Home NCERT Solutions NCERT Exemplar CBSE Sample Papers NCERT Books Class 12 Class 10

# Class 12th Physics 2018 Set1 Delhi Board Paper Solution

### Section 1

•  Q1 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? Q2 Name the electromagnetic radiations use for (a) water purification, and (b) eye surgery. Q3 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. Q4 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? Q5 Which mode of propagation is used by short wave broadcast services?

### Section 3

•  Q11 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. Q11 (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. Q12 Define the term 'conductivity' of a metallic wire. Write its SI unit. Q12 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. Q13 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). Q14 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. Q14 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. Q15 Show using a proper diagram how unpolarised light can be linearly polarised by reflection from a transparent glass surface. Q15 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. Q16 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. Q16 What kind of fringes do you expect to observe if white light is used instead of monochromatic light? Q17 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. Q18 State Bohr's postulate to define stable orbits in hydrogen atom. How does e Broglie's hypothesis explain the stability of these orbits? Q18 A hydrogen atom initially in the ground level absorbs a photon, which excites it to the n = 4 level. Determine the wavelength and frequency of the photon. Q19 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. Q19 A radioactive isotope has a half-life of 10 Years. How long will it take for the activity to reduce to 3.125%? Q20 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. Q20 Give the truth table and circuit symbol for NAND gate. Q21 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) Q22 Give three reasons why modulation of a message signal is necessary for long distance transmission. Q22 Show graphically an audio signal, a carrier wave and an amplitude modulated wave.

### Section 4

•  Q23 The teachers of Geeta’s school took the students on a study trip to a power generating station, located nearly 200 km away from the city. The teacher explained that electrical energy is transmitted over such a long distance to their city, in the form of alternating current (ac) raised to a high voltage. At the receiving end in the city, the voltage is reduced to operate the devices. As a result, the power loss is reduced. Geeta listened to the teacher and asked questions about how the ac is converted to a higher or lower voltage.   (a) Name the device used to change the alternating voltage to a higher or lower value. State one cause for power dissipation in this device.   (b) Explain with an example, how power loss is reduced if the energy is transmitted over long distances as an alternating current rather than a direct current.   (c) Write two values each shown by the teachers and Geeta. Q24 Define electric flux. Is it a scalar or a vector quantity? A point charge q is at a distance d/2 directly above the centre of a square of side ‘d’, as shown in the figure. Use Gauss’s law to obtain the expression for the electric flux through the square.   (b) If the point charge is now moved to a distance ‘d’ from the centre of the square and the side of the square is doubled, explain how the electric flux be affected. Q24 Use Gauss’ law to derive the expression for the electric field  due to a straight uniformly charged infinite line of charge density λ C/m. Q24 Draw graph to show the variation of E with perpendicular distance r from the line of charge. Q24 Find the work done in bringing a charge q from perpendicular distance r1 to r2 (r2>r1). Q25 State the principle of an ac generator and explain its working with the help of a labelled diagram. Obtain the expression for the emf induced in a coil having N turns each of cross-sectional area A,rotating with a constant angular speed (ω) in a magnetic field , directed perpenicular to the axis of rotation. Q25 An aeroplane is flying from west to east with a velocity of 900 km/hour. Calculate the the potential difference developed between the ends of its wings having a span of 20m. The horizontal component of the Earth's magnetic field is 5 x10-4 T and the angle of dip is 30º. Q25 A device X is connected across an ac source of voltage V=Vo sin ωt. The current through X is given as I=Io sin (ωt+π/2). (a) Identify the device X and write the expression for its reactance. (b) Draw graphs showing variation of voltage and current with time over one cycle of ac, for X. (c) How does the reactance of the device X vary with the frequency of the ac? Show this variation graphically. (d) Draw the phasor diagram for the device X. Q26 Draw a ray diagram to show image formation when the concave mirrow produces a real, inverted and magnified image of the object. Q26 Obtain the mirror formula and write the expression for the linear magnification. Q26 Explain two advantages of the reflecting type telescope over a refracting telescope. Q26 Define wavefront. Using Huygen’s principle, verify the laws of reflection at a plane surface. Q26 In a single slit diffraction experiment, the width of the slit is made double the original width. How does this affect the size and intensity of the central diffraction band? Explain. Q26 When a tiny circular obstacle is placed in the path of the light from a distance source, a bright spot is seen at the centre of the obstacle. Explain why.