ISC Class 12 Physics Question Paper Solution 2019

Solving the previous year question paper is the best way to prepare for the exam. It provides an idea on types of questions expected to be asked in the exam and it’s difficulty level. By solving the Past Years ISC Class 12 Physics Question Papers, students will get well worse with the exam pattern and question paper design. So, to help them, we have provided the ICSE Class 12 Physics Question Paper 2019 along with the solutions.

The ISC Class 12 Physics – Paper I (Theory) was conducted on 11th March 2019. The exam started at 2 PM, and students were allotted 3 hours of time duration to finish the paper. Students can download both the ISC Class 12 Physics Question Paper and Solution pdf 2019 from the link below.

Students can have a look at the ISC Class 12 Physics Question Paper Solution 2019 below:

ISC Class 12 Physics Question Paper Solution 2019

Question 1: Choose the correct alternative (a), (b), (c) or (d) for each of the questions given below:

(i) A closed surface in vacuum encloses charges –q and +3q. The total electric flux emerging out of the surface is:

(a) Zero

(b) 2 q/ε_o

(c) 3 q/ε_o

(d) 4 q/ε_o

(ii) What is the angle of dip at a place where the horizontal component (BH) and vertical component (BV) of earth’s magnetic field are equal:

(a) 130^o

(b) 60^o

(c) 45^o

(d) 90^o

(iii) A beam of light is incident at the polarizing angle of 35^o on a certain glass plate. The refractive index of the glass plate is:

(a) sin 35^o

(b) tan 35^o

(c) tan 35^o

(d) sin 35^o

(iv) In a gamma ray emission from nucleus:

(a) only the number of protons change.

(b) the number of protons and neutrons, both change.

(c) there is no change in the number of protons and the number of neutrons.

(d) only the number of neutrons change.

(v) The energy associated with light of which of the following colours is minimum:

(a) violet

(b) red

(c) green

(d) yellow

(B) Answer the following questions briefly and to the point.

(i) Define equipotential surface.

(ii) Calculate the net emf across A and B shown in Figure 1 below:

(iii) Why are the pole pieces of a horseshoe magnet in a moving coil galvanometer made cylindrical in shape?

(iv) What is the value of power factor for a pure resistor connected to an alternating current source?

(v) What should be the path difference between two waves reaching a point for obtaining constructive interference in Young’s Double Slit experiment?

(vi) Define critical angle for a given medium.

(vii) Name the series in the atomic spectra of the hydrogen atom that falls in the ultraviolet region.

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

(i)(b) or 2q/∈_o

(ii) (c) or 45^o

(iii) (b) or (tan 35)

(iv) (c) or No change in the number of protons and the number of neutrons

(v) (b) or Red light

(B) Answer the following questions briefly and to the point.

(i) It is the locus of points having the same potential due to a given charge distribution.

OR

Any surface on which the electric potential is the same everywhere is called equipotential surface.

(ii) 8V

(iii) To obtain radial field or θ = 900

(iv) One or cos00

(v) nλ, where n = 0,1, 2, 3…… or integral multiple of λ

(vi) Critical angle is that angle of incidence in a denser medium, for which the angle of refraction in the rarer medium is 90º.

(vii) Lyman

Question 2: In a potentiometer experiment, the balancing length with a resistance of 2Ω is found to be 100 cm, while that of an unknown resistance is 500 cm. Calculate the value of the unknown resistance.

Answer:

Question 3: A rectangular loop of area 5m², has 50 turns and carries a current of 1 A. It is held in a uniform magnetic field of 0·1T, at an angle of 30^o. Calculate the torque experienced by the coil.

Answer: τ = NIA B sin 𝜃 OR

τ = 50 × 1 ×5×0.1 sin 30 / cos 60

τ = 12.5 N m

OR

τ = 50 × 5 × 0.1 × 1 sin 60/ cos 30

τ = 21.65 N m

Question 4:

(a) An electric current I flows through an infinitely long conductor as shown in Figure 2(a) below. Write an expression and direction for the magnetic field at point P.

OR

(b) An electric current I flows through a circular loop as shown in Figure 2(b) below. Write an expression and direction for the magnetic field at the centre of the loop at point P.

