ISC Class 12 Physics Question Paper Solution 2020

Students who are searching for the ISC Class 12 Physics Question Paper Solutions are on the right page. Here, we have provided the solved Class 12 Physics ISC Question Paper 2020 PDF along with detailed solutions. Practising the questions and then going through the answers will help them understand the answer writing skills to fetch more marks. Moreover, they will also find the answers to the questions which they find difficult to solve.

Download ISC Class 12 Physics Question Papers Solutions 2020 PDF

The ISC Class 12 Physics (Paper 2) 2020 exam was conducted on 6th February 2020. The exam started at 9 am, and students were allotted 3 hours to complete the paper. They can download the ISC Class 12 Physics Question Paper Solution 2020 PDF from the link below.

Apart from the theory paper, students can also download the ISC Class 12 Physics Practical Question Paper 2020 pdf below.

ISC Class 12 Physics Practical Question Paper 2020 PDF

Students can have a look at the ISC Class 12 Physics Question Paper Solution 2020 below. They can also access the year wise Solved ISC Class 12 Physics Solved Previous Year Question Papers for more practice.

Difficult Topics of ISC Class 12 Physics 2020 Question Paper

Below we have listed down the topics which students found difficult and confusing while attempting the ISC 2020 Physics Question Paper.

• Gauss’ Theorem
• Kirchhoff’s Law
• Numerical problems on Potentiometer
• Curie temperature
• Angle of dip
• Numerical problems on alternating current
• Ray diagrams of lenses and mirrors
• Sign convention in lenses/compound microscope
• Labelling of ray diagrams and graphs
• Diffraction
• Davisson – Germer experiment
• Mean life and Half-life of a radioactive substance
• Binding energy
• Pair production
• Modulation and demodulation
• Zener diode, common emitter amplifier their circuit diagrams and biasing, and other related topics
• Relation between “alpha” and “beta” of a transistor
• Alternating and direct current phenomena
• Phase and phase difference
• Impedance and inductance in ac circuit
• Refraction and Scattering
• Polarizer and Analyzer
• Absorption and Emission Spectrum
• Pair Production and Pair annihilation
• Forward and reverse biasing
• Numerical problems on mass defect
• Symbol of PNP / NPN transistor and Zener diode
• Communication system

ISC Class 12 Physics Question Paper 2020 With Solutions

Question 1:

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

(i) A point charge ‘q’ is kept at each of the vertices of an equilateral triangle having each side ‘a’. Total electrostatic potential energy of the system is:

(ii) Curie temperature is the temperature above which:

(a) a ferromagnetic substance behaves like a paramagnetic substance.

(b) a paramagnetic substance behaves like a diamagnetic substance.

(c) a ferromagnetic substance behaves like a diamagnetic substance.

(d) a paramagnetic substance behaves like a ferromagnetic substance.

(iii) In an astronomical telescope of refracting type:

(a) Objective should have a small focal length.

(b) Objective should have large focal length.

(c) Eyepiece should have large focal length.

(d) Both objective and eyepiece should have a large focal length.

(iv) In photoelectric effect experiment, the slope of the graph of the stopping potential versus frequency gives the value of:

(a) h/e

(b) h

(c) e/h

(d) hc/e

(v) In a nuclear reactor, cadmium rods are used as:

(a) Control rods

(b) Fuel rods

(c) Coolant

(d) Moderator

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

(i) State Gauss’ theorem.

(ii) A metallic wire having a resistance of 20Ω is bent in order to form a complete circle. Calculate the resistance between any two diametrically opposite points on the circle.

(iii) How can a moving coil galvanometer be converted into a voltmeter?

(iv) Write Biot-Savart’s law in vector form.

(v) What is the phase difference between any two points lying on the same wavefront?

(vi) Name the physical principle on the basis of which optical fibres work.

(vii) What is Pair production?

Answer: (A) (i) (c) or

(ii) (a) or a ferromagnetic substance behaves like a paramagnetic substance.

(iii) (b) or objective should have large focal length.

(iv) (a) or h/e

(v) (a) or Control rods

(B) (i) Gauss’ theorem:

Electric flux passing through a closed surface/ Gaussian surface is a ratio of the net charge enclosed by the surface to the permittivity of vacuum. OR

It is 1/∈₀ the net charge enclosed by the surface. OR

(ii) Two 10 Ω wires are in parallel as shown in the diagram below

(iii) By connecting a high resistance in series with the galvanometer.

OR

(v) Zero or no phase difference.

(vi) Total Internal Reflection / (TIR)

(vii) Pair production:

Production of an electron – position pair from a gamma ray (or radiation)

OR

Conversion of a gamma ray photon into an electron – position pair.

OR

Question 2:

(a) A uniform copper wire having a cross sectional area of 1mm² carries a current of 5A. Calculate the drift speed of free electrons in it.

(Free electron number density of copper = 2×10²⁸/m³.)

