ISC Class 12 Physics Question Paper Solution 2018

The ISC Class 12 Physics – Paper I (Theory) was conducted on 20th February 2018. 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 2018. Solving the paper and then going through the solution pdf will help students in identifying their mistakes and thus improve their performance for the board exam. They can download the ISC Class 12 Physics Question Paper and Solution pdf 2018 from the link below.

Students can also access the Solved ISC Class 12 Physics Previous Year Question Papers at one place. They can have a look at the ISC Class 12 Physics Question Paper Solution 2018 below.

Solution pdf in view scrollable format

Difficult Topics of ISC Class 12 Physics Paper 2018

Topics which students found difficult while solving the Physics 2018 paper are mentioned below:

• Combination of capacitors
• Kirchhoff’s Law
• Motional emf
• Power Factor
• Numerical problem based on alternating current
• Sign convention in telescopes or in lenses
• Huygen’s wave theory
• Einstein’s photoelectric equation
• Binding energy
• Nuclear fusion and β-emission
• Modulation
• Zener diode

Confusing ISC Class 12 Physics Questions 2018

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

• Curie’s Law
• Magnetic Flux (Either 00 or 900)
• Magnetic field in a coil
• A.C. circuits
• Polarizer and Analyzer
• Threshold Wavelength and threshold frequency
• Pair Production

ISC Class 12 Physics Question Paper Solution 2018

Question 1:

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

(i) The order of coloured rings in a carbon resistor is red, yellow, blue and silver. The

resistance of the carbon resistor is:

1. 24×10⁶ Ω ± 5%
2. 24×10⁶ Ω ± 10%
3. 34×10⁴ Ω ± 10%
4. 26×10⁴ Ω ± 5%

(ii) A circular coil carrying a current I has radius R and number of turns N. If all the three, i.e. the current I, radius R and number of turns N are doubled, then, magnetic field at its centre becomes:

1. Double
2. Half
3. Four times
4. One fourth

(iii) An object is kept on the principal axis of a concave mirror of focal length 10 cm, at a distance of 15 cm from its pole. The image formed by the mirror is:

1. Virtual and magnified
2. Virtual and diminished
3. Real and magnified
4. Real and diminished

(iv) Einstein’s photoelectric equation is:

1. E_max = hλ – 𝜑₀
2. E_max = hc/λ – 𝜑₀
3. E_max = hv + 𝜑₀
4. E_max = hc/𝜆 + 𝜑₀

(v) In Bohr’s model of hydrogen atom, the radius of the first orbit of an electron is r₀. Then, radius of the third orbit is:

1. r₀/9
2. r₀
3. 3r₀
4. 9r₀

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

(i) In a potentiometer experiment, balancing length is found to be 120 cm for a cell E₁ of emf 2V. What will be the balancing length for another cell E₂ of emf 1·5V? (No other changes are made in the experiment.)

(ii) How will you convert a moving coil galvanometer into a voltmeter?

(iii) A moving charged particle q travelling along the positive x-axis enters a uniform magnetic field B. When will the force acting on q be maximum?

(iv) Why is the core of a transformer laminated?

(v) Ordinary (i.e. unpolarised) light is incident on the surface of a transparent material at the polarising angle. If it is partly reflected and partly refracted, what is the angle between the reflected and the refracted rays?

(vi) Define coherent sources of light.

(vii) Name a material which is used in making control rods in a nuclear reactor.

Answer: (A)

(i) (b) or 24×10⁶ Ω ± 10%

(ii) (a) or Double

(iii) (c) or Real and magnified

(iv) (b) or hc/λ – 𝜑₀

(v) (d) or 9 r₀

(B)

(iii) When it is moving perpendicular to the magnetic field.

OR

90^o / π/2 / ⟂ / correct diagram/

OR

B is along Y axis or B is along Z axis.

(iv) To minimize the loss due to induced current or eddy current.

(v) 90^o / π/2 / ⟂ to each other/ shown in the diagram

(vi) Sources which emit light waves which are either in phase / which have a zero / a constant phase difference. (Same wavelength or frequency; same amplitude)

(vii) Cadmium, boron, silver and Indium

Question 2: Define current density. Write an expression which connects current density with drift speed.

Answer: Current density is defined as current per unit cross sectional area. Or 𝐼⁄𝐴

Current density, J = nv_de or nve or 𝑁𝑉e/V.

Question 3:

(a) A long horizontal wire P carries a current of 50A. It is rigidly fixed. Another wire Q is placed directly above and parallel to P, as shown in Figure 1 below. The weight per unit length of the wire Q is 0·025 N m-1 and it carries a current of 25A. Find the distance ‘r’ of the wire Q from the wire P so that the wire Q remains at rest.

