Assertion and Reason questions on class 12 physics chapter 1 electric charges and Fields

Directions: These questions consist of two statements, each printed as Assertion and Reason. While answering these questions, you are required to choose any one of the following four responses.
(a) Both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
(b) Both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
(c) Assertion is correct, Reason is incorrect
(d) Both Assertion and Reason are correct.

Q.1. Assertion: Electron move away from a region of lower potential to a region of higher potential.
Reason: An electron has a negative charge.

Answer

a

Q.2. Assertion : A metallic shield in form of a hollow shell may be built to block an electric field.
Reason : In a hollow spherical shield, the electric field inside it is zero at every point.

Answer

a

Q.3. Assertion : Electric lines of force never cross each other.
Reason : Electric field at a point superimpose to give one resultant electric field.

Answer

b

Q.4. Assertion : The Coulomb force is the dominating force in the universe.
Reason : The Coulomb force is weaker than the gravitational force.

Answer

d

Q.5. Assertion : In a cavity within a conductor, the electric field is zero.
Reason : Charges in a conductor reside only at its surface.

Answer

a

Q.6. Assertion : When bodies are charged through friction, there is a transfer of electric charge from one body to another, but no creation or destruction of charge.
Reason : This follows from conservation of electric charges.

Answer

(a) Conservation of electric charge states that the total charge of an isolated system remains unchanged with time

Q.7. Assertion : The tyres of aircraft are slightly conducting.
Reason : If a conductor is connected to ground, the extra charge induced on conductor will flow to ground.

Answer

(b) Both the statements are independently correct.

Q.8. Assertion : Some charge is put at the centre of a conducting sphere. It will move to the surface of the sphere.
Reason : Conducting sphere has no free electrons at the centre.

Answer

(a) Because of repulsion, the free electrons will mole to the outer surface.

Q.9. Assertion : Coulomb force and gravitational force follow the same inverse-square law.
Reason : Both laws are same in all aspects.

Answer

(c) Coulomb force and gravitational force follow the same inverse-square law. But gravitational force has only one sign which is always attractive, while coulomb force can be of both signs which are attractive and repulsive.

Q.10. Assertion : The coulomb force is the dominating force in the universe.
Reason : The coulomb force is weaker than the gravitational force.

Answer

(d) Gravitational force is the dominating force in nature and not coulomb’s force. Gravitational force is the weakest force. Also, Coulomb’s force > > gravitational force.

Q.11. Assertion : If there exists coulomb attraction between two bodies, both of them may not be charged.
Reason : In coulomb attraction two bodies are oppositely charged.

Answer

(b) Coulomb attraction exists even when one body is charged, and the other is uncharged.

Q.12. Assertion : A deuteron and an 􀁄-particle are placed in an electric field. If F1 and F2 be the forces acting on them and a1 and a2 be their accelerations respectively then, a1 = a2.
Reason : Forces will be same in electric field.

Answer

c

Q.13. Assertion : The property that the force with which two charges attract or repel each other are not affected by the presence of a third charge.
Reason : Force on any charge due to a number of other charge is the vector sum of all the forces on that charge due to other charges, taken one at a time.

Answer

(b) Force on any charge due to a number of other charges is the vector sum of all the forces on that charge due to the other charges, taken one at a time. The individual force are unaffected due to the presence of other charges. This is the principle of superposition of charges.

Q.14. Assertion : A metallic shield in form of a hollow shell may be built to block an electric field.
Reason : In a hollow spherical shield, the electric field inside it is zero at every point.

Answer

(a) The electrostatic shielding is possible by metallic conductor.

Q.15. Assertion : A point charge is brought in an electric field, the field at a nearby point will increase or decrease, depending on the nature of charge.
Reason : The electric field is independent of the nature of charge.

Answer

(c) The electric field will increase if positive charge is brought in an electric field.

Q.16. Assertion : Consider two identical charges placed distance 2d apart, along x-axis.

The equilibrium of a positive test charge placed at the point O midway between them is stable for displacements along the x-axis.
Reason: Force on test charge is zero.

Answer

(b) If +ve charge is displaced along x-axis, then net force will always act in a direction opposite to that of displacement and the test charge will always come back to its original position.

Q.17. Assertion : When a conductor is placed in an external electrostatic field, the net electric field inside the conductor becomes zero after a small instant of time.
Reason : It is not possible to set up an electric field inside a conductor.

Answer

(c) Statement-1 is correct. The induced field cancels the external field. Statement-2 is false. When a current is set up in a conductor, there exists an electric field inside it.

