Exams › NEET › Physics › Electrostatics
16 questions with worked solutions.
Q1. Which of the following statements is not true about Gauss's law?
Answer: Gauss's law is not much useful in calculating electrostatic field when the system has some symmetry
Gauss’s law is very useful for calculating electric fields in highly symmetric situations, such as spherical, cylindrical, or planar symmetry. So the statement saying it is “not much useful” when the system has symmetry is false.
Answer: The force decreases.
When the uncharged conducting bar is inserted, free charges in the bar rearrange so the side near each external charge acquires opposite induced charge. This polarization partially cancels the electric field between the original charges, so their mutual attraction/repulsion is reduced.
Answer: 15
When a dielectric fills only half the separation, the plate gap behaves like two capacitors in series: one with air and one with dielectric. Since the dielectric constant is 2 and each layer has half the thickness, the equivalent capacitance becomes 15 μF.
Answer: May increase if the charge is positive
A positive point charge creates an electric field pointing away from itself, so at a nearby point its contribution can either add to or oppose the original field depending on geometry. Therefore the field at that point may increase, but it is not guaranteed to always increase.
Answer: Assertion is correct but Reason is incorrect
The assertion is true because a capacitor can store only up to a certain charge before the dielectric breaks down or the voltage limit is exceeded. The reason is false because a capacitor does not store a “sufficient” or unlimited quantity of charge; its storage is finite and limited by its capacitance and operating voltage.
Answer: 0.25
A soap bubble has excess pressure from two sources: surface tension on its two surfaces and electrostatic repulsion of the surface charge. Using the given data, the total comes out to 0.25 Pa in SI units.
Answer: the electric field at the centroid of triangle is zero
At the centroid of an equilateral triangle, the three vertices are equally distant, so each charge produces a field with the same distance factor. The vector sum of the three contributions cancels for the given charge arrangement, making the net electric field zero there.
Answer: zero
For a charged conducting spherical shell, all excess charge resides on the outer surface, so the electric field inside the shell is zero. Since \(R/2 < R\), the point lies inside the shell, making the field zero regardless of the value at \(3R/2\).
Answer: \( A, D \) and \( E \) will have negative charges. B and \( C \) will have positive charges.
In a negatively charged electroscope, excess electrons reside on the conducting parts, so the knob, stem, and leaves carry negative charge overall. However, because the leaves are repelled by the excess electrons, the upper regions can become relatively positive by charge separation, matching the stated distribution.
Answer: Leaves diverge
When a negatively charged rod is brought near an uncharged gold leaf electroscope, electrons in the electroscope are repelled downward. This makes both leaves acquire the same type of charge, so they repel each other and diverge.
Q11. An electric dipole is placed in a non- uniform electric field, then
Answer: The resultant force acting on the dipole may be zero
In a non-uniform electric field, the forces on the +q and -q charges are generally unequal, so the dipole can experience a net force. However, at certain positions or field configurations, these forces can cancel, making the resultant force zero; thus it may be zero, not always.
Q12. If a charge is placed on a conductor having a pointed end, then
Answer: the charge gets accumulated at the points
On a conductor in electrostatic equilibrium, excess charge resides on the surface and concentrates where the radius of curvature is smallest. A pointed end has the greatest curvature, so charge accumulates there most strongly.
Q13. Which of the following statement are true?
Answer: Charge \( q_{3} \) applies a smaller force on charge \( q_{2} \) than on charge \( q_{1} \)
By Coulomb’s law, the magnitude of force between two charges is proportional to 1/r^2. Since q2 is farther from q3 than q1, q3 exerts a smaller force on q2 than on q1.
Answer: A touches positive plate and then moves towards negative plate and remains in contact with it.
When the neutral metallic sphere moves toward the positive plate, it is attracted by induction and on contact it acquires positive charge. A positively charged sphere is then repelled by the positive plate and attracted toward the negative plate, so it travels across and finally sticks to the negative plate after contact there.
Answer: \( 90.0 N \) n
The electric force on a charge in an electric field is given by F = qE. Substituting q = 3.00 C and E = 30.0 N/C gives F = 90.0 N.
Answer: Divergence decreases
When a positively charged rod is brought near the disc, it attracts electrons upward toward the disc. This leaves the leaves with less positive charge, so their mutual repulsion decreases and the divergence reduces.