Electric Flux and Electric Dipole

Correct answer: A. It is double

Explanation: For a dipole at a distance much larger than its separation, the far-field expressions are E_axial = (1/4πε₀)(2p/r³) and E_equatorial = (1/4πε₀)(p/r³) in magnitude. Their ratio is therefore E_axial/E_equatorial = 2. Both fields decrease as 1/r³, so the axial field is double at the same far distance.

Correct answer: A. Opposite to the dipole moment

Explanation: The dipole moment p points from the negative charge toward the positive charge. At an equatorial point, the electric-field components perpendicular to the dipole axis cancel, while the components along the axis add in the direction opposite to p. Consequently, the equatorial field is antiparallel to the dipole moment, not directed outward along the equatorial line.

Correct answer: A. One-eighth of the earlier value

Explanation: In the far-field approximation, the dipole field varies as E ∝ 1/r³. If the distance changes from r to 2r, then E′/E = (r/2r)³ = (1/2)³ = 1/8. Therefore, the new axial field magnitude is one-eighth of its original value, provided the point remains in the far-field region.

Correct answer: A. Faster, because dipole field decreases with the cube of distance

Explanation: Two equal and opposite charges separated by a small distance form an electric dipole with zero net charge. At a sufficiently large distance, the leading monopole terms cancel, and the remaining dipole field varies approximately as 1/r³. In contrast, the field of a single non-zero point charge varies as 1/r². Because 1/r³ decreases more rapidly than 1/r² as r increases, the dipole field falls faster. Thus option A is correct; the other options use the wrong distance dependence.

Correct answer: A. Along the dipole moment

Explanation: The dipole moment p points from the negative charge to the positive charge. On the far axial line, the field of the nearer charge is stronger, and the vector contributions from the two charges combine in the direction of p on the positive axial side. Equivalently, the far axial field is Eaxial ≈ (1/4πε₀)(2p/r³), directed along p. Thus A is correct; B describes the far equatorial direction, C is not axial, and D is false.

Correct answer: A. Opposite to the dipole moment

Explanation: Let the dipole moment p point from the negative charge to the positive charge. At a point on the equatorial line, the components of the two fields perpendicular to the dipole axis cancel by symmetry, while their components along the axis add in the direction opposite to p. The far-field result is Eequatorial ≈ −(1/4πε₀)p/r³. Therefore A is correct; the field is not zero and is not directed outward along the equatorial line.

Correct answer: A. From positive charge to negative charge

Explanation: This is the equatorial-line case of an electric dipole. The point is equally distant from the positive and negative charges, so the two field magnitudes are equal. The vertical components point oppositely and cancel. The horizontal component from the positive charge points away from positive, toward the negative charge; the negative charge’s field also points toward negative. Hence the net field is from positive to negative.

Correct answer: A. One eighth

Explanation: For a short electric dipole observed at a far point on its axial line, the field varies approximately as E_axial ∝ 1/r³. If the distance changes from r to 2r, the new field is proportional to 1/(2r)³ = 1/(8r³). Therefore E_new/E_old = 1/8. The one-fourth choice would describe an inverse-square dependence, not the far-field dipole dependence.

Correct answer: A. One twenty-seventh

Explanation: For a short electric dipole at a far equatorial point, the field magnitude is proportional to p/r³, where p is the dipole moment and r is the distance from its centre. If r becomes 3r, the new field is E' = E/(3³) = E/27. Thus it becomes one twenty-seventh of the original field. One-ninth would correspond to an inverse-square law, while one-third would correspond to an inverse-distance law.

Correct answer: A. One twenty-seventh of the earlier value

Explanation: For a far point on the axis of a dipole, the field magnitude is approximately E_axial = 2kp/r³, where p is the dipole moment. The important dependence is the inverse cube: E ∝ 1/r³. If the distance changes from r to 3r, the new field is proportional to 1/(3r)³ = 1/(27r³). Thus the field becomes E/27, so option A is correct; inverse-square and inverse-first-power choices are not applicable here.

Correct answer: A. Components along the equatorial line

Explanation: Take a point on the perpendicular bisector, called the equatorial line. It is equally distant from +q and −q, so the two individual field magnitudes are equal. Their components parallel to the equatorial line point in opposite directions and cancel. Their components along the dipole axis point in the same net direction, opposite to the dipole moment, and therefore add. Thus only the equatorial-line components cancel.

Correct answer: A. Add all parts with signs

Explanation: Electric flux is obtained by integrating the field component through an oriented surface: Φ_total = ∮ E · dA. For a closed surface, dA is conventionally outward. Flux directed outward contributes positively, while inward flux contributes negatively, so all elemental contributions must be added algebraically with their signs. Adding magnitudes alone would calculate total absolute flux, not net flux, and could hide cancellation required by Gauss’s law.

Correct answer: A. Because the two charges are at different positions and their fields do not cancel completely

Explanation: A dipole consists of equal and opposite charges separated by a finite distance. At a distant point, the two fields are nearly equal and opposite, so their leading contributions largely cancel; this makes the resultant field small. They are generated from slightly different positions and are not generally identical in magnitude and direction, so complete cancellation does not occur. The net charge is zero, but the dipole moment is not, making option A correct.

