List of top Physics Questions asked in CBSE CLASS XII

(i) Explain the following statements giving reason:

(I) An equipotential surface through a point is normal to the electric field at that point.

(II) When a dielectric is placed in an external electric field, the electric field inside the dielectric is less than that outside it.

(III) The potential difference between the plates of a charged parallel plate capacitor decreases when its plates are brought closer.

(ii) Obtain an expression for the work done to dissociate the system of three charges \(q\), \(-4q\) and \(2q\) placed at the vertices A, B and C respectively of an equilateral triangle of side 'a'.

Draw a plot showing the variation of potential energy of a pair of nucleons as a function of their separation $(r)$. Using this plot, show that the nuclear force is (i) attractive for $r > r_0$, and (ii) repulsive for $r < r_0$.

A series combination of circuit elements $X$ and $Y$ is connected across an ac source. It is found that the voltage is ahead of current in phase by $\dfrac{\pi}{4}$ radian. When element $Y$ is replaced by element $Z$, the current leads the voltage by $\dfrac{\pi}{4}$ radian.

(a) Identify the elements $X$, $Y$ and $Z$.

(b) What will the phase angle and power factor for the circuit be if $X$, $Y$ and $Z$ were connected in series across the same ac source? What can you say about the current that flows in the circuit in this case?

A metallic rod of $1\ \text{m}$ length is rotated with a frequency of $40\ \text{rev/s}$, with one end hinged at the centre and the other end at the circumference of a circular metallic ring of radius $1\ \text{m}$, about an axis passing through the centre and perpendicular to the plane of the ring. A constant and uniform magnetic field of $0{\cdot}5\ \text{T}$ parallel to the axis is present in the region. The value of emf induced between the centre and the metallic ring is close to:

The velocity of the photoelectrons emitted from a surface depends upon the:

Calculate the de Broglie wavelength of a neutron having kinetic energy of $150\ \text{eV}$.

Define the term 'critical angle'. A small bulb is placed at the bottom of a tank containing a liquid of refractive index $\mu$ at a depth $H$. It is observed that the light emerges from a circular area of radius $r$ of the surface. Obtain the expression for $r$ in terms of $H$ and $\mu$.

A plano-convex lens, made of glass $(n = 1{\cdot}5)$ has a curved surface whose radius is $40\ \text{cm}$. The image formed of an object is of the same size as that of the object. The distance of the object from the lens is:

An electric dipole $\vec{P} = 2qa\hat{i}$ is placed in $x$-$y$ plane, centred at the origin. Let $E_1$ and $E_2$ be the magnitudes of electric field at a point distant $r\ (\gg a)$ from its centre, along its axis and on its equatorial plane, respectively. Then the value of $\left(\dfrac{E_1}{E_2}\right)$ is:

Name the electromagnetic waves which are (i) also known as &lsquo;heat waves&rsquo;, (ii) used in LASIK eye surgery, (iii) used to destroy cancer cells. Also write their wavelength range.

(a) Draw electric field lines and equipotential surfaces for a system of two equal positive charges separated by some distance.

(b) Using the drawing in (a) above, explain that electric field need not be a constant over each point of an equipotential surface.

According to Bohr model of hydrogen atom, the time period of an electron in its $n^{\text{th}}$ orbit is proportional to:

A point charge of $5\ \mu\text{C}$ is located at $(1\ \text{cm},\,1\ \text{cm})$ in $x$-$y$ plane. A second charge of $6\ \mu\text{C}$ is brought from infinity to point $(-3\ \text{cm},\,4\ \text{cm})$. The work done in the process is:

(a) Why is manganin used in wire-wound standard resistors? Calculate the temperature at which the resistance of a wire increases by $25\%$ of its resistance at room temperature $27{\cdot}5\,^{\circ}\text{C}$. The temperature coefficient of resistance of the material of the wire is $0{\cdot}004\,^{\circ}\text{C}^{-1}$.

(b) How does the resistivity of a semiconductor depend on temperature? Show it on a plot.

A ray of light is incident at an angle $2A$ on one of the faces of a glass prism of refracting angle $A$. After refraction from this face, the ray strikes the opposite face which is silvered. The reflected ray from this face retraces on its path. Trace the ray diagram and find the relation between the refractive index of material of prism and the angle of prism.

Two identical circular loops $A$ and $B$ of metal wire are arranged on a table close to each other. Loop $A$ is connected to a battery and the current in it increases with time. In response, the loop $B$:

In Bohr model of hydrogen atom, an electron is moving in an orbit, with total energy $-3{\cdot}4\ \text{eV}$. Its angular momentum will be:

A person is standing facing geographic north and a vertical wire in front of him is carrying a current in downward direction. The direction of magnetic field at a point on the right side of wire is:

(i) Define mutual inductance of a pair of coils. Write its SI unit.

(ii) A long solenoid of radius $R$ and length $L$ has $n$ turns per unit length. A circular loop of radius $r\;(<R)$ is placed inside at the centre of the solenoid such that its axis coincides with the axis of the solenoid. Obtain the mutual inductance of the solenoid and the loop.

(a) Consider the following nuclides: ${}^{12}_{6}\text{C}$, ${}^{198}_{80}\text{Hg}$, ${}^{14}_{6}\text{C}$, ${}^{197}_{79}\text{Au}$. Group them into isotopes and isotones.

(b) How does the size of a nucleus depend on its mass number $A$? Hence prove that the density of nucleus is a constant, independent of $A$, for all nuclei.

(a) An electric field $\vec{E} = E_{0}\hat{i}$ exists in a region of space. Draw three equipotential surfaces in the region.

(b) Two point charges $-q$ and $+q$ are located at points $(-a, 0, 0)$ and $(a, 0, 0)$ respectively. Find the electrostatic potential at point $(x, 0, 0)$ where $x \gg a$.

A straight long wire lying along $y$-axis, carries a current of $1\ \text{A}$ along $-y$ direction. The magnetic field due to the conductor at a point $(50\ \text{cm},\,0,\,0)$ will point along:

(a) Why does one prefer to view the image formed at infinity than that formed at near point in microscope/telescope?

(b) Consider lenses $L_1$, $L_2$ and $L_3$ as specified in the following table. Which of them will you select as objective and eyepiece for constructing best possible (i) telescope (ii) compound microscope? Give reason for your answer.

(a) Draw the V-I characteristics of silicon diode.

(b) Explain the following terms:

(i) minority carrier injection in forward bias
(ii) breakdown voltage in reverse bias