Physics NEB 2082 question paper

a) A ballet dancer sometimes stretches and sometimes folds her arms during her performance, why? Justify. [2]

b) Establish a relation between torque and moment of inertia for a rigid body. [2]

c) Why do we prefer a wrench of longer arm over a wrench of shorter arm? [1]

OR

a) Show that the total energy of the particles executing SHM is constant. [3]

b) Draw a graph showing the variation of potential energy of particle in SHM with displacement. [1]

c) The time period of simple pendulum on the surface of earth is T. What will be its time period on the Moon's surface? [1]

a) Define laminar flow and turbulent flow of liquid. [2]

b) Water flows steadily through a horizontal pipe of non-uniform cross section. If the pressure of water is $4\times 10^4\mathrm{N}{\mathrm{m}}^{-2}$ at a point where the velocity of flow is $2m/s$ and cross section is $200\mathrm{c}{\mathrm{m}}^2$. Calculate the pressure at a point where cross section reduces to $50\mathrm{c}{\mathrm{m}}^2$. (density of water = $1000kg/{\mathrm{m}}^3$) [3]

a) A Carnot's engine has 25% efficiency with a sink at 9°C. By how many degrees should the temperature of the source be increased in order to raise the efficiency to 50%? [3]

b) State first law of thermodynamics. Does it follow the principle of conservation of energy? Explain. [2]

a) Discuss the effect of temperature and pressure on velocity of sound in a gas. [2]

b) A train is approaching a cliff at 10 m/s. The driver sounds a whistle of frequency 600 Hz. What will be the frequency of echo as heard by the driver? (Velocity of sound in air = 340 m/s⁻¹) [3]

a) State the two Kirchhoff’s laws of electrical circuit. [2]

b) Obtain an expression for the balanced condition of Wheatstone bridge using Kirchhoff’s laws. [3]

a) State Lenz’s law. [1]

b) Plot a graph to show the variation of induced emf in a coil rotating in a uniform magnetic field with time. [1]

c) Derive an expression for the energy stored in an inductor. [3]

a) Write the symbol and truth table of NAND gate. [2]

b) Explain the full wave rectification using two P-N junction diodes. [3]

a) An oil drop of mass $3\times 10^{-11}$ g and of radius $2\times 10^{-4}$ cm carries 10 excess electrons. What is its terminal velocity when i) falling in a region in which there is no electric field? [1] ii) falling in an electric field of $3\times 10^5\mathrm{V}{\mathrm{m}}^{-1}$ directed downward? (Viscosity of air = $1.8\times 10^{-5}\mathrm{N}\mathrm{S}{\mathrm{m}}^{-2}$) [1]

b) In Thomson's method for determining specific charge of an electron, why is the electric field kept perpendicular to magnetic field? Justify. [2]

c) What is the use of X-rays in Millikan's oil drop experiment? [1]

OR

a) Define decay constant and half life. [2]

b) Derive the equation: $N=N_0{e}^{-\lambda t}$ for radioactive process where symbols have their usual meanings. [3]

a) Define threshold wavelength.

b) A metal has a work function of 2eV. Calculate the maximum speed of the emitted photoelectrons when it is illuminated by radiation of wavelength 150nm. What is the least frequency of the radiation for which electrons are emitted? (Given: Planck's constant $h=6.62\times 10^{-34}$ Js, $m=9.1\times 10^{-31}$ Kg, $e=1.6\times 10^{-19}$ C)

c) State Heisenberg's Uncertainty principle.

d) Obtain an expression for velocity of an electron in nth orbit of hydrogen atom.

OR

a) Explain the variation of thermoelectric emf with the temperature of the hot junction of a thermocouple.

b) A potentiometer circuit is shown in figure. If the total length of potentiometer wire is 4m and has resistance 10Ω and the balance point is obtained at a length of 240cm, calculate the value of unknown emf E.

c) Use ampere's circuital law to find the magnetic field due to a straight current carrying conductor.

• b) Guitar string of length $1.5\mathrm{m}$ is made of steel of density $7800kg/{\mathrm{m}}^3$ and Young's modulus $2\times 10^{11}N/{\mathrm{m}}^2$. It produces an elastic strain of $1\%$ in the string. Calculate [3]
  • i) stress developed in the string.
  • ii) frequency of second mode of vibration.
• c) The given figure shows standing wave formed in an open organ pipe. [figure in the original paper]
  • i) Which mode of vibration is shown ? [1]
  • ii) Obtain the frequency of vibration for this mode. [1] OR
• a) Prove that bright and dark fringes are equally spaced in Young's double slit experiment. [3]
• b) A parallel beam of monochromatic light is incident on diffraction grating having $500lines/mm$ to get second order maxima at $30^{\circ }$. Calculate the wavelength of light used. [3]
• c) State and prove Brewster's law. [2]

• a) The figure below shows a series LCR circuit. [figure in the original paper] If the frequency of an ac source is $\frac{500}{\pi }\mathrm{Hz}$ then,
  • i) Calculate the phase angle between current and voltage. [2]
  • ii) Which one leads in phase, current or voltage ? Justify. [1]
• b) What is meant by impedance of an ac circuit ? [1]
• c) Two parallel wire P and Q carrying currents $I_1$ and $I_2$ are placed in the air at a distance r apart as shown in figure. [figure in the original paper]
  • i) Obtain an expression for the magnitude of force per unit length between them. [2]
  • ii) What is the nature of force in this case ? [1]
  • iii) What will be the force per unit length between two wires if distance between wires is halved, keeping all other quantities constant ? [1] OR
• a) Explain the variation of thermoelectric emf with the temperature of the hot junction of a thermocouple. [3]
• b) A potentiometer circuit is shown in figure. [figure in the original paper] If the total length of potentiometer wire is $4\mathrm{m}$ and has resistance $10\Omega$ and the balance point is obtained at a length of $240\mathrm{cm}$, calculate the value of unknown emf E. [3]
• c) Use ampere's circuital law to find the magnetic field due to a straight current carrying conductor. [2]