Correct answer: A. Non-uniform electric field

Explanation: The governing idea is that a dipole experiences equal and opposite forces in a perfectly uniform electric field, so its net translational force is zero, although it may experience torque. In a non-uniform field, the field strength at the positive and negative charges differs; therefore the two forces do not cancel completely. The resulting net force can pull the dipole toward the stronger-field region. Thus option A is correct; options B and C cannot produce this stated pull, and option D is unrelated to an electrostatic dipole force.

Correct answer: A. Both are in the same direction

Explanation: The dipole moment p is defined from the negative charge toward the positive charge. On the axial line, the fields produced by the two charges combine in the direction of p on the far side corresponding to the positive-charge direction. In the far-field limit, the axial field is proportional to 2kp/r^3 and points along the dipole moment. Hence option A is correct. Option B describes neither the axial result nor the general dipole rule, while C and D are false.

Correct answer: A. Because perpendicular components cancel and remaining components add in the opposite direction

Explanation: Consider a point on the perpendicular bisector of the dipole. The two charge fields have symmetric components: the components perpendicular to the dipole axis cancel, while the components along the axis point in the same net direction. That surviving direction is from the positive-charge side toward the negative-charge side, opposite to p, which is defined from negative to positive charge. Therefore A is correct. The field is not zero, and the charges are unlike, so B, C, and D fail.

Correct answer: A. Both decrease inversely as cube of distance

Explanation: For a dipole observed at a distance much greater than its separation, the leading 1/r^2 contributions from the two opposite charges largely cancel. The remaining dipole term varies as 1/r^3. Specifically, the far axial magnitude is approximately 2kp/r^3 and the far equatorial magnitude is approximately kp/r^3; their numerical factors differ, but their distance dependence is the same. Therefore A is correct, while B, C, and D contradict the dipole-field equations.

Correct answer: A. Fields of opposite charges partially cancel in a dipole

Explanation: A point charge produces a monopole field, E = kq/r^2, because there is only one net source charge. An electric dipole contains equal and opposite charges separated by a small distance, so at a far observation point their leading 1/r^2 field contributions partially cancel. The first surviving term is the dipole term, proportional to p/r^3. Thus A gives the physical reason; B and C misdescribe a dipole, and D is false.

Correct answer: A. No, positive and negative parts may add to zero

Explanation: Gauss’s law states that the net flux through a closed surface is Phi = Q_enclosed/epsilon_0. A complete dipole has charges +q and -q, so Q_enclosed = 0 and the algebraic total flux is zero. However, electric field lines can cross different parts of the surface in opposite directions: outward flux is positive and inward flux is negative. These contributions cancel only in the total, not necessarily locally. Hence A is correct.

Correct answer: A. No, the field may be parallel to the surface

Explanation: Electric flux through an open surface is defined by Phi = integral of E dot dA. It measures only the component of the electric field normal to the surface. If a nonzero field lies entirely parallel to the surface, E dot dA is zero at each point and the flux is zero even though the field exists. Cancellation of positive and negative normal components can also make total flux zero. Therefore A is correct, not B.

Correct answer: A. Axial field magnitude is twice the equatorial field

Explanation: For a short dipole at distance r much greater than its separation, the far-field magnitudes are E_axial = (1/4πε₀)(2p/r³) and E_equatorial = (1/4πε₀)(p/r³). Taking their ratio gives E_axial/E_equatorial = 2. Hence the axial field magnitude is twice the equatorial magnitude, so option A is correct. The fields are not equal or zero at a finite far point; “far” means the dipole approximation is valid.

Correct answer: A. Charge magnitude must be suitably large

Explanation: The magnitude of a dipole moment is p = qd, where q is the magnitude of either charge and d is the separation between the charges. If d becomes very small while p must remain finite and nonzero, q must increase in such a way that the product qd remains finite. Thus the charge magnitude must be suitably large in the limiting idealisation. Option A is correct; setting q to zero would make p zero, and separation is essential.

Correct answer: A. The charge magnitude must be zero, or the statement is inconsistent

Explanation: The governing relation for an electric dipole is p = qd, where q is the magnitude of either charge and d is the separation between the two equal and opposite charges. If d is non-zero but p = 0, then q must be zero. However, a physical dipole requires two non-zero opposite charges, so the description is inconsistent as a real dipole. Therefore, option A is correct; the other choices contradict p = qd.

Correct answer: A. Whether the surface is open or closed and what net charge it encloses

Explanation: The first step is to identify the geometrical nature of the surface and the quantity being asked. For a closed surface, Gauss’s law gives the total electric flux as Φ = Q_enclosed/ε₀, so the net enclosed charge is essential. An open surface requires direct evaluation of E·dA and is not determined solely by enclosed charge. Dipole direction may matter in some field questions, but colour and charge names are irrelevant. Thus option A is the best first identification.

Correct answer: A. Because force magnitudes on the two charges may no longer be equal

Explanation: For a dipole in a uniform field, the positive and negative charges experience forces of equal magnitude, qE, in opposite directions, so their vector sum is zero, although a torque may remain. In a non-uniform field, the two charges occupy different positions and generally experience different field magnitudes, E+ and E−. Their forces qE+ and qE− no longer cancel completely, producing a net force. Thus option A is correct.