Continuous Charge Distribution

Correct answer: A. It cannot be obtained safely by directly applying the ordinary external-point formula

Explanation: For an ideal line charge, evaluating the field at a point on the line is a singular situation: the distance from the field point to a charge element can approach zero, making the integral divergent or undefined in the ideal model. Therefore option A is correct. The midpoint symmetry argument applies to suitable off-line points, not automatically to a point lying on the charged line.

Correct answer: B. Six coulombs per metre

Explanation: Linear charge density is defined as λ = Q/L. Initially, λ = 12 C/4 m = 3 C/m. Halving the length gives L′ = 2 m, while the total charge remains Q′ = 12 C. Hence the new density is λ′ = Q′/L′ = 12 C/2 m = 6 C/m. The density doubles because the same charge is confined to half the length. Therefore option B is correct; option A is only the original density.

Correct answer: A. It becomes double

Explanation: Surface charge density is σ = Q/A. Let the original values be Q and A, so σ = Q/A. After the change, Q′ = 6Q and A′ = 3A. Therefore σ′ = Q′/A′ = 6Q/(3A) = 2Q/A = 2σ. The increase in charge is twice the relative increase in area, so the density becomes double. Option A is correct.

Correct answer: C. It becomes double

Explanation: For a volume distribution, total charge is Q = ρV, where ρ is volume charge density and V is volume. If ρ′ = 4ρ and V′ = V/2, then Q′ = ρ′V′ = (4ρ)(V/2) = 2ρV = 2Q. Thus the total charge becomes twice its original value. It is not merely four times, because the volume simultaneously decreases by a factor of two. Option C is correct.

Correct answer: D. Linear density is increasing with position

Explanation: Linear density is charge per unit length, λ = ΔQ/Δl. Since all four parts have equal length, comparing their charges directly compares their linear densities: the part with greater charge has greater λ. Because the charge increases successively from one part to the next, λ increases as position advances along the distribution. Uniform density would require equal charges in equal lengths. Thus D is correct.

Correct answer: B. When fields are in different directions

Explanation: Electric field is a vector quantity. The field contribution dE from each small charge element has both magnitude and direction, so contributions cannot be added as ordinary signed numbers when they point in different directions. They must be resolved into components and combined vectorially. If all contributions are collinear and share a direction, algebraic addition may be valid. Therefore B is correct.

Correct answer: A. Potential is scalar and field is vector

Explanation: At the centre of a uniformly charged ring, every element produces an electric-field contribution that is opposed by the contribution from the diametrically opposite element. These vector contributions cancel, so the net field is zero. Potential is scalar: each element contributes k dq/R with the same sign for a ring of one charge sign, so the contributions add to a generally non-zero value. Hence A is correct.

Correct answer: B. Because transverse components cancel but the axial component remains

Explanation: Consider a point on the symmetry axis of a uniformly charged ring. For every charge element, the transverse component of its field is cancelled by the corresponding element on the opposite side of the ring. However, the components parallel to the axis point in the same axial direction at an off-centre point and therefore add. The net axial field is consequently non-zero. Thus B is correct.

Correct answer: A. Because the field can depend on the actual spatial distribution

Explanation: Electric field is found by vector addition of contributions from each charge element: dE = (1/4πε₀)(dq/r²) in the appropriate direction. An average density may give total charge, but it does not specify where the charge lies. In a non-uniform distribution, different locations produce different distances and directions, so two distributions with the same average can produce different fields. Therefore option A is correct.

Correct answer: A. Whether opposite elements have equal magnitude and distance

Explanation: The governing idea is superposition together with symmetry. Contributions from paired charge elements can cancel only when their magnitudes are equal and their directions are opposite; equal charge magnitude and equal distances help establish this condition. Therefore option A is correct. Colour, mass, and temperature do not determine electrostatic field cancellation in this situation.

Correct answer: B. Field may become non-zero because symmetry is broken

Explanation: For a complete uniformly charged ring, every charge element has an opposite partner at the same distance, so their electric fields cancel at the centre. Removing even a small part removes the corresponding contribution and destroys that pairing symmetry. Hence the remaining field is generally non-zero, making option B correct. The other choices describe unrelated or false consequences.

Correct answer: A. Because equal small parts on both sides will no longer exist

Explanation: Electric-field cancellation on the perpendicular bisector relies on pairing equal charge elements at equal distances, with their relevant components pointing in opposite directions. Removing material from only one end destroys those equal pairs, so the previous cancellation argument is no longer generally valid. Thus option A is correct. The total charge need not be zero, the rod does not become a ring, and electric field remains a vector.

Correct answer: A. Field of each small element depends on distance

Explanation: By Coulomb’s law, the electric field due to a small charge element is proportional to dq/r², where r is its distance from the observation point, and its direction also depends on position. In a continuous distribution, different elements can have different r values, so their contributions must be integrated with the correct distance and direction. Hence option A is correct; distance neither removes density nor changes charge sign.

