Electric Flux and Electric Dipole

Correct answer: B. From negative charge to positive charge

Explanation: Electric dipole moment is defined as a vector quantity p = qd, where d is directed from the negative charge to the positive charge. Consequently, its direction is from −q to +q, so option B is correct. It is not defined from positive to negative charge, and it need not always be opposite to the external electric field. Its orientation depends on the actual placement of the dipole, not on an arbitrary direction.

Correct answer: A. Product of charge and separation

Explanation: For an electric dipole made of charges +q and −q separated by distance d, the magnitude of the dipole moment is p = qd. Here q is the magnitude of either charge, not the algebraic sum of the two charges, whose net charge is zero. Therefore option A is correct. The quantity is a vector, directed from the negative charge to the positive charge, and its SI unit is coulomb metre. The other expressions have incorrect physical meaning or dimensions.

Correct answer: A. Coulomb metre

Explanation: The magnitude of electric dipole moment is defined by p = qd, where q is electric charge and d is the separation between the two charges. In SI, q is measured in coulombs and d in metres, so the unit of p is C·m, or coulomb metre. Thus option A is correct. Newton measures force, joule measures energy, and volt measures potential difference; none of these has the dimensions of dipole moment.

Correct answer: A. Two equal-magnitude charges of opposite signs separated by a small distance

Explanation: An electric dipole is a system of two point charges having equal magnitude and opposite signs, +q and −q, separated by a small distance. Its dipole moment is defined as p = qd, directed from the negative charge to the positive charge. Two like charges do not form a dipole, and a single or neutral particle has no separated charge pair. Therefore option A gives the complete definition and is correct.

Correct answer: C. From negative charge to positive charge

Explanation: Electric dipole moment is defined as the vector p = qd, where d is directed from the negative charge to the positive charge. Therefore its conventional direction is from negative to positive charge, independent of whether an external electric field is present. Option A reverses the definition, while B and D are not universally valid because the field direction and vertical direction can vary. Hence option C is correct.

Correct answer: D. Product of charge and separation distance

Explanation: For an electric dipole consisting of charges +q and −q separated by distance d, the magnitude of the dipole moment is p = qd. Thus both the magnitude of either charge and the separation distance are required. Electric field is not part of this definition; it appears in expressions for torque or potential energy. Therefore option D is correct, with SI unit coulomb metre.

Correct answer: D. A pair of equal and opposite charges separated by a small distance

Explanation: An electric dipole is a system of two point charges having equal magnitudes and opposite signs, separated by a small but finite distance. If the charges are +q and −q and their separation is d, the dipole moment has magnitude p = qd and points from the negative charge toward the positive charge. Therefore option D gives the complete definition. Two similar charges or one charge alone do not form a dipole.

Correct answer: C. It remains the same

Explanation: The magnitude of the electric dipole moment is p = qd, where q is the magnitude of either charge and d is the separation between the charges. If q becomes 2q and d becomes d/2, the new moment is p′ = (2q)(d/2) = qd = p. Therefore the dipole moment remains unchanged. It does not double or become four times because the two changes cancel exactly.

Correct answer: B. Twelve times

Explanation: For an electric dipole, p = qd. The new charge is q′ = 4q and the new separation is d′ = 3d. Hence p′ = q′d′ = (4q)(3d) = 12qd = 12p. Therefore the dipole moment becomes twelve times its original value. Addition would incorrectly give seven, while considering only one changed quantity would give four or three.

Correct answer: C. It remains the same

Explanation: The magnitude of the electric dipole moment is p = qd, where q is the magnitude of either charge and d is the separation between the charges. If q changes to 2q and d changes to d/2, the new moment is p′ = (2q)(d/2) = qd = p. Thus the increase in charge exactly compensates for the decrease in separation, so the dipole moment remains unchanged.

Correct answer: B. Fifteen times

Explanation: For an electric dipole, the magnitude of dipole moment is p = qd. If the charge becomes q′ = 5q and the separation becomes d′ = 3d, then p′ = q′d′ = (5q)(3d) = 15qd = 15p. Therefore the dipole moment becomes fifteen times its original value. The factors must be multiplied, not added, so eight times is not correct.

Correct answer: A. A measure of the electric field passing through a surface

Explanation: Electric flux describes the amount of electric field crossing a specified surface. For a uniform field and a plane surface, it is given by Φ = EA cos θ, where E is field strength, A is area and θ is the angle between the field and the area vector. Thus option A is correct. Current, mass and colour are unrelated quantities, so options B, C and D are not valid meanings of flux.

