Physics MCQs for Class 12 with Answers Chapter 1 Electric Charges and Fields

Electric Charges and Fields MCQ

1 . If a body is charged by rubbing it, its weight

1. Always decreases slightly
2. Always increases slightly
3. May increase slightly or may decrease slightly
4. Remains precisely the same

2. Safety fuse should have

1. High resistance and high MP
2. High resistance and low MP
3. Low resistance and high MP
4. Low resistance and low MP

3. 3 charges +4q, Q and q are placed in a straight line of length l at points distance 0, 0.5 and 1 respectively. What should be the value of Q in order to make the net force on q to be zero?

1. -q
2. -2q
3. -q/2
4. 4q

Answer/Explanation

3. (1)

Two forces are acting on q one due to 4q and second due to Q

F1 + F2 = 0

Q = -q

4. In the winter season, a mild spark is often seen when a man touches somebody’s else’s skin. Why?

1. Due to lack of humidity and rubbing with clothes, charge accumulate on human body which is discharged via sparking
2. Due to cold, electrostatic charge on body finds a lower resistance path to the skin of other’s body
3. He static charge on sweaters worn by the two persons is different, hence discharge through sparking occurs
4. Similar to the lighting, extremely high potential exists on both the bodies and hence they discharge through sparking

5. There are two charges +2microC and -3microC. The ratio of forces acting on them will be

1. 2 : 3
2. 1 : 1
3. 3 : 2
4. 4 : 9

Answer/Explanation

5. (2)

6. When a comb is rubbed with hair, it attracts paper bits. Choose the right explanation.

1. Bits of paper gets attracted due to gravitational force
2. Due to electromagnetic effect, bits of paper are attracted
3. Comb gets charged by friction and attract bits of paper
4. None of these

7. Four charges +Q, -Q, +Q and -Q are situated at the corners of a square; in a sequence then at the centre of the square

1. E = 0, V = 0
2. E = 0, V ≠ 0
3. E ≠ 0, V = 0
4. E ≠ 0, V ≠ 0

8. A spherical conducting ball is suspended by a grounded conducting thread. A positive point charge is moved near the ball. That ball will

1. Be attracted to the point charge and swing toward it
2. Be repelled from the point charge and swing away from it
3. Not be affected by the point charge
4. None of these

9. Electric force can be

1. Always attractive
2. Always repulsive
3. Attractive and repulsive
4. None of these

10. A charge Q is divided into two parts of q and Q-q. If the coulomb repulsion between them when they are separated , is to be maximum, the ratio of Q/q is

1. 2
2. ½
3. 4
4. ¼

Answer/Explanation

10. (1)

Let the charges be separated by a distance ‘d’

Maximum force will be got after differentiating the force equation

For maximum or minimum force

Q – 2q = 0

Q = 2q

Q/q = 2

11. Two neutrons are placed at some distance apart from each other. They will

1. Attract each other
2. Repel each other
3. Neither repel nor attract each other
4. Depends on the distance between the two nucleus

12. Two bodies X and Y carry charges -6.6µC and -5µC. How many electrons should be transferred from X to Y so that they acquire equal charges?

1. 2 x 10¹²
2. 5 x 10¹⁴
3. 5 x 10¹²
4. 5 x 10¹³

Answer/Explanation

12. (3)

X has excess charge of -1.6 x 10^-6 C composed to Y

X has to give a charge -0.8 x 10^-6 C if charge on both have to be the same

The charge on one electron is -1.6 x 10^-19 C

The number of electrons required to transfer a charge of -0.8 x 10^-6 C =

13. An electric charge is held at rest in a region filled with a non-uniform magnetic field. As soon as it is left free

1. It will move in the direction of the field
2. It will move opposite to the direction of the field
3. It will move perpendicular to the direction of the field
4. It will remain at rest

14. Which one of the following is a bad conductor?

1. Acid
2. Coal
3. Distilled water
4. Human body

15. SI unit of electric flux is

1. Voltmeter
2. Joule/metre
3. Newton
4. None

16. A charge is placed at the edge of a cube of side length ‘a’. Calculate the electric flux through each other face of the cube.

