CBSE Class 12 Physics Revision Assignment Set A

1. State Coulomb’s law and write its vector form.

2. Calculate no. of electrons that will form 1 coulomb charge. (e- = 1.6X 10^-19C)

3. How will you charge a neutral body with a negative charge if you have a positively charged body? What is the name of the process? How many such bodies can be charged?

4. Calculate force between two protons of charge 1.6X 10^-19C separated by 5X10^-15m.(nucleus)

5. Calculate the ratio of coulomb to gravitational forces between an electron and a proton. me=9.1 X10^-31kg, m_p= 1.7X10^-27kg, r=5.3X10^-11m, qp=qe=1.6X10^-19C (H-atom)

6. What is a dipole? What is the dipole moment if q= 1.2μC and distance between them = 2cm. What is its direction? If this dipole is replaced by an ideal dipole what will be its dipole moment?

7. Calculate the electric field due to a dipole at a distance r on its equator? State the condition for which this expression is valid?

8. State Gauss’s Law. Using this find an expression for electric field due to a plane sheet of charge.

1. What is dielectric strength of a medium? Give its value for vacuum. 

2. What is the physical importance of the line integral of an electrostatic field? 

3. What is the work done in moving a test charge q through a distance of 1 cm along the equatorial axis of an electric dipole? 

4. A uniform electric field E exists between two charged plates as shown in the figure. What would be the work done in moving a charge ’q’ along the closed rectangular path ABCDA? 

5. Derive an expression for the electric potential at a point along the axial line of an electric dipole.

6. Two parallel wires carrying current in same direction attract each other. What about two beams of electrons travelling parallel, and in same direction to each other? 

7. Give two factors by which voltage sensitivity of a moving coil galvanometer can be increased. 

8. What kind of material is used to make core of an electric transformer? Justify your answer

9. Two point charges q1 and q2 at a separation r in vacuum produce a force F on each other. What will be their separation in an oil of a relative permittivity 16 so that the force between them remains F only? 

10. Derive an expression for the total work done in rotating an electric dipole through an angle θ in a uniform electric field. 

11. Two dielectric slabs of dielectric constants K1and K2 have been filled in between the plates of a capacitor as shown in Figure. What will be the capacitance of the capacitor? 

12. State Ohm’s Law. The sequence of bands marked on a carbon resistor is brown, black, brown, gold. What is the value of the resistance? 

13. Find the value of the unknown resistance X, in the following circuit, if no current flows through the section AO. Also calculate the current drawn by the circuit from the battery of emf 6 V and n negligible internal resistance. 

14. Using Ampere’s circuital law, obtain an expression for the magnetic field along the axis of a current carrying solenoid of length l and having N number of turns. 

15. The given figure shows tracks of three charged particles 1,2 and 3 in a uniform electric field. Give the signs of the three charges. Which particle has highest charge to mass ratio?

16. If the dip circle is set at 450 to the magnetic meridian, then the apparent dip is 300. Calculate the true dip. 

17. A galvanometer has a resistance of 5 ohm and a full scale deflection is produced by 15 mA. What resistance must be connected to enable the galvanometer to read 1.5 V? 

18. N identical resistors, each having a resistance R, when connected in series have an effective resistance of X Ω. When these resistors are connected in parallel, their effective resistance is Y Ω. What is the relation between R, X and Y? 

19. A capacitor of capacitance C is charged fully by connecting it to a battery of e.m.f. It is then disconnected from the battery. If the separation between the plates of the capacitor is doubled, what will be effect on 
(a) charge stored by the capacitor,
(b) potential difference across it,
(c) field strength between the plates,
(d) energy stored by the capacitor?

20. Find the equivalent capacitance of the combination of capacitors between the points A and B as shown. Also calculate the total charge that flows in the circuit when a 100 V battery is connected between the points A and B. 

21. State gauss’s law in electrostatics. Using this law derive an expression for the electric field due to a uniformly charged infinite plane sheet.

