ISC Class 12 Physics Question Paper Solution 2015

The ISC Class 12 Physics – Paper I (Theory) was conducted on 10th March 2015. The exam started at 2 PM. The paper was of 3 hours of time duration and 70 Marks. Here, we have made available the ISC Class 12 Physics Question Paper Solution 2015. Solving the ISC Class 12 Physics Previous Year Question Papers and then analysing the answer sheet by referring to the solution pdf will help in identifying the mistakes done. Students need to pay attention to the areas in which they have made mistakes. Working on them will improve their performance in the board exam. Students can download the ISC Class 12 Physics Question Paper and Solution pdf 2015 from the link below.

Science Stream students can also access the Solved ISC Class 12 Previous Year Question Papers with Solutions for all subjects compiled at one place. They can have a look at the ISC Class 12 Physics Question Paper Solution 2015 below.

Difficult Topics of ISC Class 12 Physics Paper 2015

Topics which students found difficult while solving the Physics 2015 paper are mentioned below:

• Equivalent resistance of a circuit.
• Equivalent capacitance of a given circuit.
• Mosley’s law
• Potentiometer.
• Hysteresis.
• Time constant of an LR circuit.
• Lens mirror combination.
• Resolving power of a telescope.
• Polarisation of light.
• Reflection of light by Huygens theory.
• Drawing and use of labelled graphs.
• Elements of earth’s magnetic field.
• Derivation of E in the broad side position of an electric dipole.
• Derivation of B at the Centre of a circular coil of N turns.
• Drawing a ray diagram of a compound microscope.

Confusing ISC Class 12 Physics Questions 2015

Physics concepts between which students got confused during the exam are mentioned below.

• Capacitors in series and parallel.
• Resistors in series and parallel
• Cells in series and parallel
• Ordinary electric circuit and Wheatstone bridge.
• End on position and broad side position of an electric dipole.
• Biot-savart’s law and Ampere’s circuital law.
• Long-sightedness and short sightedness.
• Reflection of light and refraction of light by Huygens’s wave theory.
• Spherical aberration and chromatic aberration.
• Pair production and pair annihilation.
• Interference of light and diffraction of light.
• LR dc circuit and LR ac circuit.
• Electrostatic potential and potential energy.
• Magnetic susceptibility of paramagnetic, ferromagnetic and diamagnetic materials.
• Effect of temperature on resistance of metals and nonmetals.

ISC Class 12 Physics Question Paper Solution 2015

Question 1:

A. Choose the correct alternative (a), (b), (c) or (d) for each of the questions given below:

(i) A short electric dipole (which consists of two point charges, +q and –q) is placed at the centre O and inside a large cube (ABCDEFGH) of length L, as shown in Figure 1. The electric flux, emanating through the cube is:

1. q/4πϵ₀L
2. Zero
3. q/2πϵ₀Lm
4. q/3πϵ₀L

(ii) The equivalent resistance between points a and f of the network shown in Figure 2 is:

1. 24 Ω
2. 110 Ω
3. 140 Ω
4. 200 Ω

(iii) A moving electron enters a uniform and perpendicular magnetic field. Inside the magnetic field, the electron travels along:

1. a straight line
2. a parabola
3. a circle
4. a hyperbola

(iv) A fish which is at a depth of 12 cm in water (μ = 𝟒𝟑) is viewed by an observer on the bank of a lake. Its apparent depth as observed by the observer is:

1. 3 cm
2. 9 cm
3. 12 cm
4. 16 cm

(v) If E_p and E_k represent potential energy and kinetic energy respectively, of an orbital electron, then, according to Bohr’s theory:

1. E_k = -E_p/2
2. E_k = -E_p
3. E_k = -2E_p
4. E_k = 2E_p

B. Answer all questions given below briefly and to the point:

(i) What is meant by the term Quantization of charge?

(ii) A resistor R is connected to a cell of emf e and internal resistance r. Potential difference across the resistor R is found to be V. State the relation between e, V, R and r.

(iii) Three identical cells each of emf 2V and internal resistance 1Ω are connected in series to form a battery. The battery is then connected to a parallel combination of two identical resistors, each of resistance 6Ω. Find the current delivered by the battery.

