**CBSE Class 12 Physics Important Questions: **Before appearing for the CBSE Class 12 Physics board exam 2022 tomorrow, it is very important that students practise these important questions. Practising these questions will help you a lot in scoring the best marks in exams. In the last moments before the exam, you must focus on not missing any important topic and revising all important topics. Check the complete set of important questions here.

## CBSE Class 12 Physics Paper Design

The paper will contain eighteen MCQ of 1 mark each, seven questions of two marks each, five questions of three marks each, three long questions of five marks each and two case study based questions of 4 marks each. The questions would be divided into 5 sections. There will be 35 questions in all.

## CBSE Class 12 Physics Important Questions

### MULTIPLE CHOICE QUESTIONS

**1 The electric potential on the axis of an electric dipole at a distance ‘r from it’s centre is V. Then the potential at a point at the same distance on its equatorial line will be**

(i) 2V

(ii) -V

(iii) V/2

(iv) Zero

**2 A wire of length L carrying a current I can be turned into a circular loop of N turns. For what value of N, will the magnetic moment of this current-carrying loop be maximum?**

- one
- 4πL
- infinite
- (Magnetic moment is a constant for a given L and is independent of N)

**3 Two concentric and coplanar circular loops P and Q have their radii in the ratio 2:3. Loop Q carries a current 9 A in the anticlockwise direction. For the magnetic field to be zero at the common centre, loop P must carry**

(i) 3A in clockwise direction

(ii) 9A in clockwise direction

(iii) 6 A in anti-clockwise direction

(iv) 6 A in the clockwise direction.

**4 Two statements are given-one labelled Assertion (A) and the other labelled**

**Reason (R). Select the correct answer to these questions from the codes (A), (B), (C), and (B) as given below.**

**Assertion (A) : The curve between the binding energy per nucleon versus mass number droops at high mass numbers (A>170) as well as at low mass numbers (A<30).**

**Reason (R) : Nuclei with middle mass numbers (30<A<170) have higher binding energy per nucleon.**

- Both A and R are true and R is NOT the correct explanation of A.
- Both A and R are true and R is the correct explanation of A.
- A is false and R is also false.
- A is true but R is fals

**5 If the magnetizing field on a ferromagnetic material is increased, its permeability**

(i) decreases

(ii) increases

(iii) remains unchanged

(iv) first decreases and then increases

**6 Assuming that the momentum of an electron is measured with complete accuracy, that is., the corresponding uncertainty in its momentum being zero, what is the uncertainty in a simultaneous measurement of the electron’s position?**

- zero
- unity
- infinitely large
- some finite value between unity and infinity

**7 Under ideal conditions, consider two different sources of light producing identical waves that happen to be in phase with each other. The two sources are placed at the corners of a square. They broadcast waves uniformly in all directions.**

**Which of the following locations of the two sources will ensure that the waves always produce constructive interference at the center of the square?**

- any two corners of the square
- only the adjacent corners of the square
- only corners across the diagonal of the square
- one source at the corner and the other at the center

**8 A 5 ohm resistor, a 5 mH inductor and a 5 μF capacitor, joined in series resonate with an ac source of frequency ωo. If only the resistance is changed to 10 ohm, the circuit resonates at a frequency ω1. If only the inductor is changed to 20 mH, the circuit resonates at a frequency ω2. Find the ratio ω1/ω2.**

- 0.5
- 1
- 2
- 4

**9 Which of the following statement is NOT true about the properties of electromagnetic waves?**

(I) These waves do not require any material medium for their propagation

(ii) Both electric and magnetic field vectors attain the maxima and minima at the same time

(iii) The energy in electromagnetic wave is divided equally between electric and magnetic fields

(iv) Both electric and magnetic field vectors are parallel to each other

**10 The work function for a metal surface is 4.14 eV. The threshold wavelength for this metal surface is:**

(i) 4125 Å

(ii) 2062.5 Å

(iii) 3000 Å

(iv) 6000 Å

### 2 MARK QUESTIONS

1 A biconvex lens made of a transparent material of refractive index 1.25 is immersed in water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Justify your answer.

2 The critical angle for the total internal reflection of diamond in air is 24o. State whether the two statements given here are correct or incorrect. Give a reason for your answer.

(a) The critical angle for total internal reflection of diamond is more than 24o when surrounded by water.

