Uttarakhand Ubse Class 12 Physics — Real Previous-Year Papers

[FIGURE: Ray diagram at a plane interface between medium n₁ (rarer) and medium n₂ (denser), n₂>n₁, showing an incident ray, a reflected ray and a refracted ray.] When monochromatic light is incident on a surface separating two transparent media (first medium is rarer and second medium is denser) then some light is reflected back into first medium and remaining light is refracted in second medium. In this case - (i) Frequency of incident, refracted and reflected light is same. (ii) Frequency of incident and reflected light is same but frequency of refracted wave is decreased. (iii) Frequency of incident and reflected light is same but frequency of refracted light is increased. (iv) Frequency of incident light and refracted light is same but frequency of reflected light is changed.

Formula for total energy of the electron in the nth stationary state of the hydrogen atom is - (i) -(11.2)/(n²) eV (ii) +(11.2)/(n²) eV (iii) -(13.6)/(n²) eV (iv) +(13.6)/(n²) eV

If mass number of a nucleus is 'A', then its radius will be - (i) R=R₀A1/3 (ii) R=R₀A4/3 (iii) R=R₀A³ (iv) R=R₀A2/3

Assertion (A): The speed of electromagnetic waves in vacuum is same for all wavelengths. Reason (R): Formula for the speed of electromagnetic waves in vacuum is c=dfrac1√(μ₀epsilon₀). (i) Both A and R are correct and R is correct explanation of A. (ii) Both A and R are correct but R is not the correct explanation of A. (iii) A is correct but R is incorrect. (iv) Both A and R are incorrect.

Assertion (A): The majority charge carriers in n-type semiconductor are electrons. Reason (R): An n-type semiconductor is formed by doping an intrinsic semiconductor with trivalent impurity. (i) Both A and R are correct and R is correct explanation of A. (ii) Both A and R are correct but R is not the correct explanation of A. (iii) A is correct but R is incorrect. (iv) Both A and R are incorrect.

[FIGURE: A circle representing a uniformly charged conductor with + charges around its surface, labelled V=V0 on the surface and V=? at a point inside.] If electric potential at the surface of a uniformly charged conductor is V₀ then what will be value of electric potential inside this conductor?

Write mathematical expression of Gauss's law for magnetism.

Which electromagnetic wave has the highest energy?

In half wave rectification, what is the output frequency, if the input frequency is 50 Hz?

Read the following passage carefully and answer the questions given below - Light entering in a dark room through a narrow gap under or around a closed door often appears to bend and spread. This is due to diffraction, the narrow gap acts like a slit, and light wave bends around the edges. This phenomenon is a small but clear demonstration of diffraction. (a) What would be the approximate size of sharp edge or opening compared to the wavelength of light for diffraction to be clearly observed?

Read the following passage carefully and answer the questions given below - Light entering in a dark room through a narrow gap under or around a closed door often appears to bend and spread. This is due to diffraction, the narrow gap acts like a slit, and light wave bends around the edges. This phenomenon is a small but clear demonstration of diffraction. (b) Why can the diffraction not be explained by ray-optics?

Evaluate the statement that Coulomb's law agrees with the Newton's third law of motion.

In a parallel plate capacitor with air between the plates has a capacitance of 8 pF. If the distance between the plates is reduced by half and space between the plates is filled with a dielectric substance, its capacitance becomes 96 pF. Find the dielectric constant of the dielectric substance. **OR** Two point charges q₁=4q and q₂=-2q are placed at a distance r apart. If 25% of the charge is transferred from q₁ to q₂, then what will be electrostatic force between q₁ and q₂.

[FIGURE (A): graph of resistivity ρ vs temperature T, curving upward (increasing) — a conductor. FIGURE (B): graph of resistivity ρ vs temperature T, curving downward (decreasing) — a semiconductor.] In figure (A) and (B) relation between resistivity and temperature is shown for two different materials. Identify and write which of two graphs represent conductor and semiconductor respectively. Also write an expression showing relation between resistivity and temperature of conductor.

