M }$
$\rightarrow$ dividing the equation by $\mu_0$,
$\therefore \frac{\vec{B}}{\mu_0}=\frac{\overrightarrow{B_0}}{\mu_0}+\vec{M}$
$but \frac{\overrightarrow{B_0}}{\mu_0}=\overrightarrow{ H }-$ Which is a vector quantity called magnetic intensity.$\therefore \frac{\vec{B}}{\mu_0}=\vec{H}+\vec{M}$
$\therefore \vec{B}=\mu_0(\vec{H}+\vec{M})$
$\rightarrow$ Magnetic intensity $(\overrightarrow{ H })$ has same dimensions as $\overrightarrow{ M }$ and its unit is $\frac{ A }{m}$ $($ or A.m ${ }^{-1} ).$
View full question & answer→
Question 62 Marks
Provide the comparison (analogy) between Electro Statics and Magnetism.
Answer
Electro Statics Magnetism
1Permittivity of free space $($ vaccum $)-\varepsilon_0$1Permeability of free space (vacuum) $-\mu_0$
2Constant $\frac{1}{4 \pi \varepsilon_0}$2Constant $\frac{\mu_0}{4 \pi}$
3Electric Charge $q$3Pole Strength $q_m$
4Electric dipole moment
$p=2 a q$
Direction : $-q$ to $+q$		4Magnetic dipole moment
$m=2 l q_m$
Direction : S to N
5Electric Field ( $\vec{E}$ )5Magnetic Field ( $\vec{B}$ )
6Force acting between two stationary point charges
$F =\frac{1}{4 \pi \varepsilon_0} \cdot \frac{q_1 q_2}{r^2}$		6Magnetic force acting between two stationary magnetic poles,
$F =\frac{\mu_0}{4 \pi} \cdot \frac{q_{m_1} \cdot q_{m_2}}{r^2}$
7Electric field on the axis of an electric dipole
$E =\frac{1}{4 \pi \varepsilon_0} \cdot \frac{2 p}{r^3}$		7Magnetic field on the axis of a magnetic dipole
$B =\frac{\mu_0}{4 \pi} \cdot \frac{2 m}{r^3}$
8Electric field on the equatorial axis of electric dipole
$E =\frac{-1}{4 \pi \varepsilon_0} \cdot \frac{p}{r^3}$		8Magnetic field on the equatorial axis of a magnetic dipole
$B =\frac{-\mu_0}{4 \pi} \cdot \frac{m}{r^3}$
9Torque acting on an electric dipole in uniform electric field $\vec{\tau}=\vec{p} \times \overrightarrow{ E }$9Torque acting on a magnetic dipole in uniform magnetic field $\vec{\tau}=\vec{m} \times \overrightarrow{ B }$
10Potential energy of an electric dipole in uniform electric field
$\begin{aligned}
U & =-\vec{p} \cdot \overrightarrow{ E } \\
& =-p E \cos \theta
\end{aligned}$		10Potential energy of a magnetic dipole in uniform magnetic field
$\begin{aligned}
U & =-\vec{m} \cdot \overrightarrow{ B } \\
& =-m B \cos \theta
\end{aligned}$
11Work required to be done in moving an electric dipole from angle $\theta_1$ to $\theta_2$ in uniform electric field, $W =p E \left(\cos \theta_1-\cos \theta_2\right)$11Work required to be done in moving a magnetic dipole from angle $\theta_1$ to $\theta_2$ in uniform magnetic field, $W = mB \left(\cos \theta_1-\cos \theta_2\right)$
View full question & answer→
Question 72 Marks
Using the equation $\overrightarrow{ B }=\mu_0(\overrightarrow{ H }+\overrightarrow{ M })$, Derive the equation $\overrightarrow{ B }=\mu \overrightarrow{ H }$ in case of a long solenoid carrying current I and having its interior filled with magnetic material.
Answer
→Magnetisation ( $\overrightarrow{ M }$ ) is proportional to magnetic intensity $(\vec{H})$.
$\therefore \vec{M} \propto \vec{H}$
$\therefore \overrightarrow{ M }=\chi \overrightarrow{ H }$
Where the proportionality constant $\chi$ (or $\chi_m$ ) is a dimension less quantity, called the Magnetic Susceptibility.
→Its value depends on the type of material and its temperature.
