CBSE 2025, Set 2 Solutions
CBSE Physics (2025)

A galvanometer can be converted into an ammeter of desired range by connecting a:

(a) Define the term 'drift velocity' of conduction electrons in a conductor.\n(b) A conductor of length $l$ and area of cross-section $A$ is connected across an ideal battery of emf $V$. Derive the formula for the current density in terms of relaxation time $\tau$.

A ray of light is incident at an angle $i$ on a parallel sided glass slab of thickness '$d$' and gets refracted into the slab at angle $r$. Draw a ray diagram to show its path as its emerges out of the slab. Hence, obtain an expression for the lateral shift of the ray. Under what condition will the shift be minimum?

Describe briefly Geiger-Marsden scattering experiment. Depict the graph showing the variation of number of scattered particles detected with the scattering angle. How did this graph lead to the discovery of the nucleus ?

Find the Q value of the following nuclear reaction:\n${}_{6}^{12}\text{C} + {}_{6}^{12}\text{C} \longrightarrow {}_{10}^{20}\text{Ne} + {}_{2}^{4}\text{He}$\nIs this reaction exothermic or endothermic ?\nGiven:\n$m({}_{6}^{12}\text{C}) = 12.00000\text{ u}$\n$m({}_{10}^{20}\text{Ne}) = 19.992439\text{ u}$\n$m({}_{2}^{4}\text{He}) = 4.002603\text{ u}$\n$1\text{ u} = 931\text{ MeV/c}^2$

Name the electromagnetic wave used (i) in radar, (ii) in eye surgery and (iii) as diagnostic tool in medicine. Write their wavelength range also.

(a) (i) Draw a ray diagram to show the image formation by a compound microscope. Obtain the expression for the total magnification of the microscope when the final image is formed at infinity.\n(ii) In a compound microscope, an object is placed at a distance of $1.5\text{ cm}$ from the objective of focal length $1.25\text{ cm}$. The eyepiece has a focal length of $5\text{ cm}$. The final image is formed at infinity. Calculate the distance between the objective and the eyepiece.

(b) (i) Using Huygens' principle, explain the refraction of a plane wavefront, propagating in air, at a plane interface between air and glass. Hence verify Snell's law.\n(ii) Use mirror formula to deduce that a convex mirror always produces a virtual image of an object kept in front of it.

(a) (i) The electric field in a region is given by $\vec{E} = 40x \hat{i}\text{ N/C}$. Find the amount of work done in taking a unit positive charge from a point $(0, 3\text{m})$ to the point $(5\text{m}, 0)$.\n(ii) A charge $Q$ is distributed over two concentric hollow spheres of radii $r$ and $R (R > r)$ such that their surface charge densities are equal. Find: (I) the electric field, and (II) the potential at their common centre.

(b) (i) Obtain an expression for the electric field due to a dipole of dipole moment $\vec{p}$ at a point on its equatorial plane and specify its direction. Hence, find the value of electric field: (I) at the centre of the dipole $(r=0)$, and (II) at a point $r >> a$, where $2a$ is the length of the dipole.\n(ii) An electric field $\vec{E} = (10x + 5)\hat{i}\text{ N/C}$ exists in a region in which a cube of side $L$ is kept as shown in the figure. Here $x$ and $L$ are in metres. Calculate the net flux through the cube.\n\n

(a) (i) Write the principle of working of an ac generator. Draw its labelled diagram and explain its working.\n(ii) A resistor of $400\ \Omega$, an inductor of $(\frac{5}{\pi})\text{ H}$ and a capacitor of $(\frac{50}{\pi})\ \mu\text{F}$ are joined in series across an ac source $v = 140 \sin(100\pi)t\text{ V}$. Find the rms voltages across these three circuit elements. The algebraic sum of these voltages is more than the rms voltage of source. Explain.

Assertion (A): A hole is an apparent free particle with effective positive electronic charge.\n\nReason (R): A hole is not necessarily a vacancy left behind by an electron in the valence band.

Assertion (A): In a reflecting telescope, the image does not have chromatic aberration.\n\nReason (R): Chromatic aberration occurs only due to refraction of light through an optical medium.

An ac source is connected to a resistor and an inductor in series. The voltage across the resistor and inductor are $8\text{ V}$ and $6\text{ V}$ respectively. The voltage of the source is:

(b) (i) Write the principle of working of a transformer. With the help of a labelled diagram, explain the working of a step-up transformer.\n(ii) An ideal transformer is designed to convert $50\text{ V}$ into $250\text{ V}$. It draws $200\text{ W}$ power from an ac source whose instantaneous voltage is given by $v_i = 20 \sin(100\pi)t\text{ V}$. Find:\n(I) rms value of input current.\n(II) expression for instantaneous output voltage.\n(III) expression for instantaneous output current.

A vertically held bar magnet is dropped along the axis of a copper ring having a cut as shown in the diagram. The acceleration of the falling magnet is:\n\n

State Lenz's law. A rod MN of length $L$ is rotated about an axis passing through its end M perpendicular to its length, with a constant angular velocity $\omega$ in a uniform magnetic field $\vec{B}$ parallel to the axis. Obtain an expression for emf induced between its ends.

Define 'self-inductance' of a coil. Derive an expression for self-inductance of a long solenoid of cross-sectional area $A$ and length $l$, having $n$ turns per unit length.