1. Q / 4𝛜_o
2. Q / 24𝛜_o
3. Q / 2𝛜_o
4. Q / 15𝛜_o

17. Electric flux per unit solid angle is defined as

1. Electric force
2. Electric field intensity
3. Electric potential
4. Electric power

18. An electric dipole is placed in a uniform electric field. The net electric force on the dipole.

1. Is always zero
2. Depends on the orientation
3. Depends on the dipole moment
4. Is always finite but not zero

19. Induction is possible

1. Only in conductor
2. Only in insulator
3. Both in conductor and insulator
4. None of these

20. When the separation between two charges is increased the electric potential energy of the charges

1. increases
2. decreases
3. remains the same
4. may increase or decrease

21. If a positive charge is shifted from a low potential region to high potential region, the electric potential energy

1. decreases
2. remains the same
3. may increase or decrease
4. increases

22. Mark out the correct options.

1. Total positive charge of the universe is constant
2. Total negative charge of the universe is constant
3. Total number of charged particles in the universe is constant
4. The total charge of the universe is constant

23. A Proton and an electron are placed in a uniform electric field

1. the electric forces acting on them will be equal
2. the magnitudes of the forces will be equal
3. their acceleration will be equal
4. the magnitude of their acceleration will be equal

24. An electric dipole is placed in an electric field generated by a point charge.

1. The net electric force on the dipole must be zero
2. The net electric force on the dipole maybe zero
3. The torque on the dipole due to field must be zero
4. The torque on the dipole due to the field may be zero

25. Electric lines of force

1. exist everywhere
2. exist only in the immediate vicinity of electric charges
3. exist only when both positive and negative charges and near one and other
4. are imaginary

26. Which of the following statements is not true about Gauss’s law

1. Gauss’s law is not much useful in calculating electrostatic field when the system has some symmetry
2. Gauss’s law is based on the Inverse Square dependence on distance contained in the Coulomb’s law
3. Gauss’s law is true for any closed surface
4. The term Q on the right side of gases law includes the sum of all charges in closed by the surface

27. Two infinite plate parallel sheets separated by a distance D have equal and opposite uniform charge densities σ. Electric field at a point between the sheets is

1. σ/2𝛜_o
2. σ/𝛜_o
3. Zero
4. Depends on the location of the point

28. The electric field at a point on axial line of a dipole and direction of the dipole moment

1. Will be parallel and in the same direction
2. Will be in opposite direction
3. Will be perpendicular
4. Are not related

29. If an electron has an initial velocity in a direction different from that of an electric field, the path of the electron is:

1. a straight line
2. a circle
3. an ellipse
4. a parabola

30. Electric lines of force about a negative point charge are

1. circular anticlockwise
2. circular clockwise
3. radial, inwards
4. radial, outwards

31. If Ea be the electric field strength of a short dipole at a point on its axial line and Ee that on the equatorial line at the same distance, then

1. Ee=2Ea
2. Ee=Ea
3. Ea=2Ee
4. Ee=4Ea

32. The surface considered for Gauss’s law is called

1. Closed surface
2. Spherical surface
3. Gaussian surface
4. Plane surface

33. A hemisphere is uniformly charged positively. The electric field at a point on a diameter away from the centre is directed:

1. perpendicular to the diameter
2. parallel to the diameter
3. at an angle tilted towards the diameter
4. at an angle tilted away from the diameter

34. The electric field at a point 5 cm from a long line charge of density 2.5 x 10^-6 cm^-1 is

1. 9 x 10³ NC^-1
2. 9 x 10⁴ NC^-1
3. 9 x 10⁴ NC^-1
4. 9 x 10⁴ NC^-1

35. The field at a distance r from a long straight wire of charge per unit length ƛ is

1. kƛ / r²
2. kƛ / r
3. kƛ / 2r
4. 2kƛ / r

36. F is the force between two charges. If the distance between them is tripled, then the force between the charges will be

1. F
2. F / 3
3. F / 9
4. F / 27

37. An attractive force of 9N acts between +5C and -5C at some distance. These charges are allowed to touch each other and are then again placed at their initial position. The force acting between them will be

