 Home Board Paper Solutions Text Book Solutions Articles NCERT Exemplar Solutions

# Class 12th Physics 2018 Set3 Delhi Board Paper Solution

### Section 1

•  Q1 Which mode of propagation is used by short wave broadcast services? Q2 Four nuclei of an element undergo fusion to form a heavier nucleus, with release of energy. Which of the two - the parent or the aughter nucleus - would have higher binding energy per nucleon? Q3 Name the electromagnetic radiations use for (a) water purification, and (b) eye surgery. Q4 Draw graphs showing variation of photoelectric current with applied volatge for two incident radiations of equal frequency and different intensities. Mark the graph for the radiation of higher intensity. Q5 A proton and an electron travelling along parallel paths enter a region of uniform magnetic field, acting perpendicular to their paths. Which of them will move in a circular path with higher frequency?

### Section 3

•  Q11 Draw the typical input and output charateristics of an n-p-n transistor in CE configuration. Show how these charateristics can be used to determine (a) the input resistance (r1) and (b) current amplification factor (β) Q12 A student wants to use two p-n junction diodes to convert alternating current into direct current.Draw the labelled circuit diagram shw would use and explain how it works. Q12 Give the truth table and circuit symbol for NAND gate. Q13 Explain the processes of nuclear fission and nuclear fusion by using the plot of binding energy per nucleon (BE/A) versus teh mass number A. Q13 A radioactive isotope has a half-life of 10 Years. How long will it take for the activity to reduce to 3.125%? Q14 Give three reasons why modulation of a message signal is necessary for long distance transmission. Q14 Show graphically an audio signal, a carrier wave and an amplitude modulated wave. Q15 If one of two identical slits producing interference in Young's experiment is covered with glass, so that the light intensity passing through it is reduced to 50%, find the ratio of the maximum and minimum intensity of the fringe in the interference pattern. Q15 What kind of fringes do you expect to observe if white light is used instead of monochromatic light? Q16 State Bohr's postulate to define stable orbits in hydrogen atom. How does e Broglie's hypothesis explain the stability of these orbits? Q16 A hydrogen atom initially in the ground state absorbs a photon which excites it to the n = 4 level, Estimate the frequency of the photon. Q17 Show using a proper diagram how unpolarised light can be linearly polarised by reflection from a transparent glass surface. Q17 The figure shows a ray of light falling normally on the face AB of an equilateral glass prism having refractive index 3 / 2, placed in water of refractive index 4 / 3. Will this ray suffer total internal reflection on striking the face AC? Justify your answer. Q18 A symmetric biconvex lens of radius of curvature R and made of glass of refractive index 1.5, is placed on a layer of liquid placed on top of a plane mirror as shown in the figure. An optical needle with its tip on the principal axis of the lens is moved along the axis until its real, inverted image coincides with the needle itself. The distance of the needle from the lens is measured to be x. On removing the liquid layer and repeating the experiment, the distance is found to be y. Obtain the expression for the refractive index of the liquid in terms of x and y. Q19 An iron ring of relative permeability μr has windings of insulated copper wire of n turns per metre. When the current in the windings is I, find the expression for the magnetic field in the ring. Q19 The susceptibility of a magnetic material is 0.9853. Identify the type of magnetic material.Draw the modification of the field pattern on keeping a piece of this material in a uniform magnetic field. Q20 A bar magnet of magnetic moment 6 J/T is aligned at 60° with a uniform external magnetic field of 0.44 T. Calculate (a) the work done in turning the magnet to align its magnetic moment (i) normal to the magnetic field. (ii) opposite to the magnetic field, and (b) the torque on the magnet in the final orientation in case (ii). Q21 Define the term 'conductivity' of a metallic wire. Write its SI unit. Q21 Using the concept of free electrons in a conductor, derive the expression for the conductivity of a wire in terms of number density and relaxation time. Hence obtain the relation between current density and the applied electric field E. Q22 Four point charges Q, q, Q, and q are placed at the corners of a square of side 'a' as shown in the figure. Find 1. The resultant electric force on a charge Q, and 2.Potential energy of this system. Q22 (a) Three point charges q, -4q and 2q  are placed at  the vertices of an equilateral triangle ABC of side 'l' as shown in the figure. Obtain the expression for  the magnitude of the resultant electric force acting on the charge q. (b) Find out the amount of the work done to separate the charges at infinite distance.