Continuous Charge Distribution

Correct answer: B. Charge is considered spread continuously over an object

Explanation: The governing concept is modelling charge as a continuous distribution. Instead of treating an object as a collection of separate point charges, physics assumes that charge is smoothly spread along a line, over a surface, or throughout a volume. Hence option B is correct. Option A describes a point charge, while C incorrectly says charge must vanish and D confuses electric charge with magnetism. The model is useful for integration and field calculations.

Correct answer: A. For charge spread over a line-like object

Explanation: Linear charge density applies when charge is distributed along a one-dimensional path, such as a thin wire or line. It is defined by λ = dQ/dl, meaning charge per small length; for a uniform distribution, λ = Q/L. Therefore option A is correct. Surface charge density uses area, σ = dQ/dA, and volume charge density uses volume, ρ = dQ/dV, so B and C describe different quantities.

Correct answer: B. When charge is distributed over a surface

Explanation: Surface charge density
\(\sigma\) represents charge per unit area on a surface: \(\sigma = \frac{dQ}{dA}\). Therefore, it is used for charge spread over a surface, such as a charged plate or spherical shell. Charge distributed along a wire is described by linear charge density \(\lambda\), while charge spread through a volume is described by volume charge density \(\rho\).

Correct answer: C. Charge distributed throughout a three-dimensional volume

Explanation: Volume charge density describes charge distributed through the three-dimensional interior of a material or region. It is defined as ρ = dQ/dV, or charge per unit volume. Thus option C is correct. A line distribution requires linear density λ = dQ/dl, and a surface distribution requires surface density σ = dQ/dA. A point charge is represented separately, not by volume density over a finite region.

Correct answer: A. Linear charge density

Explanation: A long, thin wire is ideally treated as a line, so its charge is specified per unit length. Therefore, the appropriate quantity is linear charge density,  ext{λ}=rac{dq}{dl} ext{}. Surface charge density applies to charge spread over a surface, whereas volume charge density applies to charge distributed through a three-dimensional body.

Correct answer: B. Surface charge density

Explanation: Charge distributed over a surface is expressed by the surface charge density, \(\sigma\). It is defined as \(\sigma = \frac{dq}{dA}\); for a uniformly distributed total charge, \(\sigma = \frac{Q}{A}\). Its SI unit is \(\mathrm{C\,m^{-2}}\). Linear charge density applies to one-dimensional distributions such as a wire, whereas volume charge density applies to charge distributed through a volume.

Correct answer: C. Volume charge density

Explanation: When charge is distributed throughout the volume of an object, volume charge density is used. It is given by
ho = rac{Q}{V}
, where Q is the total charge and V is the volume. Linear charge density applies to a one-dimensional distribution such as a wire, whereas surface charge density applies to charge spread over a surface.

Correct answer: A. Charge per unit length

Explanation: Linear charge density means the amount of electric charge present per unit length of a line or wire. For a small element it is written as λ = dQ/dl; for uniform charge on a wire, λ = Q/L. Therefore option A is correct. Charge per unit area is surface density, charge per unit volume is volume density, and charge per unit time is electric current rather than linear density.

Correct answer: B. Charge per unit area

Explanation: Surface charge density is the amount of charge distributed over each unit area of a surface. It is represented by σ and defined for a small surface element as σ = dQ/dA; for a uniform surface, σ = Q/A. Hence option B is correct. Option A refers to linear density, option C to volume density, and option D is not the standard measure used for an electric charge distribution on a surface.

Correct answer: C. Charge per unit volume

Explanation: Volume charge density means the amount of electric charge contained in each unit of three-dimensional volume. It is denoted by ρ and, locally, is defined as ρ = dQ/dV; for a uniform distribution it can be written as ρ = Q/V. Therefore option C is correct. Linear density uses length, surface density uses area, and speed has no role in defining volume charge density.

Correct answer: A. The charge density remains constant throughout the distribution.

Explanation: The governing idea is charge density: a continuous distribution is uniform when its density has the same value at every relevant position. Thus, equal lengths, equal areas, or equal volumes contain equal charges for linear, surface, or volume distributions respectively. Option A states this definition. Option B describes a non-uniform distribution, while a point charge and zero total charge do not define uniformity.

Correct answer: B. Charge density varies with position.

Explanation: The governing concept is non-uniform charge density. In such a continuous distribution, the amount of charge per unit length, area, or volume is not identical everywhere; mathematically, the density depends on position, such as ρ = ρ(r). Therefore option B is correct. Option A defines a uniform distribution, while options C and D are not necessary consequences of non-uniformity.