(b) The sparkle of the diamond increases remarkably when placed in water

3 A narrow slit is illuminated by a parallel beam of monochromatic light of wavelength λ equal to 6000 Å and the angular width of the central maximum in the resulting diffraction pattern is measured. When the slit is next illuminated by light of wavelength λ’, the angular width decreases by 30%. Calculate the value of the wavelength λ’

4 Estimate the number of dark fringes on the either side of the central maximum that can be produced by diffraction set up with slit of width 5 x 10-6 m and incident light of wavelength 600 nm.

5 The short wavelength limit for the Lyman series of the hydrogen spectrum is 913.4 A0. Calculate the short wavelength limit for the Balmer series of the hydrogen spectrum.

6 By what factor does kinetic and potential energy of an electron in a hydrogen atom change as it moves from n = 1 to n =3?

7 A capacitor consists of two parallel plates, with an area of cross-section of 0.001 m2, separated by a distance of 0.0001 m. If the voltage across the plates varies at the rate of 108 V/s, determine the value of displacement current through the capacitor.

### 3 MARK QUESTIONS

1 Two long straight parallel conductors carrying currents I1 and I2 are separated by a distance d. If the currents are flowing in the same direction, show how the magnetic field produced by one exerts an

attractive force on the other. Obtain the expression for this force and hence define 1 ampere.

2 (a) Give reason: A radiation of wavelength λ < λthreshold incident on a metal sphere placed on an insulated stand results in the emission of photoelectrons for some time and then stops.

(b) In the photoelectric experiment apparatus containing the collector and the emitter plate, a saturated photoelectron current is observed. If an external electric field is applied in the direction opposite to the motion of the photoelectrons, what is the change observed in each of the following? Give reasons.

- The saturation value of the photocurrent
- The kinetic energy of the photoelectrons striking the collector plate

3 An a.c. source generating a voltage ε = ε0 sin ωt is connected to a capacitor of capacitance C. Find the expression for the current I flowing through it. Plot a graph of ε and I versus t to show that the current is ahead of the voltage by π/2.

4 a. A glass container contains hydrogen atoms with all its atoms in their ground states. The container is irradiated with electromagnetic waves containing wavelengths corresponding to Lyman, Balmer and Paschen series. The electromagnetic waves exiting the glass container are found to have certain strong absorption spectral lines. Identify one or more series to which these absorption spectral lines would correspond to. Explain. Assume that once an electron absorbs a photon and jumps to a higher level, it does not absorb more photons to jump to even higher le

5 Radiation of frequency 10^15 Hz is incident on three photosensitive surfaces A, B and C. Following observations are recorded:

Surface A: no photoemission occurs

Surface B: photoemission occurs but the photoelectrons have zero kinetic energy.

Surface C: photo emission occurs and photoelectrons have some kinetic energy.

Using Einstein’s photo-electric equation, explain the three observations.

6 Coil 1 has self-inductance L1 which is 3 times the self-inductance L2 of coil 2. If during a certain instant, the rate of increase in current and the power dissipated in these two coils is the same, then determine the ratio of

(a) their induced voltages

(b) currents

(c) energy stored in the two coils at that instant.

7 The ground state energy of hydrogen atom is -13.6 eV. The photon emitted during the transition of electron from n=3 to n=1 state, is incident on a photosensitive material of unknown work function. The photoelectrons are emitted from the material with the maximum kinetic energy of 9eV .Calculate the threshold wavelength of the material used.

### 5 MARK QUESTIONS

1 (a)Three charges –q, Q and –q are placed at equal distances on a straight line. If the potential energy of the system of these charges is zero, then what is the ratio Q:q?

(b)(i) Obtain the expression for the electric field intensity due to a uniformly charged spherical shell of radius R at a point distant r from the centre of the shell outside it.

(ii) Draw a graph showing the variation of electric field intensity E with r, for r > R and r < R.

2 A parallel plate capacitor of capacitance C is charged to a potential V by a battery. Q is the charge stored on the capacitor. Without disconnecting the battery, the plates of the capacitor are pulled apart to a larger distance of separation.

What changes will occur in each of the following quantities? Will they increase, decrease or remain the same? Give an explanation in each case.

(a) Capacitance

(b) Charge

(c) Potential difference

(d) Electric field

(e) Energy stored in the capacitor

3 (a)Draw equipotential surfaces for (i)an electric dipole and (ii) two identical positive charges placed near each other.

(b) In a parallel plate capacitor with air between the plates, each plate has an area of 6 x 10-3m2 and the separation between the plates is 3 mm.

(i) Calculate the capacitance of the capacitor.

(ii) If the capacitor is connected to a 100V supply, what would be the charge on each plate?

(iii) How would charge on the plate be affected if a 3 mm thick mica sheet of k=6 is inserted between the plates while the voltage supply remains connected ?