OR

(b) Calculate force per unit length acting on the wire B due to the current flowing in the wire A. (See Figure 2 below)

Answer:

Question 4:

(i) Explain Curie’s law for a paramagnetic substance.

(ii) A rectangular coil having 60 turns and area of 0·4m2 is held at right angles to a uniform magnetic field of flux density 5Å~10-5T. Calculate the magnetic flux passing through it.

Answer: (i) Magnetic susceptibility of a paramagnetic substance varies inversely with its absolute temperature.

(ii) ⏀ = BAN

= 60 × 5 × 10^-5 × 0·4

= 1·2×10^-3 Wb

OR

If the angle between the magnetic field and area vector is taken to be 90⁰.

Then, ⏀ = BAN cos 90⁰ = 0

Question 5: What is motional emf? State any two factors on which it depends.

Answer: (i) It is the emf induced in a conductor (rod / disc) due to its motion in a magnetic field.

(ii) (a) length of the rod

(b) magnetic flux density

(c) velocity (with which it is moved)

(d) Sin θ (where θ is the angle between the rod and the field)

Question 6: (i) What is the ratio of the speed of gamma rays to that of radio waves in vacuum?

(ii) Name an electromagnetic wave which is used in the radar system used in aircraft navigation.

Answer: (i) 1 OR 1:1 OR equivalent

(ii) Short Radio waves or Microwaves

Question 7: A biconvex lens made of glass (refractive index 1·5) has two spherical surfaces having radii 20 cm and 30 cm. Calculate its focal length.

Answer:

Question 8: State any two differences between primary rainbow and secondary rainbow.

Answer: Any two of the following:

1. Total internal reflection takes place once in primary rainbow and twice in secondary rainbow.

2. Primary rainbow has violet colour on the inner edge whereas the secondary rainbow has red colour on the inner edge.

OR

Primary rainbow has red colour on the outer edge whereas the secondary rainbow has violet colour on outer edge.

3. Primary rainbows are brighter than secondary rainbows.

4. Secondary rainbow is larger than the primary rainbow.

5. Semi vertical angle for primary rainbow is 41⁰ to 43⁰ and that for a secondary rainbow is 51⁰ to 54⁰.

Question 9:

(i) State de Broglie hypothesis.

(ii) With reference to photoelectric effect, define threshold wavelength.

Answer:

(i) Moving particles can behave like waves.

OR

Moving particles can show wave nature

OR

Moving Particles show / exhibit dual nature

OR

𝜆 = ℎ /p

𝑝𝑝 where p is momentum

OR

𝜆 = ℎ/𝑚ν

(ii) Maximum or the largest wavelength of incident radiation / light for which photoelectric effect takes place / observed / metal emits electrons.

Question 10: Calculate the minimum wavelength of the spectral line present in Balmer series of hydrogen.

Answer:

Question 11:

(a) What is meant by pair annihilation? Write a balanced equation for the same.

OR

(b) What is meant by the terms half-life of a radioactive substance and binding energy of a nucleus?

Answer: (a) When a particle / electron meets an antiparticle / positron, they destroy / each other, creating energy. Annihilate.

(b) Half-life: It is that time in which a given amount of radioactive substance becomes half.

OR

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

Binding energy of a nucleus: It is the minimum amount of energy required to separate all the nucleons/protons and neutrons of a nucleus from each other.

Question 12: In a communication system, what is meant by modulation? State any two types of modulation.

Answer: Modulation: Superposition or overlapping of a modulating wave /signal / low frequency wave on a carrier wave / high frequency wave is called modulation.

Any two of the following:

1. Amplitude modulation or AM

2. Frequency modulation or FM

3. Phase modulation or PM

Question 13: Obtain an expression for intensity of electric field at a point in end on position, i.e. axial position of an electric dipole.

Answer:

Question 14: Deduce an expression for equivalent capacitance C when three capacitors C₁, C₂ and C₃ are connected in parallel.

Answer:

Question 15:

(a) 𝜀₁ and 𝜀₂ are two batteries having emf of 34V and 10V respectively and internal resistance of 1Ω and 2Ω respectively. They are connected as shown in Figure 3 below. Using Kirchhoff’s’ Laws of electrical networks, calculate the currents I₁ and I₂.

(b) An electric bulb is marked 200 V, 100 W. Calculate electrical resistance of its filament. If five such bulbs are connected in series to a 200 V supply, how much current will flow through them?

Answer:

Question 16:

(a) For any prism, prove that:

where the terms have their usual meaning.

