# Physics Important Long Questions Collection | Class 12

## Electricity and Magnetism: Important Long Questions

1. Discuss the mechanism of metallic conduction. Derive J = nev where ‘J’ is current density, ‘e’ is electronic charge and ‘v’ is drift velocity.

2. What do you mean by a shunt? Describe its use in converting a galvanometer into an ammeter.

3. What is a galvanometer? How can you convert it into voltmeter? Why should the resistance of a voltmeter be high?

4. Define emf, terminal potential difference and internal resistance. Derive the relation between them for a circuit.

5. What is heating effect of current? Deduce an expression for the heat developed in a wire by the passes of electric current?

6. State and explain Joule’s law of heating of electric current. Discuss how they are verified experimentally.

7. State and explain Kirchhoff’s laws of current and voltage. Explain how these laws are used to obtain the balanced condition of Wheatstone’s bridge.

8. What is a Wheatstone bridge? Obtain the balanced condition for the bridge. Explain how resistance can be measured by a meter bridge.

9. Discuss the principle of the potentiometer and use it to compare the emf’s of two cells

10. Discuss the principle of potentiometer and use it to determine the internal resistance of a cell.

11. Explain what do you mean by Seebeck Effect? How does thermoelectric e.m.f. vary with the temperature?

12. State Faraday’s laws of electrolysis. Discuss the experiment to verify them.

13. Derive an expression for force on a current carrying conductor placed in uniform magnetic field.

14. State Biot-Savart law. Derive the formula for the magnetic field at the centre of a circular coil carrying current. Explain why the magnetic field at the centre of the coil disappears when the circular coil is made infinitely large.

15. State Biot-Savart law. Derive an expression for the magnetic field produced by a current carrying circular coil at any point on the axis of the coil.

16. Derive the formula for the magnetic field at a point due to a long straight current carrying conductor using Biot-Savart law.

17. Derive the formula for the magnetic field at a point due to a long straight current carrying conductor using Ampere’s law.

18. Derive the formula for the magnetic field at a point due to a Toroid using Ampere’s law.

19. Using Biot-Savart law, derive an expression for magnetic field intensity due to a long solenoid.

20. State and explain Ampere’s theorem and hence use it to find the magnetic field intensity due to a long current carrying solenoid.

21. Derive an expression of force per unit length between two parallel conductors separated by a distance ‘r’ and carrying currents I1 and I2 in the same direction.

22. Derive an expression of force per unit length between two parallel conductors separated by a distance ‘r’ and carrying currents I1 and I2 in the opposite direction.

23. Find an expression for torque on rectangular coil in a uniform magnetic field.

24. Describe with the help of a diagram, the principle, construction and working of a moving coil galvanometer.

25. What is Hall Effect? Derive an expression for Hall voltage and Hall coefficient.

26. State Faraday’s law of electromagnetic induction. Derive an expression for the e.m.f. induced in a straight conductor moving at right angle to the direction of a uniform magnetic field.

27. State and explain Faraday’s law of electromagnetic induction. Derive an expression for the emf induced in a coil rotating in a uniform magnetic field.

28. State Lenz’s law and explain how this law leads to the conservation of energy principle.

29. Describe the theory and working of an a.c. generator.

30. Define self-inductance and derive an expression for it, also derive an expression for factors affecting self-inductance.

31. Define self inductance. Derive an expression for energy stored in an inductor.

32. An alternating emf is applied across a capacitor. Show that the current in it leads to the applied emf by 90°.

33. Discuss the phase relationship between the voltage and current in the AC circuit containing inductance and resistance. What is power factor of the circuit?

34. Discuss the phase relationship between the voltage and current in the a.c. circuit containing capacitor and resistor in series and hence derive an expression for the impedance of the circuit.

35. Derive the expression for impedance and current in LCR series circuit. Also find the resonant frequency.

## Waves: Important Long Questions

1. What are stationary waves? Prove that the distance between any two consecutive nodes in a stationery wave is $\frac{\lambda }{2}$.

2. What is the Newton’s formula for the velocity of sound? What correction was made by Laplace?

3. Derive an expression for the velocity of sound in a medium by dimensional method. Discuss the effect of various factors on the velocity of sound in air.