Current in a circuit falls from 2.0 A to 1.0 A in 0.1 s. If an average emf of 100 V is induced, calculate the self inductance of the circuit.

[FIGURE: An AC source E=E₀sinω t connected in series with a pure inductor L, forming a simple single-loop circuit.] Draw a phasor diagram for AC circuit shown in above figure.

How can a prism bend a light ray by 90° using total internal reflection? Explain with the help of a diagram. **OR** A magician makes a glass lens (n=1.47) disappear when placed in a liquid. What is the refractive index of the liquid and why does the lens become invisible?

Using Huygen's principle, explain how waves originating from a point source propagate in a medium.

The work function for certain metal is 4.2 eV. Will the metal give photoelectric emission for incident radiation of 330 nm? Explain with mathematical calculations.

Define nuclear fission and state its significance in the generation of nuclear energy.

A pure semiconductor has equal electron and hole concentration of 6×10⁸m⁻³. After doping with a suitable impurity the electron concentration rises to 9×10¹²m⁻³. (i) What type of extrinsic semiconductor is formed after doping? (ii) Calculate new concentration of holes.

Read the following passage carefully and answer the questions given below - Light entering in a dark room through a narrow gap under or around a closed door often appears to bend and spread. This is due to diffraction, the narrow gap acts like a slit, and light wave bends around the edges. This phenomenon is a small but clear demonstration of diffraction. (c) Explain the phenomenon of diffraction on the basis of wave theory of light.

State Gauss's law of electrostatics and prove it. **OR** Derive the expression for the potential energy of an electric dipole placed in uniform external electric field.

A charge particle enters in a uniform magnetic field with its velocity perpendicular to magnetic field - (i) Describe how the magnetic field affects the direction of motion of this particle. (ii) Derive the formula for the radius of the path followed by this particle in magnetic field.

What is moving coil galvanometer? State its principle. **OR** Mention any three types of energy losses in a transformer and explain how these losses can be minimized.

Two convex lenses are made from different materials (n₁=1.5 and n₂=1.6). If both the lenses have same radii of curvature and are kept in same medium, then analyse which lens will have larger focal length and why?

If a plane wave is entering from a denser medium to a rarer medium, explain the refraction of wave with the help of Huygen's wave theory.

Draw the labelled diagram of the circuit required for the experimental study of the photoelectric effect and write important conclusion drawn from the experiment.

Write the formula for the energy of a photon. A hydrogen atom initially in the ground level absorbs a photon which excites it to the n=4 level. Determine the wavelength and frequency of this photon. **OR** Explain the process of nuclear fusion with a suitable example. Mention its two applications.

Explain the working of a p-n junction diode in forward bias condition. Draw a labelled circuit diagram showing the forward biasing of this diode.

What is electric dipole? Explain its physical significance. Derive the expression for the electric field due to electric dipole at a point located in its equatorial plane. **OR** Using Kirchhoff's rules related to electric circuits, derive the condition for balanced state of any Wheatstone bridge. For the following Wheatstone bridge circuit, determine the value of unknown resistance R in the balanced state - [FIGURE: A Wheatstone-bridge diamond circuit with four vertices (left, top, right, bottom). Arms: left-to-top = 20 Ω, top-to-right = R (unknown), right-to-bottom = 20 Ω, bottom-to-left = 10 Ω. A galvanometer (shown as a circle with an arrow, marked 80 Ω) is connected across the bridge diagonal between the top and bottom vertices. A battery E is connected across the left and right vertices via the outer bottom wire.]

Explain the concept of magnetic domains in ferromagnetic materials and analyse how their alignment leads to permanent magnetism. Compare the magnetic behavior of ferromagnetic, paramagnetic and diamagnetic materials in the presence of an external magnetic field. **OR** Under what condition does resonance occur in a series LCR circuit? Analyse the effect of resonance on current and impedance. Evaluate why resonance is not possible in pure RL or RC circuit.