1. infinite
2. 9 x 10⁹ N
3. 1 N
4. zero

38. SI unit of volume charge density is

1. cm^-1
2. cm^-2
3. cm^-3
4. cm^-4

39. The resultant electric field at centre of a ring due to ring is zero. Which of the following is incorrect

1. The total charge of the ring may be zero, although everywhere
2. The charge on the ring must be uniformly distributed
3. The charge on the ring may be distributed non-uniformly
4. Total charge on the ring may be positive

40. If two charged particles of same mass and charge are projected in a uniform electric field with the same speed, then

1. Both have same momentum at any instant
2. Both have same kinetic energy at any instant
3. Both have same magnitude of momentum at any instant
4. They may move on a straight line

41. A hollow sphere of sphere of charge does not produce electric field at any

1. Interior point
2. Outer point
3. Beyond 2m
4. Beyond 10cm

42. Charges Q each are placed at each of two opposite corners of a square. Charges q each are placed at each of the other two corners. Find the relation between q and Q when the net force on Q is zero.

1. q = – 3Q / 2√2
2. q = + Q / 2√2
3. q = – Q / √2
4. q = – Q / 2√2

43. A point charge q1 exerts a force F upon another point charge q2. If a third charge q3 be placed quite close to the charge q2 then the force that charge q1 exerts on the charge q2 will be

1. F
2. >F
3. <F
4. Zero

44. The number of electrons present in 1C of charge is _____

1. 4.25 x 10¹⁸
2. 6.25 x 10¹⁸
3. 3.25 x 10¹⁸
4. 2.25 x 10¹⁸

45. In Coulomb’s law, the constant of proportionality has units as

1. N
2. Nm²
3. NC²/m²
4. Nm²/C²

Assertion and Reasoning MCQ

Codes

(a) Both A and R are true and R is the correct explanation of A

(b) Both A and R are true and but R is not a correct explanation of A

(c) A is true but R is false

(d) A is false, but R is true

1 . Assertion (A) The charge on any body can be increased or decreased in terms of e

Reason (R) Quantization of charge means that the charge of a body is integral multiple of e

2. Assertion (A) The properties that the force with which two charges attract or repel each other are not affected by the presence of a third charge

Reason (R) Forces on any charge due to the number of other charges is the vector sum of all the forces on that charge due to other charges, taken one at a time.

3. Assertion (A) When we rub a glass rod with Silk, the rod gets negatively charged and the Silk gets positively charged

Reason (R) On rubbing, electrons from Silk cloth move to the glass rod

4. Assertion (A) Consider two identical charges placed distance 2d apart, along the 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(R) Force on test charge is zero

5. Assertion (A) Coulomb’s force is the dominating force in the universe

Reason (R) Coulomb’s force is weaker then the gravitational

6. Assertion (A) When charges are shared between any two bodies no charges is really lost but some loss of energy does occur

Reason (R) Some of the energy is dissipated in the form of heat, sparkling etc

7. Assertion (A) If there exists coulomb attraction between two bodies both of them must be charged

Reason (R) In coulomb attraction of two bodies are positively charged

8. Assertion (A) The positively charged particle is placed in front of a spherical uncharged conductor. The number of lines forces terminating on the spare will be more than those emerging from it

Reason (R) The surface charge density at a point on the sphere nearest to the point charge will be negative and maximum in magnitude compared to other points on the sphere

9. Assertion (A) A point charge is brought in an electric field at a nearby point will increase or decrease depending on the nature of charge

Reason (R) The electric field is independent of the nature of charge

10. Assertion (A) On going away from a point charge or a small electric dipole, electric field decreases at the same rate in both the cases

Reason (R) Electric field is inversely proportional to cube of distance from the charge of an electric dipole

11. Assertion (A) Electrostatic field line start at positive charges and end at negative charges

Reason (R) Field lines are continuous curve without any breaks and they form a closed loop

Case Study Based MCQ

1 . A Faraday cage or Faraday Shield is an enclosure made of a conducting material. The fields within a conductor cancel out with any external fields, so the electric field within the enclosure is zero.

These Faraday cage act as a big hollow conductor; you can put things in it to shield them from electric fields.