4. What do you understand by ‘harmonics’ and ‘overtone’ in the case of organ pipe? Also prove that only odd harmonics are produced in closed ended organ pipe.

5. Describe the resonance tube experiment to determine the velocity of sound in air at Laboratory and obtain the expression of end correction.

6. Show the both harmonics, odd and even can be produced in an open organ pipe. What is end correction?

7. Define the intensity of sound and prove that, I =$\frac{1}{2}\rho v{{a}^{2}}{{\omega }^{2}}$ =$\frac{1}{2}\sqrt{rB}\text{ }{{a}^{2}}{{\omega }^{2}}$ , where the symbols have their usual meaning.

8. What are beats? Obtain the expression for the beat frequency when beats are produced by superposing two waves of slightly different frequencies.

9. Discuss the phenomenon of Doppler’s effect. Find the change in frequency when a moving source of sound passes a stationary observer.

## Optics: Important Long Questions

1. State and explain Huygen’s principle. Use the principle to show that a plane wave front incident obliquely on a plane mirror is reflected as a plane wave front so that the angle of incidence is equal to the angle of reflection.

2. Define Huygen’s principle and prove Snell’s law by the help of wave theory of light.

3. Describe Foucault’s experimental method for the measurement of the velocity of light with necessary theory.

4. Describe Michelson’s method to determine the speed of light. Write advantages of the method over Foucault’s method.

5. Prove analytically that the bright and dark fringes in young’s double slit experiment are equally spaced.

6. What is Fraunhofer diffraction? Explain the formation of maxima and minima due to diffraction? Show that the width of central maxima is inversely proportional to the distance between the two slits.

## Modern Physics: Important Long Questions

7. Describe with necessary, theory, Millikan’s oil drop experiment to determine the value of the charge associated with an electron.

8. Explain the phenomena of discharge of electricity through gases at low pressure.

9. Show that the motion of an electron in magnetic field is circular. Prove that frequency and time period are independent with the velocity of an electron.

10. What is specific charge of an electron? Describe and give necessary theory of J.J. Thomson’s method to determine the specific charge of an electron.

11. What is photoelectric effect? Derive Einstein’s photoelectric equation; define various terms used in it.

12. Describe Millikan’s experiments to verify Einstein’s photoelectric equation.

13. What is an extrinsic semiconductor? Explain the formation of potential barrier and depletion region in a p-n junction diode.

14. Explain with neat diagram, the working mechanism of a full wave rectifier using junction diodes. How the output changes when a filter circuit is used?

15. What is Zener breakdown? Describe how a Zener diode can be used as a voltage regulator?

16. How is an N-P-N transistor formed? Discuss the input and output characteristics of the transistor in CE configuration.

17. What are Bohr’s postulates of hydrogen atom? Derive an expression for the radius of Bohr’s nth orbit.

18. Starting from Bohr’s postulates, obtain an expression for the energy of the electron in nth orbit of the hydrogen atom.

19. What is laser? Describe the construction and working principle of He-Ne laser.

20. Describe the modern method of productions of X-rays. Discuss crystal diffraction.

21. Derive Bragg’s law and explain how this law is used to determine the crystal plane spacing.

22. Define binding energy. How does binding energy per nucleon vary with mass number? What is its significance

23. What is nuclear fission? How energy is released in nuclear fission reaction?

24. Write down the schemes for nuclear fusion and nuclear fission. How can the release energy in any of these reactions estimated? What do you mean by Q-value of a nuclear reaction?

25. State laws of radioactive disintegration and show that the number of atoms of a given radioactive substance decreases exponentially with time. Also, derive a relation between decay constant and half-life of a radioactive substance.

26. Define half-life and decay constant of a radioactive substance. Establish a relation between them.

27. Name the quarks you know. Also present the quark combinations of baryon and meson groups of particles.

28. State Hubble’s law and use it to determine the expression for critical density.

29. Give a brief account on:

a. Global energy consumption pattern and demand

b. Energy crisis

c. Water pollution in Nepal

d. Air pollution in Nepal

e. Ozone layer

f. Solar energy