Remember :         Value of
→For paramagnetic materials $\rightarrow \chi_m$ is small and positive.
→For Diamagnetic materials $\rightarrow \chi_m$ is small and negative.
→Now, $\vec{B}=\mu_0(\vec{H}+\vec{M})$ (given equation) substituting value of $\overrightarrow{ M }$ in the above equation,
$\begin{aligned}
\quad \overrightarrow{ B } & =\mu_0(\overrightarrow{ H }+\chi \overrightarrow{ H }) \\
\therefore \quad \overrightarrow{ B } & =\mu_0 \overrightarrow{ H }(1+\chi) \\
\therefore \quad \vec{B} & =\mu_0 \mu_r \overrightarrow{ H } \\
\therefore \quad \overrightarrow{ B } & =\mu \overrightarrow{ H }
\end{aligned}$
Where $\mu_r=1+\chi$ and $\mu=\mu_0 \cdot \mu_r$ →$\mu$ is called the magnetic permeability of the material (Substance/given medium.)
View full question & answer→
Question 82 Marks
Explain about super conductor materials with respect to their magnetic properties.
Answer
→Super conductors are most exotic diamagnetic materials.
→Basically they are metals, cooled to very low temperatures which exhibit both perfect conductivity and perfect diamagnetism. When such materials are placed in magnetic field, all field lines are completely expelled. Hence, the magnetic field inside the substance $B =0$.
→For super conductors, susceptibility $\chi=-1$ and hence $\mu_r=0$.
→A super conductor repels a magnet and is repelled by magnet.
→The phenomenon of perfect diamagnetism in super conductors is called the Meissner effect, after the name of its discoverer.
→Super conducting magnets are used in variety of situations. For example, for running Mag-lev (magnetically levitated) superfast trains.
View full question & answer→
Question 92 Marks
What does the equivalence / similarity of magnetic field lines of a bar magnet and a solenoid indicate (suggest)?
Answer
→(i) A bar magnet also has circulatory currents just like a solenoid, which has circulatory currents in a large number.
Image
(ii) If a bar magnet is broken into (cut into) two pieces, each behaves as an independent magnet.
Image
(iii) Magnetic field lines in a solenoid are continuous, like a bar magnet.
(iv) The magnetic field lines in the region inside a bar magnet are from S to N , whereas in the outside region they are from N to S .
$\rightarrow$
→Similarly, the magnetic field lines in a solenoid emerge (come out of) from one face of a solenoid and enter into the other face, forming a closed loop.
(v) The magnetic field of the solenoid is the same as the magnetic field at a point on the axis of the magnet is,
$B =\frac{\mu_0}{4 \pi} \cdot \frac{2 m}{r^3} .$
View full question & answer→
Question 102 Marks
Explain what happens when iron filings are sprinkled on a sheet of glass placed over a short bar magnet.
Answer
→When iron filings are sprinkled on a sheet of glass placed over a short bar magnet, the arrangement of iron filings we get is shown in the figure.
Image
→The pattern seen in the arrangement shows magnetic field lines. The pattern suggests that the magnet has two poles similar to the positive and negative charge of an electric dipole.
→The poles are designated as :
(1) North pole and (2) South pole
→When the bar magnet is suspended freely in the horizontal plane, these poles point approximately towards the geographic north and south poles, respectively.
View full question & answer→
Question 112 Marks
Give some commonly known ideas regarding magnetism.
Answer
→(i) The earth behaves as a magnet with the magnetic field pointing approximately from the geographic south to north.
(ii) When a bar magnet is freely suspended, it points in the north-south direction. The tip which points to the geographic north is called the north pole and the tip which points to the geographic south is called the south pole of the magnet.
(iii) There is a repulsive force when north poles (OR south poles) of two magnets are brought close together. Conversely, there is an attractive force between the north pole of one magnet and the south pole of the other.
(iv) We cannot isolate the north, or south pole of a magnet. If a bar magnet is broken into two halves, we get two similar bar magnets with somewhat weaker properties. Unlike electric charges, isolated magnetic north and south poles known as magnetic monopoles do not exist.
(v) It is possible to make magnets out of iron and its alloys.
View full question & answer→
Question 122 Marks
Explain hard ferromagnetic and soft ferromagnetic materials.