OR

(b) An electric bulb is rated as 250V, 750W. Calculate the:

(i) Electric current flowing through it, when it is operated on a 250V supply.

(ii) Resistance of its filament.

Answer: (a)

OR

(b)

(ii) I = V/R = 250/83.3 = 3.00 A

Question 3: Write an expression for force per unit length between two long current carrying wires, kept parallel to each other, in vacuum and hence define an ampere, the SI unit of current.

Answer:

Definition of ampere: One ampere is that current which while flowing through each of two long conductors kept parallel to each other 1m apart in vacuum (attract or repel each other with) experience a force of 2 ×10^-7 N m^-1.

Question 4:

(i) Define angle of dip.

(ii) State the relation between magnetic susceptibility (χ) and relative permeability(μ_r) of a magnetic substance.

Answer: (i) Angle of dip: It is the angle made by the magnetic axis of a freely suspended magnetic needle with the horizontal.

OR

It is the angle between the direction of the earth’s magnetic field and the horizontal in the magnetic meridian at that place.

OR

It is the angle made by the freely suspended magnet with the horizontal.

(ii) μ_r= 1+ χ

Question 5:

(a) Figure 1 below shows a metallic rod MN of length l = 80cm, kept in a uniform magnetic field of flux density B = 0·5T, on two parallel metallic rails P and Q. Calculate the emf that will be induced between its two ends, when it is moved towards right with a constant velocity v of 36 km/hr.

(b) When current flowing through one coil changes from 0 Amp to 15 Amp in 0·2 s, an emf of 750 V is induced in an adjacent coil. Calculate the coefficient of mutual inductance of the two coils.

Answer:

Question 6:

(i) State any one use of infrared radiations.

(ii) State any one source of ultraviolet radiations.

Answer:

(i) 1.To relieve muscular pain.

2. In greenhouses to keep plants warm.

3. In warfare, to look through haze.

4. For night vision.

5. Solar Heater or Cooker

6. Photography Or any other correct application

(Or any other correct use)

(ii) Sun Or Arc (lamp) Or Vacuum spark Or Ionised gases

(Or any other correct use)

Question 7: Where will you keep an object in front of a:

(i) Convex lens in order to get a virtual and magnified image?

(ii) Concave mirror to get a real and diminished image?

Answer:

(i) Between optical centre and focus. Or Between O and F Or distance less than f

(ii) Beyond centre of curvature. Or Beyond C Or Beyond 2F Or beyond R

Question 8: Draw a labelled graph of angle of deviation versus angle of incidence (i) for a prism.

Answer: Axes correctly labelled i.e. (δ) on Y axis and (i) on X axis

Question 9:

(i) State de Broglie hypothesis.

(ii) What conclusion can be drawn from Davisson and Germer’s experiment?

Answer: (i) The waves associated with moving material particles are called matter waves / de Broglie waves.de Broglie wavelength (λ)associated with a moving particle is related to its momentum p as

λ=h/p

or λ=h/mv

(Or any other correct statement)

(ii) Moving electrons behave like waves/ moving particles exhibit dual nature.

Or

Wave nature of moving particles.

Or

Electron Diffraction / verified and confirmed the wave nature of electrons

(Or any other correct statement)

Question 10: Calculate binding energy of oxygen nucleus

from the data given below:

Answer:

Question 11: For a radioactive substance, write the relation between:

(i) Half life (T) and disintegration constant (λ).

(ii) Mean life (τ) and disintegration constant (λ).

Answer:

Question 12: With reference to communication systems, what is meant by:

(i) modulation?

(ii) demodulation?

Answer: (i) The process of superimposition/ overlapping of low frequency (signal wave)/ modulating wave/audio wave on a high frequency (carrier wave), in order to transmit the signal to a large distance is called modulation.

(ii) The process of separating low frequency (signal wave)/ modulating wave/audio wave from the modulated wave is called demodulation.

Question 13: Show that intensity of electric field E at a point in broadside on position is given by:

where the terms have their usual meaning.

Answer:

Question 14: A parallel plate capacitor is charged by a battery, which is then disconnected. A dielectric slab having dielectric constant (relative permittivity) K, is now introduced between its two plates in order to occupy the space completely.

State, in terms of K, its effect on the following:

(i) The capacitance of the capacitor.

(ii) The potential difference between its plates.

(iii) The energy stored in the capacitor.

Answer:

(i) Capacitance becomes K times. Or Increases K times.

(ii) Potential difference becomes 1/K times. Or Decreases by K times.

(iii) Energy becomes 1/K times. Or Decreases by K times.

Question 15:

(a) E₁ and E₂ are two batteries having emfs of 3V and 4V and internal resistances of 2Ω and 1Ω respectively. They are connected as shown in Figure 2 below. Using Kirchhoff’s Laws of electrical circuits, calculate the currents I₁ and I₂.