Correct answer: C. 12 coulomb per cubic metre

Explanation: For a uniform volume distribution, the governing formula is ρ = Q/V. Half of 72 C is 36 C, and one-third of 9 m³ is 3 m³. Hence ρnew = 36/3 = 12 C m⁻³, so option C is correct. The original density was 72/9 = 8 C m⁻³; simply halving the charge or changing only one quantity would produce a distractor, but both changes must be applied together.

Correct answer: D. 12:6

Explanation: For each uniformly charged wire, total charge is Q = λL. Therefore Q₁:Q₂ = (λ₁L₁):(λ₂L₂). Substituting the given ratios gives Q₁:Q₂ = (4×3):(3×2) = 12:6, so option D is the direct calculated ratio. Although 12:6 can be simplified to 2:1, that simplified value is not listed among the options after revision; the other choices do not equal the calculated ratio. Thus D is unambiguous.

Correct answer: A. Because symmetric opposite charge elements no longer remain

Explanation: For the original symmetric rod, pairs of charge elements produce field components that cancel along the rod and add along the perpendicular direction. Cutting off one end removes the matching elements on that side, so the cancellation is incomplete. The remaining unequal contributions can produce a component along the rod and rotate the resultant field direction. Therefore option A is correct.

Correct answer: A. Because the field depends on distances and directions of local charges

Explanation: Average linear density can reproduce the net charge through Q = λ_avg L, but electric field is obtained by vector integration, dE = (1/4πε₀) dq/r², including both distance and direction from every charge element to the observation point. Different charge arrangements can have the same Q yet different fields. Thus option A is correct; the other choices deny valid concepts.

Correct answer: A. Complete circular symmetry is absent

Explanation: Electric field is a vector, so cancellation requires equal contributions in opposite directions. In a complete uniformly charged ring, each small charge element has a diametrically opposite element producing an equal and opposite field at the centre. A partial arc lacks this complete pairing, so the vector contributions generally leave a resultant field. Therefore option A is correct.

Correct answer: A. The opposite semicircular half is absent

Explanation: For a complete uniformly charged ring, corresponding elements on opposite sides produce equal electric fields in opposite directions, giving complete cancellation at the centre. A semicircular arc contains only one half of that ring, so the matching opposite half is missing. Its field contributions therefore have a nonzero resultant, directed along the symmetry axis of the semicircle. Hence option A is correct.

Correct answer: A. Consider both the sign and the direction

Explanation: The total charge is obtained by the signed integral Q = ∫σ dA, so positive and negative surface regions must be added algebraically rather than replaced by magnitudes. The electric field is a vector, and the sign of each element determines the direction of its contribution; the contributions must then be added vectorially. Therefore both sign and direction are essential, making option A correct.

Correct answer: A. No; the charge distribution can still produce an electric field

Explanation: Net charge Qnet = 0 only states that positive and negative charges cancel algebraically when integrated over the whole distribution. Electric field depends on the positions as well as the signs of the charges, so separated charges can produce a nonzero field even when their total is zero; an electric dipole is a standard example. Thus option A is correct, and zero net charge must not be confused with zero field everywhere.

Correct answer: A. Distribution type, nature of density, symmetry, and direction

Explanation: A reliable method begins by identifying whether the charge is distributed along a line, over a surface, or throughout a volume. Next determine whether the relevant density is uniform or variable, then use symmetry to simplify the integration or vector addition, and finally track direction. Thus option A gives the correct order. Total charge alone is insufficient, while colour, temperature, sound, and memorised units do not determine the electrostatic calculation.

Correct answer: B. The local density is zero at every point

Explanation: For a linear distribution, average linear density is related to the signed total charge divided by the total length. A zero average can result from cancellation between positive and negative portions, so the net charge may be zero while the local density varies from point to point. Therefore, option B is not always correct. Options A, C, and D are compatible with signed charge addition.

Correct answer: A. Because the field depends on the positions and directions of local charges

Explanation: Average density summarizes charge over a region, but the electric field follows Coulomb’s law as a vector sum of contributions from every local charge element. Each contribution depends on its magnitude, position, distance, and direction relative to the observation point. Different non-uniform distributions can share the same average density yet produce different fields. Hence option A is correct; average density is meaningful but insufficient, and electric field is a vector.

Correct answer: B. Add the charges of small length elements

Explanation: For a variable linear charge density, the charge per unit length is not constant, so the shortcut Q = lambda L cannot be used with one fixed lambda. Divide the distribution into small elements dx; each contributes dq = lambda(x) dx. Adding all contributions gives Q = integral lambda(x) dx over the full length. Therefore B is correct. Using one density, only the maximum, or zero would discard the actual variation.