CBSE EXAMINATION PAPER-2025

Extrinsic semiconductors are made by doping pure or intrinsic semiconductors with suitable impurity. There are two type of dopants used in doping, Si or Ge, and using them p-type and n-type semiconductors can be obtained. A p-n junction is the basic building block of many semiconductor devices. Two important processes occur during the formation of a p-n junction : diffusion and drift. When such a junction is formed, a 'depletion layer' is created consisting of immobile ion-cores. This is responsible for a junction potential barrier. The width of a depletion layer and the height of potential barrier changes when a junction is forward-biased or reverse-biased. A semiconductor diode is basically a p-n junction with metallic contacts provided at the ends for application of an external voltage. Using diodes, alternating voltages can be rectified.

The figure shows the voltage (V) versus the current (I) graphs for a wire at two temperatures T₁ and T₂. One can conclude that:

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

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

A proton and an α-particle enter with the same velocity v in a uniform magnetic field B such that v⊥B. The ratio of the radii of their paths is:

The work function of a material is 2.21 eV. Which of the following cannot produce photoelectrons from it?

Assertion (A): In a reflecting telescope, the image does not have chromatic aberration.

Reason (R): Chromatic aberration occurs only due to refraction of light through an optical medium.

The value of magnetic field at point O in the given figure is :

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

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 :

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

Inside a nucleus, the nuclear forces between proton and proton, proton and neutron, neutron and neutron are 𝐹ₚₚ, 𝐹ₚₙ and 𝐹ₙₙ respectively. Then :

Assertion (A) : A hole is an apparent free particle with effective positive electronic charge.

Reason (R) : A hole is not necessarily a vacancy left behind by an electron in the valence band.

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.

Assertion (A) : The binding energy per nucleon is practically constant for mass number in the range (30 < A < 170).

Reason (R) : Nuclear forces between the nucleons for mass numbers in the range (30 < A < 170) are not short-range.

A right angled isosceles glass prism ABC is kept in contact with an equilateral triangular prism DBC as shown in the figure. Both prisms are made of the same glass of refractive index 1·6. Trace the path of the ray MN incident normally on face AB as it passes through the combination.

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 point source of light in air is kept at a distance of 12 cm in front of a convex spherical surface of glass of refractive index 1·5 and radius of curvature 30 cm. Find the nature and position of the image formed.

What is the difference between 'emf' and 'terminal voltage' of a cell ?

Two cells of emfs E₁ and E₂ and internal resistances r₁ and r₂ are connected in parallel. Derive an expression for the emf and internal resistance of the equivalent cell.

A rectangular loop carries a current of 1 A. A straight long wire carrying 2 A current is kept near the loop in the same plane as shown in the figure.

Find (i) the torque acting on the loop, and (ii) the magnitude and direction of the net force on the loop.

(a) Consider the so-called 'D-T reaction' (Deuterium-Tritium reaction). In a thermonuclear fusion reactor, the following nuclear reaction occurs:

(b) Show that the nuclear density is independent of mass number.

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.