Electric Charges and Fields — Class 12 MCQs with Answers

Electric Charges and Fields — important questions & MCQs with answers (Class 12 Physics)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   The SI unit of electric charge is:

   AAmpereBTeslaCVoltDCoulomb
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   Answer: Coulomb

   Explanation: SI charge unit traces to current: 1 coulomb = 1 ampere flowing for 1 second (C = A·s). Ampere and volt measure current and potential, not charge — easy to confuse with those base units.

2. Q2 of 6Easy

   The SI unit of electric field is:

   AC/mBC·mCN/CDJ/C
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   Answer: N/C

   Explanation: Field is force per charge, E = F/q, giving units N/C — algebraically identical to V/m since work per charge over distance gives the same combination. Don't mistake it for C/m, a charge density, not a field.

3. Q3 of 6Medium

   Gauss's law states that electric flux through a closed surface equals:

   Aq × ε₀Bε₀ · ACq · ADq / ε₀
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   Answer: q / ε₀

   Explanation: Gauss's law: Φ = q_enclosed/ε₀ — total flux through any closed surface depends only on enclosed charge, and this is what lets symmetric charge distributions be solved for E without integrating Coulomb's law directly.

4. Q4 of 6Medium

   Assertion (A): The electrostatic repulsion between two protons is about 10³⁶ times stronger than the gravitational attraction between them. Reason (R): The Coulomb force between two point charges acts along the line joining them.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: Both A and R are true but R is not the correct explanation of A

   Explanation: A is true: the ratio of the two forces for two protons is about 1.3 × 10³⁶. R is true: the Coulomb force acts along the line joining the charges. But the direction of the force says nothing about how strong it is compared with gravity, so R does not explain A.

5. Q5 of 6Hard

   Electric field on the axis of an electric dipole at distance r (r ≫ a) is:

   A2kp / r³Bkp / r³Ckp / r²Dkp / r
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   Answer: 2kp / r³

   Explanation: The axial field of a short dipole comes from adding the fields of +q and −q along the axis and expanding for r ≫ a: the result is E = 2kp/r³ — twice the equatorial value at the same distance.

6. Q6 of 6Hard

   Assertion (A): If the net electric flux through a closed surface is zero, the electric field is zero at every point of the surface. Reason (R): The net electric flux through a closed surface is zero if the surface encloses no net charge.

   ABoth A and R are true and R is the correct explanation of ABBoth A and R are true but R is not the correct explanation of ACA is true but R is falseDA is false but R is true
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   Answer: A is false but R is true

   Explanation: A is false: a closed cylinder placed in a uniform field has zero net flux, because flux entering one end equals flux leaving the other, yet the field is non-zero everywhere on it. R is true: this follows directly from Gauss's law.

Electric Charges and Fields notes: key concepts (Class 12 Physics)

Electrostatics from first principles: the three properties of charge, Coulomb's law and how a medium weakens it, the electric field and its lines, the electric dipole and the torque it feels, and Gauss's law with the three standard symmetries it makes trivial.

Properties of electric charge

Charge is additive, is conserved in every process, and is quantised in multiples of the elementary charge e = 1.6 × 10⁻¹⁹ C.

Quantisation of charge

Any observable charge is an integral multiple of e, so a body can never carry a charge like 1.5e. Free quarks, with fractional charge, are never observed in isolation.

Conservation of charge

The total charge of an isolated system is constant. Charging by friction transfers electrons between bodies; it never creates or destroys charge.

Coulomb's law

The force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation, acting along the line joining them.

Effect of a medium

Placing a dielectric of constant K between charges reduces the force by a factor of K, since the medium polarises and partially cancels the field.

Superposition principle

The force on a charge from several others is the vector sum of the individual pairwise forces, each computed as though the others were absent.

Electric field

The force per unit positive test charge at a point, a vector field with units of N C⁻¹ or equivalently V m⁻¹. It exists whether or not a test charge is present.

Electric field lines

Start on positive charge and end on negative, never intersect (the field has one direction at each point), and are denser where the field is stronger.

Electric dipole

A pair of equal and opposite charges separated by a small distance — the model for polar molecules such as water and HCl.

Electric dipole moment

The product of one charge and the separation, a vector pointing from the negative to the positive charge, with unit coulomb-metre.

Field of a dipole

Falls off as the CUBE of distance, faster than a point charge's inverse square. The axial field is twice the equatorial field at the same distance.

Torque on a dipole

A uniform field exerts no net force on a dipole but does exert a torque p × E that tends to align it with the field.

Electric flux

The number of field lines crossing a surface, E·A. It is a scalar and is maximum when the surface is perpendicular to the field.

Gauss's law

The total flux through any closed surface equals the enclosed charge divided by ε₀. Charges outside the surface contribute exactly zero net flux.

Applications of Gauss's law

Yields the field of an infinite wire, an infinite sheet and a charged shell in a line of working — provided the chosen surface matches the symmetry.

Field inside a conductor

Zero everywhere inside a conductor in electrostatic equilibrium, so any excess charge resides entirely on the outer surface.