Any electrical shock cage receives, pass harmlessly around the outside of the cage.

i) Which of the following material can be made Faraday cage

1. Plastic
2. Glass
3. Copper
4. Wood

ii) Example of real world Faraday cage is

1. Cars
2. Plastic box
3. Lighting rod
4. Metal rod

iii) What is the electrical force inside a Faraday cage when it is struck by lightning

1. The same as the lightning
2. Half that of the lightning
3. Zero
4. A quarter of the lightning

iv) An isolated point charge + q is placed inside a Faraday cage. It’s surface must have charge equal to

1. Zero
2. +q
3. -q
4. +2q

2. Gauss theorem is mainly used to find out the electric flux linked to a closed surface. It does not depend upon the shape or size of the surface. According to this theorem, the electric flux linked to a closed surface is equal to 1/𝛜_o times the charge enclosed by the surface. Let we have a charge q, now if we want to find out the net flux linked to a closed surface around it them,

Electric flux ɸ = ∲_sE.ds = q / 𝛜_o

i) Gauss theorem is used to find out:

1. Electric force
2. Electric flux
3. Electric potential
4. None of these

ii) This theorem is applied over a ________ surface:

1. Closed surface
2. Open surface
3. Both 1 and 2
4. None of these

iii) Gauss theorem does not depends upon the………of surface:

1. Shape
2. Size
3. Area
4. Both 1 and 2

iv) If we increase the charge enclosed by the surface then electric flux will:

1. Increases
2. Decreases
3. Remain same
4. Both 1 and 2

v) Net flux linked to a closed surface around a charge particle is times the charge.

1. 𝛜_o
2. 1/𝛜_o
3. 𝛜_o²
4. None of these

3. A system of closely spaced electric charge forms a continuous charge distribution. To find the field of a continuous charge distribution, we divide the charge into infinitesimal charge elements. Each infinitesimal charge element is then considered as a point charge and electric field dE is determined due to this charge at given point. The net field at the given point is the summation of fields of all the elements i.e., E = ∫dE

i) How many electrons must be added to an isolated spherical conductor of radius 20 cm to produce an electric field 1000 N/C just outside the surface?

1. 2.77 x 10²⁰
2. 2.77 x 10¹⁰
3. 1.77 x 10¹⁰
4. 5.4 x 10¹⁰

ii) A circular annulus of inner radius r and outer radius R has a uniform charge density ‘a’. What will be the total charge on the annulus?

1. a (R²-r²)
2. πa (R²-r²)
3. a (R-r)
4. πaR²

iii) What is the dimension of linear charge density?

1. [ATL^-1]
2. [AT^-1L]
3. [ATL]
4. [A^-1T^-1L]

4. Microwave oven works on the principle of torque acting on an electric dipole. The food we consume has water molecules which are permanent electric dipoles. Oven produces microwaves that are oscillating electromagnetic fields and produce torque on the water molecules. Due to this torque on each water molecule, molecules rotate very fast and produce thermal energy. Thus, heat generated is used to heat the food.

i) An electric dipole is placed at an angle of 300 to a uniform electric field. The dipole will experience a torque as well as & translational force.

1. a torque as well as & translational force.
2. a torque only
3. a translational force only in the direction of the field
4. a translational force only in a direction normal to direction of the field

ii) An electric dipole is placed in a nonuniform electric field, what acts on it?

1. only torque
2. only force
3. both 1 and 2
4. none of these

iii) An electric dipole of moment p in placed in a uniform electric field E . The maximum torque experienced by the dipole is

1. pE
2. p/E
3. E/p
4. p.E

iv) Let Ea be the electric field due to a dipole in its axial plane distant I and let Eq be the field in the equatorial plane distant l. The relation between Ea and Eq is

1. Ea = Eq
2. Ea = 2Eq
3. Eq = 2Ea
4. Ea = 3Eq

v) A point P lies on the perpendicular bisector of an electric dipole of dipole moment p. If the distance Of P from the dipole is r (much larger than the size of the dipole) then the electric field at P is proportional to:

1. p^-1 and e^-2
2. p and r^-2
3. p² and r^-3
4. p and r^-3