(iv) State how magnetic susceptibility is different for the three types of magnetic materials, i.e. diamagnetic, paramagnetic and ferromagnetic materials.

(v) An emf of 2V is induced in a coil when current in it is changed from 0A to 10A in 0·40 sec. Find the coefficient of self-inductance of the coil.

(vi) How are electric vector (𝐸⃗ ), magnetic vector (𝐵⃗ ) and velocity vector (𝑐 ) oriented in an electromagnetic wave?

(vii) State any two methods by which ordinary light can be polarised.

(viii) A monochromatic ray of light falls on a regular prism. What is the relation between angle of incidence and angle of emergence in the case of minimum deviation?

(ix) What type of lens is used to correct long-sightedness?

(x) State any one advantage of using a reflecting telescope in place of a refracting telescope.

(xi) State Moseley’s law.

(xii) Wavelengths of the first lines of the Lyman series, Paschen series and Balmer series, in the hydrogen spectrum are denoted by λL, λP, and λB, respectively. Arrange these wavelengths in increasing order.

(xiii) What is the significance of binding energy per nucleon of a nucleus of a radioactive element?

(xiv) Write any one balanced equation representing nuclear fission.

(xv) What is the difference between analogue signal and digital signal?

Answer: A. (i) (b) OR Zero

(ii) (c) OR 140 (Ω)

(iii) (c) OR circle

(iv) (b) OR 9(cm)

(v) (a) OR Ek = -Ep/2

B. (i) Charge on a body is an integral or exact multiple of the elementary charge OR

q =( ± ) ne (where n is an integer)

(ii)

Any other correct relation containing all four quantities, i.e. r, R, e and V.

(iii) 1A, with some working. I = 𝑬 / 𝑹+𝒓 OR 𝟔 / 𝟑+𝟑 OR circuit with correct values.

(iv) (Susceptibility) is small and negative for diamagnetic material OR χ< 0

(Susceptibility) is small and positive for paramagnetic material OR χ> 0

(Susceptibility) is very large and positive for ferromagnetic material OR χ>> 0

(v) 0·08 H , with correct substitution/formula.

and other correct units

(vi) They are mutually perpendicular (to each other)/ orthogonal

OR

(vii) Any two of the following:

1. Using a polarizer or a Polaroid or a tourmaline lamina/crystal
2. By reflection (at a surface of a transparent material)
3. By double refraction / NICOL prism/dichroic/anisotropic/quartz/calcite
4. Pile of glass plates/refraction
5. By scattering
6. Selective absorption

(viii) (They are) equal OR i = e or i₁= i₂ or by diagram

(ix) A convex lens or converging lens or diagram

(x) Image is free from spherical aberration OR

Image is free from chromatic aberration OR

Image is brighter/sharper/easier to install/less distorted (or free) /better quality.

(xi) √𝒗 α z

OR

Statement: Square Root of the frequency of (K) X rays varies directly with atomic number (of the target element)]

(xii) λ_L, λ_B and λ_p OR

λ_p > λ_B > λ_L OR

λ_L < λ_B < λ_p

(xiii) It gives us an idea of the stability of the nucleus.

(xiv) ¹₀n + ²³⁵₉₂U → ¹⁴⁸₅₇(La) + ⁸⁵₃₅(Br) +3 ¹₀n

OR any other correct balanced equation

(xv)

Statement i.e. analogue signal has infinite number of values or many continuously varying values whereas digital signal has only two values.

OR Analogue – sinusoidal wave

Digital – square (pulse) or rectangular wave

Question 2:

(a) Derive an expression for intensity of electric field at a point in broadside position or on an equatorial line of an electric dipole.

(b) Two point charges of 10C each are kept at a distance of 3m in vacuum. Calculate their electrostatic potential energy.

Answer:

Question 3:

(a) Four capacitors, C₁, C₂, C₃ and C₄ are connected as shown in Figure 3 below. Calculate equivalent capacitance of the circuit between points X and Y.

(b) Draw labelled graphs to show how electrical resistance varies with temperature for:

(i) a metallic wire.