Correct answer: A. To divide the distribution into differential parts and use integration to find the total electric effect

Explanation: A continuous charge distribution is divided into infinitesimal charge elements such as \(dq\). The small contribution to electric field or potential due to each \(dq\) is calculated. By the principle of superposition, integrating all these contributions gives the total electric field or potential. Option B may be used only as an approximation in special situations, whereas the differential-element method is generally used for distributed charge.

Correct answer: A. Principle of superposition

Explanation: A continuous charge distribution is divided into infinitesimal charge elements \(dq\). The electric field \(d\vec{E}\) due to each element is calculated, and all contributions are added vectorially; for a continuous distribution, this addition is performed by integration. This is based on the principle of superposition. The principle of inertia concerns resistance to changes in motion, not the vector addition of electric fields.

Correct answer: A. Uniform linear continuous charge distribution

Explanation: When the thickness of the ring is negligible, charge is distributed along its one-dimensional circumference. Hence, it is a linear continuous charge distribution. Since the charge is spread equally around the ring, it is a uniform linear continuous charge distribution. A surface charge distribution would instead be spread over a two-dimensional surface.

Correct answer: B. It decreases

Explanation: Average linear charge density is defined by
ext{λ}_{ ext{avg}}=rac{Q}{L}
, where
Q
is the total charge and
L
is the wire’s length. If
Q
remains constant while
L
increases,
ext{λ}_{ ext{avg}}
decreases. It does not remain unchanged because the same charge is distributed over a greater length.

Correct answer: B. It decreases

Explanation: The magnitude of the average surface charge density is \(\sigma_{\text{avg}}=Q/A\). Here, the total positive charge \(Q\) is constant while the area \(A\) increases. Therefore, \(Q/A\) decreases, so the magnitude of the average surface charge density decreases. It does not become zero because charge is still present on the plate.

Correct answer: B. It decreases.

Explanation: Average volume charge density is defined by ρ_avg = Q/V, where Q is total charge and V is volume. Here Q is fixed, while V increases. Dividing the same numerator by a larger denominator makes the density smaller, so option B is correct. The charge itself does not become zero, and increasing volume cannot make the density infinite under these conditions.

Correct answer: A. By adding all small charge elements.

Explanation: A continuous distribution is treated as a very large number of infinitesimal charge elements. The total charge is the sum, or mathematically the integral, of these elements: Q = ∫dq. For example, with volume density, dq = ρ dV and Q = ∫ρ dV. Thus option A is correct; the other choices have no physical relation to calculating total charge.

Correct answer: A. Charge contained in an infinitesimally small part of the distribution

Explanation: An infinitesimal charge element \(dq\) is the charge contained in an extremely small part of a continuous distribution. To find the electric field or potential due to the complete distribution, the contributions of all such \(dq\) elements are added using integration. Option B denotes the total charge \(Q\), not a small element of it.

Correct answer: A. When charge is spread over an extended object and its size cannot be neglected

Explanation: A continuous charge-distribution model is used when charge is spread over a wire, surface, or volume and the effects of different parts of the object are important. Therefore, option A is correct. In option B, the object's size is negligible compared with the relevant distance, so the point-charge approximation is more appropriate.

Correct answer: A. Point charge

Explanation: If the observation distance is much greater than the size of the object, differences in distance from its various parts become negligible. Its total charge can then be considered concentrated at one point, so it is treated as a point charge. Surface or volume charge distributions are used when the object's size is significant.

Correct answer: A. To describe the distribution and concentration of spread-out charge.

Explanation: Total charge alone tells how much charge exists, but not how that charge is spread through space. Charge density supplies this missing information: linear density λ = dQ/dl, surface density σ = dQ/dA, and volume density ρ = dQ/dV. Therefore option A is correct. The other choices describe impossible or unrelated operations and are not purposes of charge density.

Correct answer: A. Coulomb per metre (C/m)

Explanation: Linear charge density measures charge distributed along a line, so it is defined as λ = Q/L or, locally, λ = dQ/dl. Charge is measured in coulombs and length in metres; hence the SI unit is C/m, making option A correct. C/m² belongs to surface density, C/m³ to volume density, and N/C is the unit of electric field.

Correct answer: B. Coulomb per square metre (C/m²)

Explanation: Surface charge density describes charge spread over a two-dimensional surface. It is defined as σ = Q/A, or locally σ = dQ/dA. Since charge is measured in coulombs and area in square metres, the SI unit is C/m², so option B is correct. C/m is for a line distribution, C/m³ for a volume distribution, and J/C is a unit of electric potential.