Continuous Charge Distribution

Correct answer: A. Transverse components of symmetric elements are equal and opposite

Explanation: Consider a point on the axis of the ring and choose two charge elements placed symmetrically about the plane through the axis. They are at equal distances from the point, so their individual field magnitudes are equal. Their transverse components point in opposite directions and cancel, whereas their axial components point in the same direction and add. Therefore A correctly explains the zero transverse component; B, C, and D are false.

Correct answer: A. They are equal and opposite for symmetric elements

Explanation: At any point on the perpendicular bisector, a small element on one side of the rod has a matching element at the same distance on the other side. For a uniform rod, both elements carry equal charge and produce equal field magnitudes. Their components parallel to the rod are oppositely directed, so those components cancel pairwise. The perpendicular components point the same way and add. Hence A is correct; the other options misuse charge or vector ideas.

Correct answer: A. Because symmetric elements on the two sides will no longer have equal charge

Explanation: Pairwise cancellation requires more than opposite directions: the paired field contributions must also have equal magnitudes. If one side of the rod has a larger linear charge density, corresponding elements on the two sides carry unequal charges. Even at equal geometric distances, their fields are then unequal, so the components parallel to the rod do not cancel completely. Option A states this broken-symmetry condition; B, C, and D are unrelated or false.

Correct answer: A. Because the density is not the same everywhere

Explanation: A uniform surface charge distribution means that equal areas carry equal charge, or equivalently that surface charge density σ is constant throughout the surface. If σ becomes larger near the edges, it depends on position and is therefore non-uniform, even if the total charge remains fixed. Option A gives the defining reason. Options B and D deny valid geometric possibilities, while C is not implied by an edge-enhanced density.

Correct answer: B. Zero

Explanation: Electric field is a vector, so both magnitude and direction must be considered when adding contributions. For two contributions with equal magnitude E but opposite directions, the vector sum is E + (−E) = 0. Hence their net contribution is zero, making option B correct. Option A would apply to equal contributions in the same direction; C and D have no basis in the stated symmetry.

Correct answer: A. They add as vectors in that direction

Explanation: Electric-field contributions must be combined using vector addition. When contributions point in the same direction, their magnitudes add along that common direction; for example, E_net = E₁ + E₂ for two aligned positive contributions. Therefore option A is correct. Cancellation occurs only for suitable opposite vectors, so B and C are incorrect, while field addition does not change the charge density, ruling out D.

Correct answer: A. Due to vector cancellation

Explanation: At the centre of a uniformly charged ring, every element has an opposite element at the same distance. Their field magnitudes are equal, but their directions are opposite, so each pair contributes E and −E, whose vector sum is zero. The ring can still have nonzero total charge; therefore option A is correct. B and C falsely claim zero charge or density, and D is wrong because the radius is finite.

Correct answer: B. Scalar quantity

Explanation: Electric potential is a scalar quantity, so contributions from charge elements are added algebraically rather than cancelled merely because their electric-field directions oppose each other. At the centre of a uniformly charged ring, every element contributes the same-sign potential, and for total charge Q and radius R, V = kQ/R, which is generally nonzero. Thus option B is correct; vector cancellation applies to electric field, not potential.

Correct answer: B. Small area

Explanation: A quantity given in coulombs per square metre is surface charge density, written as σ with units C/m². For a small surface element of area dA, the corresponding charge is dq = σ dA. Therefore the density must be multiplied by a small area, making option B correct. A uses the measure for linear density, C uses the measure for volume density, and D is unrelated to charge distribution.

Correct answer: A. Multiply volume charge density by a small volume

Explanation: A density specified per cubic metre is volume charge density, represented by ρ and measured in C/m³. For a small volume element dV, its charge is dq = ρ dV. The cubic-metre unit therefore requires multiplication by a small volume, so option A is correct. A linear density pairs with length, and surface density pairs with area; the other options mismatch the physical dimensions or omit the required element.

Correct answer: C. Linear density multiplied by small length

Explanation: A quantity specified per metre is linear charge density, denoted by λ and measured in C/m. For a small line element of length dl, the charge is dq = λ dl. Hence the correct measure is a small length, so option C is correct. Surface density requires a small area and volume density requires a small volume; option D has no role in defining charge from a linear distribution.

Correct answer: A. Two coulomb per square metre

Explanation: Surface charge density is defined by sigma = Q/A. The new charge is half of 108 C, so Q' = 54 C, while the new area is three times 9 m², so A' = 27 m². Hence sigma' = 54/27 = 2 C m^-2. Option A is correct. The other values result from failing to apply one of the stated changes or from an arithmetic error.

Correct answer: B. Twelve coulomb per cubic metre

Explanation: Volume charge density is rho = Q/V. Three-fourths of 240 C gives the new charge Q' = 180 C, and half of 30 m³ gives V' = 15 m³. Therefore rho' = 180/15 = 12 C m^-3, so option B is correct. Option A would come from an incorrect division, while the larger alternatives ignore the changed volume or charge.