(ii) a piece of carbon.

Answer: (a) (i) Equivalent capacitance of C₂ and C₃, C₅ = 12𝜇𝐹

(ii) Equivalent capacitance of C₄ and C₅, C₆ = 40𝜇𝐹 (C₆ = C₄ + C₅)

(iii) Final equivalent capacitance = 8𝜇𝐹 (C = C₁C₆/ C₁+C₆)

(b)

Question 4:

(a) Two resistors R₁=400 Ω and R₂=20 Ω are connected in parallel to a battery. If heating power developed in R₁ is 25 W, find the heating power developed in R₂.

(b) With the help of a labelled diagram, show that the balancing condition of a Wheatstone bridge is:

𝑹₁/𝑹₂ = 𝑹₃/𝑹₄

where the terms have their usual meaning.

Answer: (a)

(b)

Question 5:

(a) A 10m long uniform metallic wire having a resistance of 20Ω is used as a potentiometer wire. This wire is connected in series with another resistance of 480Ω and a battery of emf 5V having negligible internal resistance. If an unknown emf e is balanced across 6m of the potentiometer wire, calculate:

(i) the potential gradient across the potentiometer wire.

(ii) the value of the unknown emf e.

(b) (i) Explain the term hysteresis.

(ii) Name three elements of the earth’s magnetic field which help in defining earth’s magnetic field completely.

Answer: (a)

(b) (i) The phenomenon in which magnetic flux density /(B) lags behind magnetising field intensity/(H) is called hysteresis.

(ii) B_H (Horizontal component of earth’s magnetic field)

𝜹 (angle of dip) and

𝜽 (angle of declination)

Question 6:

(a) Obtain an expression for magnetic flux density B at the centre of a circular coil of radius R, having N turns and carrying a current I.

(b) A coil of self inductance 2·5H and resistance 20Ω is connected to a battery of emf 120V having internal resistance of 5 Ω. Find:

(i) The time constant of the circuit.

(ii) The current in the circuit in steady state.

Answer:

Question 7:

(a) Figure 4 below shows a capacitor C, an inductor L and a resistor R, connected in series to an a.c. supply of 220 V.

Calculate:

(i) The resonant frequency of the given CLR circuit.

(ii) Current flowing through the circuit.

(iii) Average power consumed by the circuit.

(b) In a series LCR circuit, what is the phase difference between V_L and V_C where V_L is the potential difference across the inductor and V_C is the potential difference across the capacitor?

Answer: (a)

(b) 180^o or π (radian)

OR

V_L & V_C shown clearly on phasor diagram.

Question 8:

(a) On the basis of Huygen’s Wave theory of light, show that angle of reflection is equal to angle of incidence. You must draw a labelled diagram for this derivation.

(b) State any one difference between interference of light and diffraction of light.

Answer:

(a) Correct diagram with at least one arrow, an incident or reflected ray

Or mentioned wave fronts i and r marked

Proof of 2 triangles as congruent (angle between ray and wave front is 90⁰, either shown on diagram or written mathematically)

Proving r = i

(b) Any one difference

Interference of light requires two coherent sources (slits), diffraction of light requires only one source.

OR

Many bright and dark fringes are obtained in interference

A few bright and dark fringes are obtained in diffraction OR

All bright fringes are equally bright in interference.

They are of decreasing intensities in diffraction. OR

Intensity curves diagram but axes may not be marked OR

Interference – Fringes may or may not be of equal widths

Diffraction – Fringe width varies

Question 9:

(a) Laser light of wavelength 630 nm is incident on a pair of slits which are separated by 1·8mm. If the screen is kept 80 cm away from the two slits, calculate:

(i) fringe separation i.e. fringe width.

(ii) distance of 10th bright fringe from the centre of the interference pattern.

(b) Show graphically the intensity distribution in Fraunhofer’s single slit diffraction experiment. Label the axes.

Answer:

Question 10:

(a) A point object O is placed at a distance of 15cm from a convex lens L of focal length 10cm as shown in Figure 5 below. On the other side of the lens, a convex mirror M is placed such that its distance from the lens is equal to the focal length of the lens. The final image formed by this combination is observed to coincide with the object O. Find the focal length of the convex mirror.