A straight conductor is carrying a current of $2\text{ A}$ in $+x$ direction along it. A uniform magnetic field $\vec{B} = (0.6\hat{j} + 0.8\hat{k})\text{ T}$ is switched on, in the region. The force acting on $10\text{ cm}$ length of the conductor is:

A proton and an $\alpha$-particle enter with the same velocity $\vec{v}$ a uniform magnetic field $\vec{B}$ such that $\vec{v} \perp \vec{B}$. The ratio of the radii of their paths is :

A rectangular loop carries a current of $1\text{ A}$. A straight long wire carrying $2\text{ A}$ current is kept near the loop in the same plane as shown in the figure.\n\n\n\nFind:\n(i) the torque acting on the loop, and\n(ii) the magnitude and direction of the net force on the loop.

Assertion (A): X-rays are produced when slow moving electrons are stopped by a metal target of high atomic number.
Reason (R): X-rays consist of low-energy photons.

A wire of resistance $R$, connected to an ideal battery consumes a power $P$. If the wire is gradually stretched to double its initial length, and connected across the same battery, the power consumed will be :

Two coherent waves, each of intensity $I_0$, produce interference pattern on a screen. The average intensity of light on the screen is:

If $R_s$ and $R_p$ are the equivalent resistances of $n$ resistors, each of value $R$, in series and parallel combinations respectively, then the value of $(R_s - R_p)$ is :

Inside a nucleus, the nuclear forces between proton and proton, proton and neutron, neutron and neutron are $F_{pp}$, $F_{pn}$ and $F_{nn}$ respectively. Then :

The de Broglie wavelength associated with an electron moving with energy $5\text{ eV}$ is :

The momentum (in $\text{kg m/s}$) of a photon of frequency $6.0 \times 10^{14}\text{ Hz}$ is :

A piece of a diamagnetic material, free to move when placed in a uniform magnetic field :

Assertion (A): The binding energy per nucleon is practically constant for mass number in the range $(30 < A < 170)$.\n\nReason (R): Nuclear forces between the nucleons for mass numbers in the range $(30 < A < 170)$ are not short-range.

A cell of emf $E$ and internal resistance $r$ is connected to an external variable resistance $R$. Plot a graph showing the variation of terminal voltage $V$ of the cell as a function of current $I$, supplied by the cell. Explain how the emf of the cell and its internal resistance can be found from it.

In an n-type semiconductor electron-hole combination is a continuous process at room temperature. Yet the electron concentration is always greater than the hole concentration in it. Explain.

A laser beam of wavelength $500\text{ nm}$ and power $5\text{ mW}$ strikes normally on a perfectly reflecting surface of area $1\text{ mm}^2$ of a body. It rebounds back from the surface. Find the force exerted by the laser beam on the body.

A ray of light is incident on face AB of a prism ABC with angle of prism $A$ and emerges out from face AC. The prism is set in the position of minimum deviation with angle of deviation $\delta$. Find (i) the angle of incidence and (ii) the angle of refraction on face AB.

(a) Find the intensity at a point on the screen in Young's double slit experiment, at which the interfering waves of intensity $I_0$ each, have a path difference of (i) $\frac{\lambda}{3}$ and (ii) $\frac{\lambda}{2}$.

(b) A point source of light in air is kept at a distance of $12\text{ cm}$ in front of a convex spherical surface of glass of refractive index $1.5$ and radius of curvature $30\text{ cm}$. Find the nature and position of the image formed.

Consider a capacitor of capacitance $C$, with plate area $A$ and plate separation $d$, filled with air [Fig. (a)]. The distance between the plates is increased to $2d$ and one of the plates is shifted as shown in Fig. (b). The capacitance of the new system now is:

Which of the following is a donor impurity atom for Ge?

When a pentavalent atom occupies the position of an atom in the crystal lattice of Si, four of its electrons form covalent bonds with four silicon neighbours, while the fifth remains bound to the parent atom. The energy required to set this electron free is about:

A slab (area $A$ and thickness $d_1$) of a linear dielectric of dielectric constant $K$ is inserted between charged plates (charge density $\sigma$) of a parallel plate capacitor [plate area $A$ and plate separation $d (> d_1)$] and opposite charges with charge density of magnitude $\sigma_p$ appear on the faces of the slab. The dielectric constant $K$ is given by:

An electric field $E$ is established between the plates of an air-filled parallel plate capacitor, with charges $Q$ and $-Q$. $V$ is the volume of the space enclosed between the plates. The energy stored in the capacitor is:

During the formation of a p-n junction:

In a reverse-biased p-n junction:

A slab (area $A$ and thickness $\frac{d}{2}$) of dielectric constant $K$ is inserted in a parallel plate capacitor of plate area $A$ and plate separation $d$. If $C$ and $C_0$ are the capacitances of the capacitors with and without the dielectric, then $\frac{C}{C_0}$ is:

The output frequency of a full-wave rectifier with $50\text{ Hz}$ as input frequency is :

Three capacitors A, B and M, each of capacitance $C$ are connected to a capacitor N of capacitance $2C$ and a battery as shown in the figure. If the charges on A and N are $Q$ and $Q'$ respectively, then $\frac{Q'}{Q}$ is :