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Electric Charges and Fields — NEET & JEE MCQs with answers
This chapter introduces electrostatics — the study of forces, fields and potentials due to charges at rest. It establishes the two kinds of electric charge, their fundamental properties (additivity, conservation, quantisation), Coulomb's law, the principle of superposition, the electric field concept, electric dipoles, and Gauss's law with its applications to symmetric charge distributions.
Topics in this chapter
- 1.1 Introduction
- 1.2 Electric Charge
- 1.3 Conductors and Insulators
- 1.4 Basic Properties of Electric Charge
- 1.5 Coulomb's Law
- 1.6 Forces Between Multiple Charges
- 1.7 Electric Field
- 1.8 Electric Field Lines
- 1.9 Electric Flux
- 1.10 Electric Dipole
- 1.11 Dipole in a Uniform External Field
- 1.12 Continuous Charge Distribution
- 1.13 Gauss's Law
- 1.14 Applications of Gauss's Law
Free practice questions
A wire carries a uniform linear charge density $\lambda = 3\,\mu C/m$ over a length of $4\,m$. The total charge on the wire is:
- A. $7\,\mu C$
- B. $12\,\mu C$
- C. $1.33\,\mu C$
- D. $0.75\,\mu C$
Show answer
Answer: B — $Q = \lambda \times l = (3\times10^{-6}\,C/m)(4\,m) = 12\times10^{-6}\,C = 12\,\mu C$.
NCERT Class 12 Physics Ch 1.12
An infinite plane sheet has a surface charge density $\sigma = 17.7 \times 10^{-12}\ \text{C/m}^2$. The electric field just outside the sheet is: (use $\varepsilon_0 = 8.854\times10^{-12}$)
- A. $2.0\ \text{N/C}$
- B. $1.0\ \text{N/C}$
- C. $0.5\ \text{N/C}$
- D. $17.7\ \text{N/C}$
Show answer
Answer: B — $E = \sigma/(2\varepsilon_0) = 17.7\times10^{-12}/(2 \times 8.854\times10^{-12}) = 17.7/17.708 \approx 1.0\ \text{N/C}$.
NCERT Class 12 Physics Ch 1.14
A nylon or plastic comb gets electrified on combing dry hair, but a metal spoon does not get electrified on being handled. The reason is that:
- A. A metal spoon repels all charge automatically
- B. No charge is ever produced on a metal spoon
- C. The comb is a conductor and the spoon is an insulator
- D. The charge on the metal leaks through our body to the ground, since both are conductors
Show answer
Answer: D — NCERT: 'The charges on metal leak through our body to the ground as both are conductors of electricity.' The comb, being an insulator, retains the charge.
NCERT Class 12 Physics Ch 1.3
Two point charges of 2 microC each are placed 0.2 m apart in vacuum. The force between them is:
- A. 0.36 N
- B. 0.09 N
- C. 9 N
- D. 0.9 N
Show answer
Answer: D — F = 9 x 10^9 x (2 x 10^-6)^2 / (0.2)^2 = 9 x 10^9 x 4 x 10^-12 / 0.04 = 0.036 / 0.04 = 0.9 N.
NCERT Class 12 Physics Ch 1.5
A field of $100\ \text{N/C}$ makes an angle of $45^\circ$ with the area-vector of a $1\ \text{m}^2$ surface. The flux (take $\cos 45^\circ = 0.707$) is approximately:
- A. $100\ \text{N m}^2/\text{C}$
- B. $50\ \text{N m}^2/\text{C}$
- C. $70.7\ \text{N m}^2/\text{C}$
- D. $141.4\ \text{N m}^2/\text{C}$
Show answer
Answer: C — $\phi = E S\cos\theta = 100 \times 1 \times 0.707 = 70.7\ \text{N m}^2/\text{C}$.
NCERT Class 12 Physics Ch 1.9
Which of the following is a conductor of electricity?
- A. Glass
- B. Porcelain
- C. Metal
- D. Nylon
Show answer
Answer: C — NCERT states that metals, human and animal bodies, and earth are conductors, having electric charges (electrons) comparatively free to move.
NCERT Class 12 Physics Ch 1.2
Electric charge has three basic properties. Which of the following correctly lists them?
- A. Attraction, repulsion and induction
- B. Quantisation, polarity and friction
- C. Conservation, magnetism and additivity
- D. Quantisation, additivity and conservation
Show answer
Answer: D — NCERT states electric charge has three basic properties: quantisation, additivity and conservation. Charging by rubbing and force laws are not among these three basic properties.
NCERT Class 12 Physics Ch 1.4
A charge q is placed at the centre of a sphere, giving flux φ = q/ε₀. The same charge q is now placed at the centre of a cube. The total flux through the cube is:
- A. Zero
- B. 6 times smaller than for the sphere
- C. q/ε₀, the same as for the sphere
- D. q/(6ε₀)
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Answer: C — Gauss's law holds for any closed surface no matter its shape. The enclosed charge is the same q, so the flux through the cube is also q/ε₀, identical to the sphere.
NCERT Class 12 Physics Ch 1.13
Compared with the field of a single point charge, the electric field of a dipole at large distances falls off:
- A. Faster (as 1/r³ instead of 1/r²)
- B. As 1/r⁴
- C. At the same rate (as 1/r²)
- D. Slower (as 1/r instead of 1/r²)
Show answer
Answer: A — NCERT: a single charge field goes as 1/r², while the dipole field falls off faster, as 1/r³.
NCERT Class 12 Physics Ch 1.10
Two point charges +q and +q are fixed at (−a, 0) and (a, 0). A third charge +Q is placed at (0, y) on the perpendicular bisector. The net force on +Q is directed:
- A. along the +y direction (perpendicular bisector), with zero x-component
- B. toward the charge at (a,0)
- C. zero for all y
- D. along the +x direction
Show answer
Answer: A — By symmetry, the x-components of the two repulsive forces cancel while the y-components add. So the net force on +Q points along the +y axis (away from the line of charges), directed radially outward along the perpendicular bisector. Its x-component is zero.
NCERT Class 12 Physics Ch 1.6
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