# Physics Pre-Board Examination-2020 [St. Xavier's College]

**Group 'A'**

**1. Answer in brief: (any four) [4x2 = 8]**

**a.**Why do electrons acquire a steady drift velocity after applying the electric field?

**b.**A small resistor is put in parallel with a current-carrying coil of an electromagnet. Why?

**c.**Distinguish between dia-, para-, and ferromagnetic materials in terms of relative permeability and susceptibility.

**d.**How do the resonance frequency of LCR circuit changes if the plates of the capacitor are brought closer to each other?

**e.**Why is the core of the transformer made up of a material of high permeability? Can we use it to make a permanent magnet?

**f.**Explain why two wires carrying current in the opposite direction repel each other?

**2.**Answer in brief: (any four) [4x2 = 8]

**a.**The wavelength of daily life particle is not observable. Why?

**b.**Write the truth table of OR gate and NAND gate. How the OR gate can be drawn by using NAND gate?

**c.**Explain why the mass of a nucleus is slightly less than the mass of constituent nucleons?

**d.**Discuss on acid rain with its one effect.

**e.**Gases are insulators at ordinary pressure and start conducting at low pressure. Why?

**f.**State Hubble's law. Write the value of Hubble constant with its significance.

**3.**Answer in brief: (anyone) [1x2 = 2]a. The sound produced in the air is not heard by a person deep inside the water. Why?

**b.**Where will the man hear a louder sound? At node or antinode in case of a stationary wave?

4. Answer in brief: (anyone) [1x2 = 2]a. With which color of light can a Hubble space telescope see finer detail in a distant astronomical object: red, blue, or UV-rays? Explain your answer.b. Two polaroids are placed with their transmission axis at 90

^{0}to each other and the transmitted intensity of light is zero. What happens in the intensity if one more Polaroid is placed in between them, bisecting the angle between them?

**Group 'B'**

**5. Answer in detail: (any three) [3x4 = 12]**

**a.**Derive joule's law of heating effect. Also, explain how thin law can be verified experimentally?

**b.**State Biot and Savart law. Use it to find the magnetic field due to current at the axis of the circular current-carrying conductor.

**c.**Define the thermoelectric effect. What is the importance of thermoelectric series in thermoelectric effect? Discuss the variation of thermo emf with temperature.

**d.**State Faraday's laws of electromagnetic induction. Deduce an expression for induced emf in a coil rotating in a magnetic field.

6. Answer in detail: (any three) [3x4 = 12]a. Discuss the law of photoelectric effect. How Einstein's law of photoelectric effect can be verified by using Millikan's experiment.

**b.**Using Bohr's postulates, find the energy of an electron in the n

^{th}orbit of a hydrogen atom.

**c.**What is a transistor? Discuss the input and output characteristics of NPN transistors in CE configuration.

**d.**Define radiocarbon dating. Explain how the age of ancient dead animals and plants can be estimated by using radiocarbon dating?

7. Answer in detail: (anyone) [1x4 = 4]a. Write Newton's formula of the velocity of sound in air. Discuss how Laplace corrected Newton's formula.

**b.**Define the intensity of sound. Show that the intensity of sound for a given frequency is directly proportional to the square of the amplitude of vibration.

8. Answer in detail: (anyone) [1x4 = 4]a. State Huygen's principle. Discuss how the Huygen's principle can be used to verify the laws of reflection of light.b. Define diffraction of light. Explain Fraunhofer's diffraction at a single slit.

**Group 'C'**9. Solve the following: (any two) [2x4 = 8]

a. A coil inductance 0.50H and resistance 100Ω is connected to a 240 V, 50 Hz AC supply. (a) What is the maximum current in the coil? (b) What is the time lag between the voltage maximum and the current maximum?**b. **A battery has an emf of 15.0 V. The terminal voltage of the battery is 11.6 V when it is delivering 20.0 W power to an external load resistor of R. (a) What is the value of R? (b) What is the internal resistance of the battery?**c. **An electromagnet produces a magnetic field of 0.55T in a cylindrical region of radius 2.5cm between its poles. A straight wire carrying a current of 10.8A passes through the center of this region and is perpendicular to both the axes of the cylindrical region and the magnetic field. What magnetic force is exerted on the wire?

a. An electron beam after being accelerated from rest through the potential difference of 5KV in a vacuum is allowed to impinge normally on a fixed surface. If the incident current is 50μA, determine the force exerted on the surface assuming that it brings the electron to rest. Take the mass of an electron is 90.1x10^{-31} kg.**b. **It is proposed to use nuclear fusion _{1}H^{2} + _{1}H^{2} → _{2}He^{4} in a nuclear reactor of 200MW rating. It the energy from the above reaction is used with 25% efficiency in the reactor, how many grams of the deuterium will be needed per day. The mass of _{1}H^{2} and _{2}He^{4} are 2.0140 amu and 4.0026 amu respectively.**c. **An electron of energy 20eV comes into collision with a hydrogen atom in the ground state. The atom excited to a higher state and the electron is scattered with reduced velocity. The atom subsequently returns to the ground state with the emission of a photon of wavelength 1.216x10^{-7}m. Determine the velocity of scattered electron.

**11. **A boy is sitting on a swing which is moving to an angle of 30^{0} from the vertical is blowing a whistle which has a frequency of 1kHz. The whistle is 2m from the point of support of the swing. A girl stands in front of the swing. Calculate the maximum and minimum frequency of the sound she will hear. (take the velocity of sound=330m/s and g=9.8m/s^{2 }) **[4]**

**12. **In an experiment using Young's double-slit experiment, the distance between the center of the interference patterns and the tenth bright fringe on either side is 3.44cm. The distance between the slit and the screen is 2m. If the wavelength of the light used is 5.89x10^{-7}m, determine the slit separation and angle made by central bright fringe at the slit. [4]

**Source: **Department of Physics, St. Xavier's College, Maitighar.