Correct answer: A. Two to three

Explanation: For a uniformly charged wire, total charge is Q = lambda L. Thus the charge ratio is Q1:Q2 = (lambda1 L1):(lambda2 L2) = (7 x 8):(4 x 21) = 56:84. Dividing both terms by 28 gives 2:3, so option A is correct. Using only the density ratio or only the length ratio would produce distractors B or C-like errors.

Correct answer: A. 2:3

Explanation: For a uniformly charged surface, total charge is Q = σA, where σ is surface charge density and A is area. Hence Q₁:Q₂ = (9 × 10):(5 × 27) = 90:135. Dividing both terms by 45 gives 2:3, so option A is correct. The other options either reverse the simplified ratio or use only one of the supplied ratios instead of combining density with area.

Correct answer: A. 3:5

Explanation: The total charge of a uniformly charged solid is Q = ρV. Thus Q₁:Q₂ = (5 × 18):(6 × 25) = 90:150. Dividing both terms by 30 gives the simplest ratio 3:5, so option A is correct. Options C and D are only the density and volume ratios separately; option B does not result from the required multiplication.

Correct answer: A. Opposite negative elements produce equal and opposite fields

Explanation: The governing idea is symmetry together with vector superposition. Every small negative charge element on the ring has an element diametrically opposite to it at the same distance from the centre. Their electric-field magnitudes are equal, but their directions are opposite; the negative sign reverses both directions consistently and does not remove the field. Pair by pair, the vectors cancel, so the net field at the centre is zero. Therefore option A is correct.

Correct answer: A. Vector superposition and symmetric cancellation

Explanation: The governing principle is vector superposition: the electric field produced by a continuous distribution equals the vector sum, or integral, of the fields from all its small elements. At the centre of a complete uniform ring, diametrically opposite elements produce equal-magnitude fields in opposite directions. Each individual contribution is non-zero, but the symmetric vector pairs cancel exactly. Thus the net field is zero, making option A correct; the other choices do not determine electric-field cancellation.

Correct answer: A. The total charge changes with length, but the linear density remains the same.

Explanation: The governing concept is linear charge density, defined as λ = Q/L, where Q is charge and L is length. For a uniform wire, λ has the same value throughout. Thus a part of length L has charge Q = λL, so unequal lengths contain unequal total charges, while their charge per unit length remains equal. Therefore option A is correct; B confuses total charge with density, C ignores length, and D has no physical basis.

Correct answer: A. Because the charge per unit area is the same.

Explanation: Surface charge density is defined as σ = Q/A, the charge per unit area. For a uniformly charged plate, σ is constant. If the selected area becomes larger, the total charge increases according to Q = σA, but the ratio Q/A remains unchanged. Therefore option A correctly explains the situation. Option B contradicts the stated charge, C denies the defining relation, and D is impossible for an ordinary physical area.

Correct answer: A. The total charge will be calculated using the wrong measure, such as volume instead of area.

Explanation: The governing distinction is dimensional: surface density is σ = dQ/dA and has units C m⁻², whereas volume density is ρ = dQ/dV and has units C m⁻³. Using ρ when the given quantity is σ leads the student to multiply by volume rather than area, producing an incorrect total charge and often inconsistent units. Hence A is correct; the other statements do not follow from a density misclassification.

Correct answer: A. Recheck the unit and determine the actual dimensional spread of the charge.

Explanation: The first step in a continuous-distribution problem is to identify the geometrical support of the charge and verify the unit. A quantity given per metre represents linear density λ and must be integrated with dl; a surface density is given per square metre and uses dA. If the wording and unit appear inconsistent, the statement must be interpreted or checked before calculating. Thus A is correct; direct multiplication by area may produce a dimensional error.

Correct answer: A. By multiplying the uniform volume density by the total volume.

Explanation: For charge distributed throughout the interior of a body, the relevant quantity is volume charge density ρ = dQ/dV. If ρ is uniform, integration gives Q = ∫ρ dV = ρ∫dV = ρV. The body may have an irregular boundary, but its complete volume is still the required measure; the surface length or maximum dimension is insufficient. Therefore option A is correct.

Correct answer: A. The actual area of the curved surface.

Explanation: Surface charge density is defined by σ = dQ/dA. For a uniform distribution on a curved surface, the total charge is Q = σA, where A is the actual area measured along the surface. The straight-line or projected distance generally differs from this area, and volume is not the measure for a surface distribution. Curvature may help calculate A, but it cannot replace the area itself. Hence option A is correct.

Correct answer: A. Linear charge density.

Explanation: The dimension of the charged object determines the appropriate density. A thin ring is treated as a one-dimensional distribution along its circumference, so its linear charge density is λ = dQ/dl. For a uniformly charged ring of radius R, λ = Q/(2πR). Surface density would describe charge spread over a two-dimensional area, while volume density would require a three-dimensional body. Thus A is correct.