Answer: (a) B = 𝜇_oI / 2𝜋r

Inward and perpendicular to the plane of the paper OR correctly shown diagrammatically.

(b) B = 𝜇_oI / 2r

Outward and perpendicular to the plane of the paper OR correctly shown diagrammatically.

Question 5: A transformer is used to step up an alternating emf of 200V to 440V. If the primary coil has 1000 turns, calculate the number of turns in the secondary coil.

Answer:

Question 6: State any two properties of microwaves.

Answer:

• No material medium is required for its propagation
• Travels at the speed of light in vacuum / air
• Travels at a speed of 3×10⁸ m/s
• Reflected
• Refracted
• Diffracted
• Polarized
• Interference

Question 7: Write any one use for each of the following mirrors:
(a) Convex
(b) Concave

Answer: (a) Convex:

• Rear view mirrors in vehicles.
• Used as reflectors in street lights.
• Placed at corners in hospitals, shopping malls, etc. to enable people to see what is approaching around the corner.
• Used in making lenses for sunglasses (this is done to help reflect the light of the sun away from the eyes of the person wearing the sunglasses).

(b) Concave:

• Shaving mirror
• Parabolic mirrors in astronomical telescope
• Search lights
• Head lights
• Solar reflectors
• To determine the focal length of a lens.

Question 8: The deviation produced for violet, yellow and red lights for crown glass are 3·75^o, 3·25^o and 2·86^o respectively. Calculate the dispersive power of the crown glass.

Answer:

Question 9: (a) What is meant by mass defect?
(b) What conclusion is drawn from Rutherford’s scattering experiment of α-particles?

Answer: (a) When a nucleus is formed from its constituent nucleons, the mass of the nucleus is found to be less that the total mass of the nucleons. The difference in mass is called mass defect.

OR

The difference between the real mass and the expected mass in a nuclear reaction.

OR

The difference between the mass of a nucleus and its constituents, Δ𝑀𝑀, is called the mass defect.

OR

The difference between the atomic mass (M) and mass number (A).

(b) The positive charge of the atom is concentrated in a small central core called the nucleus OR

The whole mass of the atom remains in a small region of the atom

OR

The space around the nucleus is almost empty.

Question 10: Define the following with reference to photoelectric effect:
(a) Threshold frequency (f_o)
(b) Stopping potential (V_s)

Answer: (a) Threshold frequency (f_o) is defined as the minimum cut off frequency of the incident radiation below which no emission of photoelectrons takes place.

(b)

• Stopping potential is defined as that negative voltage for which the fastest moving electrons may be stopped.
• Negative voltage for which photo electric current is stopped or becomes zero.

Question 11:

(a) The half-life of radium is 1550 years. Calculate its disintegration constant (λ).

(b) Copy and complete the following table for a radioactive element whose half-life is 10 minutes. Assume that you have 30g of this element at t = 0.

Answer: (a)

(b)

Question 12: Define frequency modulation and state any one advantage of frequency modulation (FM) over amplitude modulation (AM).

Answer: The frequency of a carrier wave is varied in accordance with the modulating wave. This is called frequency modulation.

• Reduces noise factor or FM is immune to all forms of noise existing in nature.
• FM has higher efficiency than AM
• The sound of a TV set (FM) is of much better quality than that of a radio set (AM)
• Bandwidth of FM is greater than the AM
• Amplitude of FM is constant unlike AM wave

Question 13: Obtain an expression for electric potential ‘V’ at a point in an end-on position i.e. axial position of an electric dipole.

Answer:

Question 14: Three capacitors of capacitance C₁ = 3µ, C₂ = 6µ and C₃ = 10µ, are connected to a 10V battery as shown in Figure 3 below:

Calculate:

(a) Equivalent capacitance.

(b) Electrostatic potential energy stored in the system.

Answer: (a)

(b)

Question 15:

(a) Obtain the balancing condition for the Wheatstone bridge arrangement as shown in Figure 4 below:

OR

(b) Draw a labelled circuit diagram of a potentiometer to measure the internal resistance ‘r’ of a cell. Write the working formula (derivation is not required).