Q.18. Assertion : A uniformly charged disc has a pin hole at its centre. The electric field at the centre of the disc is zero.
Reason : Disc can be supposed to be made up of many rings. Also electric field at the centre of uniformly charged ring is zero.

Answer

(a) The electric field due to disc is superposition of electric field due to its constituent ring as given in Reason.

Q.19. Assertion : Electric lines of field cross each other.
Reason : Electric field at a point superimpose to give one resultant electric field.

Answer

Assertion is Incorrect, Reason is correct. (d) Two field lines never intersect.

Q.20. Assertion : On bringing a positively charged rod near the uncharged conductor, the conductor gets attracted towards the rod.
Reason : The electric field lines of the charged rod are perpendicular to the surface of conductor.

Answer

(b) Though the net charge on the conductor is still zero but due to induction negatively charged region is nearer to the rod as compared to the positively charged region. That is why the conductor gets attracted towards the rod

Q.21. Assertion : Four point charges q1, q2, q3 and q4 are as shown in figure. The flux over the shown Gaussian surface depends only on charges q1 and q2.

Reason : Electric field at all points on Gaussian surface depends only on charges q1 and q2.

Answer

(d) Electric field at any point depends on presence of all charges.

Q.22. Assertion : On disturbing an electric dipole in stable equilibrium in an electric field, it returns back to its stable equilibrium orientation.
Reason : A restoring torque acts on the dipole on being disturbed from its stable equilibrium.

Answer

(a) The restoring torque brings it back to its stable equilibrium.

Q.23. Assertion : On going away from a point charge or a small electric dipole, electric field decreases at the same rate in both the cases.
Reason : Electric field is inversely proportional to square of distance from the charge or an electric dipole.

Answer

(d)

Q.24. Assertion : The electric flux of the electric field ∮ E.dA is zero. The electric field is zero everywhere on the surface.
Reason : The charge inside the surface is zero.

Answer

(d)

Q.25. Assertion : On moving a distance two times the initial distance away from an infinitely long straight uniformly charged wire the electric field reduces to one third of the initial value.
Reason : The electric field is inversely proportional to the distance from an infinitely long straight uniformly charged wire.

Answer

(a)

Q.26. Assertion (A): No two electric lines of force can intersect each other.
Reason (R) : Tangent at any point of electric line of force gives the direction of electric field.

Answer

(a)

Q.27. Assertion (A): Electric force acting on a proton and an electron, moving in a uniform electric field is same, where as acceleration of electron is 1836 times that of a proton.
Reason (R): Electron is lighter than proton.

Answer

(a)

Q.28. Assertion (A): As force is a vector quantity, hence electric field intensity is also a vector quantity
Reason (R): The unit of electric field intensity is newton per coulomb.

Answer

(b)

Q.29. Assertion (A) : Sharper is the curvature of spot on a charged body lesser will be the surface charge density at that point
Reason (R): Electric field is non-zero inside a charged conductor.

Answer

(d)

Q.30. Assertion (A): The surface densities of two spherical conductors of different radii are equal. Then the
electric field intensities near their surface are also equal.
Reason (R) : Surface density is equal to charge per unit area.

Answer

(b)

Q.31. Assertion (A): Three equal charges are situated on a circle of radius r such that they form on equilateral
triangle, then the electric field intensity at the centre is zero.
Reason (R): The force on unit positive charge at the centre, due to the three equal charges are represented
by the three sides of a triangle taken in the same order. Therefore, electric field intensity at centre is zero.

Answer

(a)

Q.32. Assertion (A): The electric lines of forces diverges from a positive charge and converge at a negative charge.
Reason (A): A charged particle free to move in an electric field always move along an electric line of force.

Answer

(c)

Q.33. Assertion (A): Charging is due to transfer of electrons.
Reason (R): Mass of a body decreases slightly when it is negatively charged.

Answer

(c)

Q.34. Assertion (A): Range of Coulomb force is infinite.
Reason (R): Coulomb force acts between two charged particles.

Answer

(b)

Q.35. Assertion (A): A small metal ball is suspended in a uniform electric field with an insulated thread. If high energy X-ray beam falls on the ball, the ball will be deflected in the electric field.
Reason (R): X-rays emits photoelectron and metal becomes negatively charged.

Answer

(c)

Q.36. Assertion (A): If a point charge be rotated in a circle around a charge, the work done will be zero.
Reason (R): Work done is equal to dot product of force and distance

Answer

(a)

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