Correct answer: A. Perpendicular to the surface

Explanation: The area vector represents both the magnitude of surface area and a chosen orientation. For a plane surface, its direction is defined along the normal, that is, perpendicular to the surface. This direction is needed in Φ = EA cos θ to determine the sign and magnitude of flux. Therefore option A is correct. A tangent or arbitrary in-plane direction cannot serve as the standard area-vector direction.

Correct answer: A. Maximum positive

Explanation: For a uniform field through a plane surface, electric flux is Φ = EA cos θ. If the electric field and area vector point in the same direction, θ = 0° and cos 0° = 1. Hence Φ = EA, the greatest positive value for fixed E and A. Option A is therefore correct. Flux is zero only at 90°, negative when the directions oppose, and is not always half.

Correct answer: A. Zero

Explanation: Electric flux through a plane surface is Φ = EA cos θ, where θ is measured between the electric field and the area vector. The area vector is perpendicular to the surface. Therefore, when the field is parallel to the surface, it is perpendicular to the area vector, so θ = 90° and cos 90° = 0. Thus option A is correct; the field does not cross the surface normally.

Correct answer: A. Newton metre squared per coulomb

Explanation: Electric flux is obtained from Φ = E·A. The SI unit of electric field is N/C and the unit of area is m². Multiplying them gives (N/C)m² = N m²/C. Therefore option A is the correct SI unit. N/C is the unit of electric field, C/m is not the flux unit, and J/C is the unit of electric potential, so options B, C and D are incorrect.

Correct answer: A. Scalar quantity

Explanation: Electric flux is defined by the dot product Φ = E·A. Although the electric field and area are represented by vectors, their dot product produces a scalar. Consequently, flux has magnitude and sign but no independent direction; its sign depends on the chosen surface orientation. Option A is correct. It is not a vector, not merely direction, and it is not dimensionless because its unit is N m²/C.

Correct answer: A. When the electric field is opposite to the area vector

Explanation: The sign of flux follows Φ = EA cos θ and depends on the chosen direction of the area vector. If the electric field is opposite to that vector, θ = 180° and cos 180° = −1, giving Φ = −EA for a uniform field. Thus option A is correct. A field along the vector gives maximum positive flux, a field parallel to the surface gives zero, and no surface gives no defined flux.

Correct answer: A. A pair of equal-magnitude, opposite charges separated by a small distance

Explanation: An electric dipole consists of two point charges that are equal in magnitude, opposite in sign and separated by a small distance. Its dipole moment is p = qd, directed from the negative charge to the positive charge. Therefore option A gives the complete definition. Two like charges do not form a dipole in this standard sense, and one charge alone cannot create a dipole pair, so B, C and D are incorrect.

Correct answer: A. Zero

Explanation: An electric dipole consists of two equal and opposite charges, +q and −q, separated by a small distance. Its net charge is the algebraic sum: Q = (+q) + (−q) = 0. Therefore, option A is correct. A dipole can still produce an electric field and have a dipole moment even though its total charge is zero; the other options confuse net charge with these separate properties.

Correct answer: A. From negative charge to positive charge

Explanation: The electric dipole moment is defined as the vector p = qd, where d is directed from the negative charge toward the positive charge. Thus its conventional direction is from −q to +q, making option A correct. Electric field lines generally point from positive to negative, so option B reverses the convention. The direction is not always upward or always opposite to an external field.

Correct answer: A. Coulomb metre

Explanation: The magnitude of electric dipole moment is p = qd, where q is the magnitude of either charge and d is the separation between the charges. Since q has SI unit coulomb and d has SI unit metre, p has unit coulomb metre (C m). Therefore option A is correct. N/C is the unit of electric field, while N m is a unit of torque or energy.

Correct answer: A. Magnitude of charge and distance between charges

Explanation: For an ideal electric dipole, the magnitude of dipole moment is p = qd, where q is the magnitude of either charge and d is the separation between the positive and negative charges. Hence changing either q or d changes p. Option A is correct. Colour, temperature, and mass are not variables in the defining electrostatic expression, so options B, C, and D are irrelevant.

Correct answer: A. It is the standard convention

Explanation: For every small surface element, the area vector is defined perpendicular to the surface. For a closed surface, the standard convention is to choose this normal outward, from inside to outside. This fixes the sign of flux consistently: field components along the outward normal give positive flux, while inward components give negative flux. It is not necessarily the direction of the electric field or gravity, and it applies to curved as well as plane surfaces.

Correct answer: A. Negative

Explanation: Electric flux through a small surface element is given by dΦ = E·dA = E dA cos θ. For a closed surface, dA points outward. If the electric field enters the surface, its direction is inward and therefore opposite to dA, so θ is 180° and cos θ = −1. Hence the contribution of that part is negative. It is not necessarily zero or signless.