22. Using Kirchhoff’s drawn a cell of emf 1 V and internal resistance 2/3 Ω connected to the network.

23. AB is 2 meter long uniform wire of 20 ohm resistance. The other data are given as shown in the figure B. calculate 9i) potential gradient along AB, and (ii) length AO of the wire, when the galvanometer shows no deflection. 

24. To increase the current sensitivity of a moving coil galvanometer by 50%, its resistance is increased so that the new resistance becomes twice its initial resistance. By what factor does its voltage sensitivity change? 

25. Name and define the elements of earth’s magnetic field at a place. Derive an expression for the angle of dip in terms of the horizontal component and the resultant magnetic field of the earth at a given place. 

26. Define relaxation time of electrons in a conductor. Explain how it varies with increase in temperature of a conductor. State the relation between resistivity and relaxation time. 

27. Find the total energy stored in the capacitors in the given network. 

28. With help of labeled diagram, describe the construction and working of a van de Graff generator.

29. Draw a labelled diagram of a moving coil galvanometer. State the principle on which it works. Deduce an expression for the torque acting on a rectangular current carrying loop kept in a uniform magnetic field. Write two factors on which the current sensitivity of a moving coil galvanometer depends.

30. Derive a mathematical expression for the force per unit length experienced by each of the two long current carrying conductors placed parallel to each other in air. Hence define one ampere of current.
Explain why two parallel straight conductors carrying current in the opposite direction kept near each other in air repel?

1. Draw a ray diagram to show image formation of a point object placed in front of a plane mirror.

2. Write nature of the image formed.

3. Define virtual image and state its properties.

4. While drawing ray diagram did you use 1st law of reflection ?Did you use 2^nd law of reflection? What is the necessity of 2^nd law of reflection?
1. Draw a ray diagram to show image formation of a point object placed in front of a plane mirror.

2. Write nature of the image formed.

3. Define virtual image and state its properties.

4. While drawing ray diagram did you use 1st law of reflection ?Did you use 2^nd law of reflection? What is the necessity of 2^nd law of reflection?

Q1. In the adjoining graph θ is the angle of scattering and N is no. of α particles scattered in Rutherford’s expt. Explain the graph.

Q2. Define impact parameter.

Q3.Calculate the distance of closest approach of an α particle to a nucleus having charge +ze.

Q4. Which atomic facts could not be explained using Bohr’s atomic model?

Q5. Define excitation, ionization, excitation energy and ionization energy.
If E₁ =-13.6ev, E₂ = -3.4ev and E₃ = -1.5 ev for a hydrogen atom then calculate 1st excitation energy and ionization energy of the atom]

CH -1 Electric charge and Fields
1. ‘Electric charge is quantized’. What is meant by this statement?
2. Define the term electric dipole moment. Is it scalar or vector?
3. Write the SI unit of electric flux.
4. Why no two electric lines of force can intersect each other.
5. Derive an expression for torque acting on an electric dipole in a uniform electric field.
6. A proton and an electron are placed 1.6 cm apart in free space. Find the magnitude of electrostatic force between them. What is the nature of this force?
7. Derive an expression for dipole field intensity at a point on the dipole axis.
8. State Gauss’s theorem in electrostatics. Apply this theorem to derive an expression for electric field intensity at a point near an infinitely long straight charged wire.
9. A conducting sphere of radius 10 cm has an unknown charge. If the electric field 20 cm from the center of the sphere is 1.5 x 103 N/C and points radially inwards, what is the net charge on the sphere?