(b) What is chromatic aberration? How can it be minimised or eliminated?

Answer:

(a) Using lens formula for convex lens:

(b) It is that defect of image in which coloured images are formed by a lens when an object is illuminated with white light. OR by diagram with arrows with at least one incident ray marked with white /polychromatic light.

It can be minimised by combining a convex lens with a suitable concave lens

OR by satisfying the condition: 𝝎/𝒇 + 𝝎′/𝒇′ = 0

OR using achromatic doublet/achromatic combination of lenses.

Question 11:

(a) Draw a labelled ray diagram of an image formed by a compound microscope, when the final image lies at the least distance of distinct vision (D).

(b) With regard to an astronomical telescope of refracting type, state how you will increase its:

(i) magnifying power

(ii) resolving power

Answer:

(a) First lens (marked as objective) with at least two rays from an object with an arrow on one of them. with I₁ correctly formed.

Second lens (marked as Eye piece) with two emergent rays with an arrow on one of them, with I₂ correctly formed.

Marking of f_o, f_e, u_o, u_e, D, v_o and β (any five marked correctly )

(b) (i) Magnifying power can be increased by either increasing focal length.

of objective lens i.e. f_o OR

by decreasing focal length of eyepiece i.e. f_e

(ii) Resolving power can be increased by increasing the diameter / aperture

Or size of the objective lens

Question 12:

(a) In an experiment of photoelectric effect, the graph of maximum kinetic energy EK of the emitted photoelectrons versus the frequency v of the incident light is a straight line AB as shown in Figure 6 below:

Find:

(i) Threshold frequency of the metal.

(ii) Work function of the metal.

(iii) Stopping potential for the photoelectrons emitted by the light of frequency v = 30×10¹⁴ Hz.

(b) (i) State how de-Broglie wavelength (λ) of moving particles varies with their linear momentum (p).

(ii) State any one phenomenon in which moving particles exhibit wave nature.

Answer: (a) (i) Threshold frequency (vo) = 10 x 10¹⁴ Hz

(ii) W = ( hv_o)

= 6·6 ×10^-34 × 10 × 10¹⁴

= 6·6 ×10^-19 J = 4.125 eV

(iii) E_K = 8 ×1·6 × 10^-19 J = 8eV

OR

(de Broglie) wavelength varies inversely with the linear momentum

(ii) Electron Diffraction /diffraction/G P Thomson/ Davisson-Germer Experiment /Interference.

Question 13:

(a) On the basis of Bohr’s theory, derive an expression for the radius of the nth orbit of an electron of hydrogen atom.

(b) Using the constants given on page 8 of this Paper, find the minimum wavelength of the emitted X rays, when an X ray tube is operated at 50 kV.

Answer:

Question 14:

(a) (i) Define half life of a radioactive substance.

(ii) Using the equation N = Noe-λt, obtain the relation between half life (T) and decay constant (λ) of a radioactive substance.

(b) With the help of a suitable example and an equation, explain the term pair production.

Answer:

(a) (i) It is that time in which a quantity of a radioactive substance becomes half.

OR

It is that time in which half of the given number of nuclei disintegrate.

(ii) Substitute N = 𝐍_𝐨/𝟐 and t = T

OR

𝐍_𝐨/2 = No e^-λT

λT = log_e 2 or ln 2

(b) It is that phenomenon in which a pair of an electron and a positron is produced from a gamma ray photon.

Question 15:

(a) Draw a labelled diagram of a full wave rectifier. Show how output voltage varies with time, if input voltage is a sinusoidal voltage.

(b) What is a NAND gate? Write its truth table.

Answer: (a)

(b) It is a combination of an AND gate and a NOT gate or it is a negated/ inverted or complement of an AND gate.

TRUTH table of NAND gate

ISC Class 12 Physics Question Paper Solution 2015 must have helped students in analysing their current level of exam preparation. Keep practising more questions and stay tuned to BYJU’S for the latest update on ICSE/CBSE/State Boards/Competitive exams. Also, download the BYJU’S App to get interactive study related videos.