Answer
→In some ferromagnetic materials, the magnetisation persists even when the external field is removed. Such materials are called hard ferro magnetic materials (or hard ferro magnets.) Example : Alnico, an alloy of iron, aluminium, nickel cobalt and copper is one such material.
→ Naturally occurring lode stone is also one such material.
→Such materials form permanent magnets to be used in other things as compass needle.
→The ferromagnetic materials in which magnetisation disappears on removal of external field, are called soft ferromagnetic. Example : Soft iron.
View full question & answer→
Question 132 Marks
Explain ferromagnetism and ferromagnetic substances.
Answer
→Ferromagnetic substances are those which get strongly magnetised when placed in an external magnetic field.
→The individual atoms in a ferromagnetic material possess a dipole moment as in a paramagnetic material.
→The neighbouring atoms are bound by strong attraction force. This bonding however, is limited to a small region. The atoms (molecules) in such small regions (small microscopic volumes) interact in such a way that their dipole moments are aligned in a common direction.
Such regions (macroscopic volumes) are called domain(s).
→Each domain has some net magnetisation. But the magnetisation varies randomly from domain to domain, and there is no bulk magnetisation.
→Typical domain size is 1 mm and the domain contains about $10^{11}$ atoms.

Image
→When external magnetic field $\left(\overrightarrow{ B _0}\right)$ is applied, the domains orient themselves in the direction of $\vec{B}_0$.
→Simultaneously the domain oriented in the direction of $\overrightarrow{B_0}$ grow in size. As shown in fig. (b) all domains merge gradually and make a larger/'giant' domain.
→Thus, in a ferromagnetic material, field lines are highly concentrated.
→When such materials are placed in non-uniform magnetic field, they are attracted towards strong magnetic field.
→Examples : Some Ferromagnetic materials such as iron, cobalt, nickel, gadolinium etc. have relative magnetic permeability $>1000$.
View full question & answer→
Question 142 Marks
Give the characteristics of magnetic field lines.
Answer
→Characteristics of magnetic field lines are as follows :
(i) The magnetic field lines form continuous closed loops. In the region outside a magnet, the magnetic field lines are from N to S , whereas in the region inside the magnet, the field lines are from S to N .
(ii) The tangent to the field line at a given point represents the direction of the magnetic field $(\vec{B})$ at that point.
(iii) The larger the number of field lines crossing per unit area, the stronger is the magnitude of the magnetic field $\overrightarrow{ B }$.
(iv) Two magnetic field lines do not intersect each other. Because if they do, at the point of intersection, the magnetic field would have two directions, which is not possible.
Image
View full question & answer→
Question 152 Marks
Write the history of magnet in brief.
Answer
→We know that a magnet possesses the property of attracting iron.
→Magnetic phenomena are universal in nature. The earth's magnetism predates human evolution.
→The word magnet is derived from the name of an island in Greece called 'magnesia' where magnetic ore deposits were found, around 600 BC .
→Shepherds on this island complained that their wooden shoes (which had nails) at times stuck to the ground. This property of magnets made it difficult for them to move around.
→The directional property of magnets was also known since ancient times. A thin and long piece of a magnet, when suspended freely, points in the north-south direction.
→The name loadstone (OR lode stone) was given to a naturally occuring ore of iron-magnetite, which means leading stone.
→The very first use of magnet (OR magnetic needles) was done by the Chinese for navigation on ships.
→Caravans crossing the Gobi Desert also employed magnetic needles.
→About four thousand years ago, an emperor Huang-ti got built a chariot by his craftsmen ('engineers' of today) on which they placed a magnetic figure with arms out stretched.
Image
→The figure swiveled around so that the figure of the statuette on it always pointed south. With this chariot, Huang-ti's troops were able to attack the army of enemy from the rear in thick fog, and were able to defeat them.
View full question & answer→
Question 162 Marks
Draw the figures of field lines of (a) a bar magnet, (b) a current carrying finite solenoid and (c) an electric dipole.
Answer
Image
→In all three figures above, the curves labelled
(i) and (ii) show closed Gaussian surfaces. As shown in the fig., the density of field lines is nearer in (ii) compared to (i) which means that the intensity of the field lines is nearer in (ii) compared to (i).
View full question & answer→
Generate Paper FreeAll MAGNETISM AND MATTER topics