OR

(b) When two thin lenses are kept in contact, prove that their combined or effective focal length F is given by:

where the terms have their usual meaning.

Answer: (a)

OR

(b)

Question 17:

(i) In Young’s double slit experiment, show graphically how intensity of light varies with distance.

(ii) In Fraunhofer diffraction, how is the angular width of the central bright fringe affected when slit separation is increased?

Answer: (i)

(ii) Decreases

Question 18: Write one balanced equation each to show:

(i) Nuclear fission

(ii) Nuclear fusion

(iii) Emission of β− (i.e. a negative beta particle)

Answer:

Question 19: With reference to semiconductor devices, define a p-type semiconductor and a Zener diode. What is the use of a Zener diode?

Answer:

By adding/doping a trivalent group 3 element to a pure / intrinsic crystal of a semiconductor /(silicon/ germanium)

It is that negative voltage

OR

voltage during reverse bias, at which current becomes large, current shoots up

OR

A Zener diode is a particular type of diode that, unlike a normal one, allows current to flow not only

from its anode to its cathode, but also in the reverse direction.

OR

It is designed to operate under reverse bias in the breakdown region, its p and n regions are heavily

doped.

Use of a Zener diode: Voltage regulator or voltage stabilization or to provide steady voltage OR

To convert a fluctuating voltage to a steady voltage OR to prevent appliances from getting damaged due to voltage fluctuations.

Question 20:

(a) An alternating emf of 220V is applied to a circuit containing a resistor R having resistance of 160 Ω and a capacitor ‘C’ in series. The current is found to lead the supply voltage by an angle 𝜃 = tan^-1 (3/4).

(i) Calculate:

1. The capacitive reactance
2. Impedance of the circuit
3. Current flowing in the circuit

(ii) If the frequency of the applied emf is 50 Hz, what is the value of the capacitance of the capacitor ‘C’?

OR

(b) An A.C. generator generating an emf of 𝜀𝜀 = 300 sin (100𝜋t) 𝑉 is connected to a series combination of 16𝜇F capacitor, 1H inductor and 100 Ω resistor. Calculate:

(i) Impedance of the circuit at the given frequency.

(ii) Resonant frequency f_o.

(iii) Power factor at resonant frequency f_o.

Answer:

Question 21:

(a) Draw a labelled ray diagram of an image formed by a refracting telescope with the final image formed at infinity. Derive an expression for its magnifying power with the final image at infinity.

OR

(b) (i) Using Huygen’s wave theory, derive Snell’s law of refraction.

(ii) With the help of an experiment, state how will you identify whether a given beam of light is polarised or unpolarised.

Answer: (a)

Any two incident rays, parallel to each other, with an arrow on one of them, incident

on objective, labelled as O or L₁.

Two emergent rays, with an arrow, on at least one of them, emerging from the eye

piece labelled as E or L₂.

Final image, at f₀ and f_e coinciding and α and β marked.

OR

(b) (i)

(ii) Pass the light through a polaroid or tourmaline crystal or Nicol prism. The polaroid is rotated about the incident light and the intensity of light emerging from the polaroid is noted.

(i) If there is no change in intensity of the emergent light, the given beam of light is unpolarised.

(ii) If the intensity of emergent light changes then the incident light is polarised.

OR

If the intensity of emergent light changes but never becomes zero, the given beam of light is partially polarised.

OR

If the intensity of emergent light changes and becomes zero twice in one complete revolution, the given beam of light is completely plane-polarised.

Question 22:

(a) (i) The forward characteristic curve of a junction diode is shown in Figure 4 below:

Calculate the resistance of the diode at:

1. V = 0·5 V
2. I = 60 mA

(ii) Draw separate energy band diagrams for conductors, semi-conductors and insulators and label each of them.

OR

(b) (i) The arrangement given below represents a logic gate:

Copy the following truth table in your answer booklet and complete it showing outputs at C and D.

(ii) Draw a labelled diagram of a common emitter amplifier, showing waveforms of signal voltage and output voltage.

Answer: (a)

(ii)

OR

(b)(i)

(ii)

Circuit requirements: Input circuit showing signal and battery biasing correctly

Output circuit load or R_L, and collector battery with correct biasing

Correct wave forms with phase reversal

V-t marked in at least one graph

Correct diagram with P-N-P/N-P-N transistor is acceptable with proper polarities.

We hope students must have found ISC Class 12 Physics Question Paper Solution 2018 helpful for their exam preparation. We have also compiled ISC Class 12 Previous Year Question Papers for Physics, Physics, Maths and Biology at one place for students convenience. 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.