(b) A potentiometer circuit is shown in Figure 3 below. AB is a uniform metallic wire having length of 2m and resistance of 8Ω. The batteries E₁ and E₂ have emfs of 4V and 1·5V and their internal resistances are 1Ω and 2Ω respectively.

(i) When the jockey J does not touch the wire AB, calculate:

(a) the current flowing through the potentiometer wire AB.

(b) the potential gradient across the wire AB.

(ii) Now the jockey J is made to touch the wire AB at a point C such that the galvanometer (G) shows no deflection. Calculate the length AC.

Answer: (a)

(b)

(ii)

Question 16: For two thin lenses kept in contact with each other, show that:

where the terms have their usual meaning.

Answer:

Note: All sign conventions are acceptable.

Question 17:

(a) A compound microscope consists of two convex lenses having focal length of 1·5cm and 5cm. When an object is kept at a distance of 1·6 cm from the objective, the final image is virtual and lies at a distance of 25cm from the eyepiece. Calculate magnifying power of the compound microscope in this set-up.

OR

(b) In Young’s double slit experiment, the screen is kept at a distance of 1·2m from the plane of the slits. The two slits are separated by 5mm and illuminated with monochromatic light having wavelength 600nm. Calculate:

(i) Fringe width i.e. fringe separation of the interference pattern.

(ii) Distance of 10^th bright fringe from the centre of the pattern.

Answer: (a)

(b)

Question 18:

Draw the energy level diagram of hydrogen atom and show the transitions responsible for:

(i) absorption lines of Lyman series.

(ii) emission lines of Balmer series.

Answer:

Question 19:

(i) State any one difference between energy band diagram of conductors and that of insulators.

(ii) Give a relation between α and β for a transistor.

(Derivation is not required.)

(iii) What is the advantage of an LED bulb over the filament electric bulb?

Answer:

(i) The conduction band and valence band overlap in a conductor whereas there is a forbidden gap between conduction band and valence band in a conductor.

OR

(ii)

(iii) Power consumption in an LED is much less as compared to that in a filament bulb.

OR

Lifespan of LED is much greater as compared to that of a filament bulb. OR

Less heating effect OR work effectively on low voltage. (or any correct answer)

Question 20:

(a) (i) A 400Ω resistor, a 3H inductor and a 5μF capacitor are connected in series to a 220V, 50Hz ac source. Calculate the:

(1) Impedance of the circuit.

(2) Current flowing through the circuit.

(ii) Draw a labelled graph showing the variation of impedance (Z) of a series LCR circuit versus frequency (f) of the ac supply.

OR

(b) (i) When an alternating emf e = 310 sin (100t)V is applied to a series LCR circuit, current flowing through it is i = 5 sin (100πt + π/3)A.

(1) What is the phase difference between the current and the emf?

(2) Calculate the average power consumed by the circuit.

(ii) Obtain an expression for the resonant frequency (f_o) of a series LCR circuit.

Answer: (a)(i)

OR

(ii)

(b)

Question 21:

(a) (i) Derive an expression for refraction at a single (convex) spherical surface, i.e. a relation between u, v, R, n₁ (rarer medium) and n₂ (denser medium), where the terms have their usual meaning.

(ii) Name the phenomenon due to which the sun appears reddish at sunset.

OR

(b) (i) Draw a labelled graph of intensity of diffracted light (I) versus angle in the Fraunhofer diffraction experiment for a single slit diffraction.

(ii) State the law of Malus.

(iii) How will you distinguish experimentally between ordinary light and plane polarized light?

Answer: (a)(i)

(ii) Scattering of light OR Rayleigh Scattering

OR

(b) (i)

(ii) It states that intensity I of light transmitted by a polaroid is given by

I=I₀Cos²θ

OR

In words: Where θ is the angle between the direction of incident light and optic axis of the polaroid. (angle between the plane of transmission of the analyser and the polariser)

OR

(iii) Given beam of light is passed through a polaroid or a lamina of tourmaline, which is then rotated. If the intensity of transmitted light:

(a) remains constant, incident light is ordinary light or unpolarized light.

(b) varies from 0 to maximum, or from darkness to brightness, incident light is plane polarized.

Question 22:

(a) (i) In a semiconductor diode, what is meant by potential barrier?

(ii) Draw a labelled circuit diagram of a Zener diode as a voltage regulator.

(iii) Show with the help of a diagram, how you will obtain an AND gate using only NAND gates. (Truth table is not required.)

OR

(b) (i) Draw a labelled circuit diagram of a transistor acting as a common emitter amplifier. What is meant by phase reversal?

(ii) Draw the symbol of a NAND gate and write its truth table. Useful Constant and Relation:

Answer:

(a) (i) Potential barrier: The potential difference across the depletion region is called the potential barrier.

(ii)

(iii)

OR

(b)(i)

Phase reversal means: Output waveform is out of phase to input wave by 180° or π.

(ii)

NOTE: For questions having more than one correct answer/solution, alternative correct answers / solutions, apart from those given in the marking scheme, have also been accepted.

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