Answer: (a)

I₁P = I₂ R

I₁Q = I₂S

I_g = 0

P/R = Q/S

(b)

Question 16:

(a) A ray of light is incident on a prism whose refractive index is 1·52 at an angle of 40^o. If the angle of emergence is 60o, calculate the angle of the prism.

OR

(b) Calculate the focal length of a convex lens whose radii of curvature of two surfaces is 10 cm and 15 cm respectively and its refractive index is 1·5.

Answer: (a)

(b)

Question 17: Derive the law of reflection using Huygen’s Wave Theory.

Answer:

Question 18: State any two Bohr’s postulates and write the energy value of the ground state of the hydrogen atom.

Answer:

• An electron in an atom could revolve in certain stable orbits without the emission of radiant energy.
• Electrons revolve around the nucleus only in those orbits in which their angular momentum is an integral multiple of h/2π where h is the Planck’s constant (6.6×10^-34 J s) that is L or mvr = nh/2π

•An electron might make a transition from one of its specified non-radiating orbits to another of lower energy. In doing so a photon is emitted having energy equal to the energy difference between the initial and final states. The frequency of the emitted photon is given by

f = (E_i – E_f)/h

where E_i and E_f are the energies of the initial and final states ( E_i > E_f)

(Any two Bohr’s postulate)

Energy value of the ground state of hydrogen atom, 𝐸 = −13.56 𝑒V or 13.6 𝑒V

Question 19: With reference to semi-conductors answer the following:

(i) What is the change in the resistance of the semi-conductor with increase in temperature?

(ii) Name the majority charge carriers in n-type semi-conductor.

(iii) What is meant by doping?

Answer: (i) The resistance of the semiconductor decreases with increase in temperature.

(ii) In n-type semi-conductor majority charge carriers are electrons.

(iii) Adding impurity atoms to a (pure) semi-conductor (to increase its conductivity) in a calculated manner is called doping.

OR

Adding a trivalent or a pentavalent atom to a semi-conductor (germanium or silicon) in a calculated manner is called doping.

OR

Adding a small amount of a suitable impurity to a pure semiconductor to increase its conductivity.

Question 20:

(a) (i) An alternating emf of 200V, 50Hz is applied to an L-R circuit, having a resistance R of 10Ω and an inductance L of 0·05H connected in series. Calculate:

(1) Impedance.

(2) Current flowing in the circuit.

(ii) Draw a labelled graph showing the variation of inductive reactance (XL) verses frequency (f).

OR

(b) (i) An a.c. source of emf ε = 200 sin ω is connected to a resistor of 50Ω.

Calculate:

(1) Average current (I_avg).

(2) Root mean square (rms) value of emf.

(ii) State any two characteristics of resonance in an LCR series circuit.

Answer: (a) (i)

(ii)

OR

(b) (i)

(ii)

• Z = R
• Cosφ = 1
• P = I²Z = I² R that is maximum power is dissipated in a circuit (through R) at resonance
• Current is maximum
• Resistance is minimum or impedance is minimum
• f = 1/ 2π√LC
• XL = XC OR 𝜔L = 1/ 𝜔C OR 2π f L = 1/ 2πfC

Question 21:

(a) Draw a neat labelled ray diagram showing the formation of an image at the least distance of distinct vision D by a simple microscope. When the final image is at D, derive an expression for its magnifying power at D.

OR

(b) Draw a neat labelled diagram of Young’s Double Slit experiment. Show that β = λ/d, where the terms have their usual meaning (either for bright or dark fringe).

Answer: (a)

OR

(b)

Question 22:

(a) (i) Draw a labelled circuit diagram of a half wave rectifier and give its output waveform.

(ii) Draw a symbol for NOR gate and write its truth table.

OR

(b) (i) Draw a neat circuit diagram to study the input and output characteristics of a common emitter transistor.

(ii) Draw the symbol for AND gate and write its truth table.

Answer: (a) (i)

(ii)

(b) (i)

(ii)

The ISC Class 12 Physics Question Paper Solution 2019 must have helped students with their exam preparation. Apart from the 2019 paper solution, students can also find the answers to other papers of ISC Class 12 Previous Years Questions by clicking here. Happy Learning and stay tuned to BYJU’S for the latest update on ICSE/CBSE/State Boards/Competitive exams. Also, don’t forget to download the BYJU’S App.