# Important 3 marks Question Physics Class 12th

CBSE is one of the most prestigious and oldest educational board in the country. The board is responsible for conducting the Class 10th and Class 12th examination which are usually held in the month of March every year. CBSE Class 12 is one of the most important stages of a student’s academic career. Students are suggested to focus on the important question in order to score well in their examination.

Physics is considered as one of the most important subjects in 12th standard science stream. Students who want to make a career in the engineering or medical science field must prepare well for this subject to score marks in the board exam as well as the competitive entrance exams. Solving important questions is one of the best ways to prepare for CBSE Class 12 board exam. The important 3 marks question for class 12 physics is given here so that students can prepare for the exam more effectively.

The important questions for physics are prepared by experts according to the latest syllabus of Class 12 Physics. Students are suggested to solve the questions which will make them familiar with the marking scheme and the difficulty level of the examination.

The important 3 marks question for Physics Class 12 is given below.

Question 1-Draw a graph showing the variation of de Broglie wavelength of a particle of charge q and mass m with the accelerating potential. Proton and deutron have the same de Broglie wavelength. Explain which has more kinetic energy.

Question 2-State the Lorenz’s force and express it in vector form. Which pair of vectors are always perpendicular to each other? Derive the expression for the force acting on a current carrying conductor of length L in a uniform magnetic field ‘B’.

Question 3-Explain the two processes involved in the formation of a p-n junction diode. Hence define the term ‘barrier potential.;

Question 4-State Gauss’s law in electrostatics. Derive an expression for the electric field due to an infinitely long straight uniform changed wire.

Question 5-A capacitor of unknown capacitance is connected across a battery of V volt. A charge of 360 $\mu C$ is stored in it. When the potential across the capacitor is reduced by 120 V, the charge stored in the capacitor becomes 120 $\mu C$. Calculate V and the unknown capacitance. What would have been the charge on the capacitor if the voltage increased by 120 V?

Question 6-Distinguish between sky wave and space wave modes of communication. What is the main limitation of space wave mode? Write the expression for the optimum separation between the transmitting and receiving antenna for effective reception of signals in this mode of communication.

Question 7-Derive the expression for the current density of a conductor in terms of the conductivity and applied electric field. Explain with reason how the mobility of electrons in a conductor changes when the potential difference applied is doubled keeping the temperature of the conductor constant.

Question 8-Explain the term, ‘amplitude modulation’ of a signal. For an amplitude modulated wave, the maximum amplitude is 10 V and the minimum amplitude is 2 V. Calculate the modulation index.

Question 9- An optical instrument uses eye-lens of power 16 D and objective lens of power 50 D and has a tube length of 16.25 cm. Name the optical instrument and calculate the magnifying power if it forms the final image at infinity.

Question 10-(a) Write two properties by which electric potential is related to the electric field/

(b) Two point charged $q_{1}$ and $q_{1}$, separated by a distance of $r_{12}$ are kept in an external electric field. Derive an expression for the potential energy of the system of two charges in the field.

Question 11-State Lenz’s law. Explain, by giving examples that Lenz’s law is a consequence of conservation of energy.

Question 12-A plane wavefront propagating from a rarer into a denser medium is incident at an angle of incidence i on a refracting surface. Draw a diagram showing incident wavefront and refracted wavefront. Hence verify Snell’s laws of refraction.

Question 13-Define the term magnetic moment of a current loop. Derive an expression for the magnetic field at any point along the axis of a solenoid of length 2l, and radius a, and number of terms per unit length n.

Question 14-The energy levels of an atom of element X are shown in the diagram. Which one of the level transitions will result in the emission of photons of wavelength 620 nm? Support your answer with mathematical calculation.

Question 15-A message signal of frequency 20 KHz and peak voltage of 20 volts is used to modulate a carrier signal of frequency 2 MHz and peak voltage of 40 volts. Determine

(i) Modulation index.

(ii) the side bands produced.

Draw the corresponding frequency spectrum of amplitude modulated signal.

Question 16-In a double slit experiment, the distance between the slit is 3 mm and the slits are 2 m away from the screen. Two interference pattern can be seen on the screen one due to light with wavelength 480 nm, and the other due to light with wavelength 600 nm. What is the separation on the screen between the fifth order bright fringes of the two interference patterns?

Question 17-How many electrons must be added to one plate and removed from the other so as to store 25 J of energy in a 5.0 nF parallel plate capacitor?

Question 18-A point charge +Q is placed at the centre O of an uncharged hollow spherical conductor of inner radius ‘a’ and outer radius ‘b’. Find the following:

1. The magnitude and sign of the charge induced on the inner and outer surface of the conducting shell.
2. The magnitude of electric field vector at a distance

(i) $r = \frac{a}{2}$

(ii) $r = 2b$ from the centre of the shell.

Question 19-With the help of a diagram, explain the principle of a device which changes a low voltage into a high voltage but does not violate the law of conservation of energy. Give any one reason why the device may not be 100% efficient.

Question 20- Draw a graph showing the variation of binding energy per nucleon versus the mass number A. Explain with the help of this graph, the release of energy in the process of nuclear fission and fusion.