Electromagnetic Induction and Alternating currents
1. What are eddy currents? How can they be minimized?
2. State the law that gives the polarity of induced emf.
3. Define self inductance. Write its SI unit.
4. How will the inductive reactance and capacitive reactance change on doubling the frequency of alternating current?
5. Peak value of emf of an a.c source is Eo. What is its rms value?
6. What is an impedance triangle?
7. A wire of length 0.1m moves with a speed of 10 m/s perpendicular to a magnetic field of induction 1 Wb/m2. What is the value of induced emf?
8. A current of 10 A in primary of a circuit is reduced to zero at a uniform rate in 10-3s of coefficient of mutual inductance is 3H, what is the induced emf in the secondary?
9. Derive a expression for the coefficient of mutual inductance between two long solenoids.
10. A transformer has an efficiency of 80%. It works at 4 kw and 100 volt. If the secondary voltage is 240 V, find the primary and secondary currents.
11. Explain the construction and working of a.c generator.
12. Show that average power consumed per cycle in an a.c circuit containing an ideal capacitor is zero.
13. With the help of a suitable phasor diagram, obtain a relation for impedance in a series LCR circuit.
14. Explain the term capacitative reactance. Show graphically the variation of capacitative reactance with frequency of alternating voltage.
15. Define Q factor of the circuit.

Electromagnetic waves
1. Name the electromagnetic waves in the order of increasing wavelength.
2. Write two uses of X – rays.
3. What do you mean by the transverse nature of electromagnetic waves?
4. Define displacement current and get modified form of Ampere’s circuital law.
5. Write uses of
a. Infra red
b. Gamma rays
c. Radio waves
d. Micro waves
6. What is electromagnetic spectrum?
7. Write the characteristics of electro magnetic waves.

Magnetic effect of current and magnetism
1. State curie law in magnetism.
2. Which materials have negative value of magnetic susceptibility?
3. Which material is used in making permanent magnets and why?
4. What is the direction of dipole moment?
5. What is the angle of dip at a place where horizontal and vertical components of earths field are equal?
6. What are magnetic elements at a place? Define them.
7. Describe construction, theory and working of a tangent galvanometer.
8. What is a cyclotron? Discuss its construction, working and theory.
9. A short bar magnet placed with its axis at 30o with a uniform external magnetic field of 0.15 T experiences a torque of magnitude equal to 4.5 x 10-2 J. what is the magnitude of magnetic moment of the magnet?
10. A horizontal overhead power line carries a current of 90A in east to west direction. What is the magnitude and direction of the magnetic field due to the current 1.5m below the line?

Ch 2 Electric potential and capacitance
1. The electric potential V is constant in a region. What can you say about the electric field there?
2. Draw an equipotential surface for a uniform electric field?
3. Sketch a graph to how charge Q given to a capacitor of capacity C varies with the potential difference V.
4. How will you obtain maximum capacitance from three given condensers?
5. On inserting a dielectric between the plates of a capacitor, its capacitance is found to increase 5 times. What is the relative permittivity of the dielectric?
6. Write the principle of Van de graff generator.
7. Explain what is meant by dielectric polarization.
8. Derive an expression for the energy stored in a parallel plate capacitor.
9. A 600 pF capacitor is charged by a 200 V supply. It is then disconnected from the supply and is connected to another uncharged 600 pF capacitor. How much electrostatic energy is lost in this process?
10. Three capacitors each of capacitance 9 pF are connected in series.
a. What is the total capacitance of the combination?
b. What is the potential difference across each capacitor if the combination is connected to a 120 V supply.
11. Derive an expression for potential due to an electric dipole.
12. Derive a relation between electric field and potential.

Ch 3 Current Electricity
1. State the principle of working of a potentiometer.
2. State two uses of potentiometer.
3. State Kirchhoff’s rules.
4. A cell of emf E and internal résistance r is connected across a variable resistor R. plot a graph showing the variation of terminal potential V with resistance R. predict from the graph the condition under which V becomes equal to E.
5. What is the effect of temperature on the relaxation time of electrons in a metal?
6. Find the value of carbon resistance when the sequence of bands marked on it, is brown, brown, black.
7. Establish a relation between Emf and potential difference.
8. Establish a relation between drift velocity and relaxation time of free electrons in a metallic conductor carrying a current.
9. With the help of a circuit diagram describe a method to find the internal resistance of a cell using potentiometer.
10. Find the current flowing through a copper wire of length 0.2 m, area of cross – section 1 mm², when connected to a battery of 4V. Given that electron mobility as 4.5 X 10^-6 m2v-1s-1 and charge on electron 1.6 x 10^-19C. The number density of electron in copper is 8.5 x 1028m^-3