Electrostatics - NEET Physics Questions
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Electrostatics

Question 81: moderate

A point Q lies on the perpendicular bisector of an electric dipole of dipole moment p. If the distance of Q from the dipole is r (much larger than the size of the dipole) then electric field at Q is proportional to :

1. \(p^{-1} \)and \( r^{-2}\)
2. p and \( r^{-2}\)
3. \(p^{2}\)  and \(r^{-3}\)
4. p and \( r^{-3}\)
View Answer

For a point \( Q \) on the perpendicular bisector of an electric dipole (distance \( r \) from the center, where \( r \gg \text{dipole length} \)):

1. Electric Field on Perpendicular Bisector: The electric field \( E \) at a point on the perpendicular bisector of a dipole is given by:
\[
E \propto \frac{p}{r^3}
\]

2. Dependence:
- Directly proportional to the dipole moment \( p \).
- Inversely proportional to \( r^3 \).

Answer:
The electric field at \( Q \) is proportional to:
\[
p \quad \text{and} \quad r^{-3}
\]

Question 82: moderate

The figure below shows the electric field lines due to two positive charges. The magnitudes \(E_{A},E_{B} and E_{C}\) of the electric fields at points A, B, and C respectively are related as :

 

1. \[E_{A}>E_{B}>E_{C}\]
2. \[E_{B}>E_{A}>E_{C}\]
3. \[E_{A}=E_{B}>E_{C}\]
4. \[E_{A}>E_{B}=E_{C}\]
View Answer

Number of Electric field line in space represents electric field intensity. As number of field line is maximum at A followed by B then at C. so, \(E_{A}>E_{B}>E_{C}\)

Question 83: moderate

A charge \(Q\) \(mu\text{C}\) is placed at the centre of a cube. The flux coming out from any one of its faces will be (in SI unit)

1. \(\frac{Q}{6ε_0} times 10^{-3}\)
2. \( \frac{Q}{6ε_0} times 10^{-6}\)
3. \( \frac{Q}{ε_0} times 10^{-6}\)
4. \( \frac{2Q}{3ε_0} times 10^{-3}\)
View Answer

By Gauss's law, the total flux through the cube is \(\Phi = \frac{q_{\text{enclosed}}}{ε_0}\). Since the charge is at the center, the flux through one of the six faces is \(\Phi_1 = \frac{\Phi}{6} = \frac{Q \times 10^{-6}}{6ε_0}\).

Question 84: moderate

A charge \(Q \mu\text{C}\) is placed at the centre of a cube. The flux coming out from any one of its faces will be (in SI unit)

1. \(\frac{Q}{6\epsilon_0} \times 10^{-3}\)
2. \(\frac{Q}{6\epsilon_0} \times 10^{-6}\)
3. \(\frac{Q}{\epsilon_0} \times 10^{-6}\)
4. \(\frac{2Q}{3\epsilon_0} \times 10^{-3}\)
View Answer

According to Gauss's Law, total flux through the cube is \(\phi = \frac{q}{\epsilon_0}\). Since a cube has 6 identical faces, the flux through one face is \(\phi' = \frac{\phi}{6} = \frac{Q \times 10^{-6}}{6\epsilon_0}\).

Question 85: moderate

1000 identical drops of mercury are charged to a potential of \(1 \text{V}\) each. They coalesce to form a single drop, the potential of this new drop will be

1. 100 V
2. 10 V
3. 1000 V
4. 1 V
View Answer

The potential of a coalesced drop is related to the individual potential by \(V' = n^{2/3} V\). For \(n = 1000\), \(V' = (1000)^{2/3} \times 1 = 100 \text{V}\).

Question 86: moderate

An electric dipole is placed in a uniform electric field making an angle \(30^\circ\) with electric field and it experiences torque equal to \(\tau\) in this scenario. The minimum work done in changing the orientation from \(30^\circ\) to \(60^\circ\) is equal to

1. \((\sqrt{3}-1)\tau\)
2. \(\frac{(\sqrt{3}-1)\tau}{2}\)
3. \((\sqrt{3}+1)\tau\)
4. \(\frac{(\sqrt{3}+1)\tau}{2}\)
View Answer

Torque is \(\tau = pE \sin 30^\circ = pE/2 \implies pE = 2\tau\). Work done is \(W = -pE(\cos 60^\circ - \cos 30^\circ) = pE(\cos 30^\circ - \cos 60^\circ) = 2\tau(\frac{\sqrt{3}}{2} - \frac{1}{2}) = (\sqrt{3}-1)\tau\).

Question 87: moderate

A particle of mass \(m\) carrying charge \(q\) is initially kept at rest at the origin. A uniform electric field \(E\) along \(x\)-axis is switched on. What will be its kinetic energy when its coordinates are \((a, b)\)?

1. \(qEa\)
2. \(qE\sqrt{a^2 + b^2}\)
3. \(qEb\)
4. \(2qE\sqrt{a^2 + b^2}\)
View Answer

Work done by the electric field \(\vec{E} = E\hat{i}\) is given by \(W = q\vec{E} \cdot \vec{d} = qE\hat{i} \cdot (a\hat{i} + b\hat{j}) = qEa\). By work-energy theorem, \(K_f - K_i = W \implies K_f = qEa\).

Question 88: moderate

Two charged spherical conductors of radii \( R_1 \) and \( R_2 \) (\( R_1 > R_2 \)) have equal surface charge densities placed at large distance from each other. If they are connected by a conducting wire, then

1. Charge will flow from smaller sphere to larger sphere.
2. Charge will flow from larger sphere to smaller sphere.
3. No charge flow will occur.
4. Charge flow will depend on the material of the conductors.
View Answer

The potential of a sphere with surface charge density \( \sigma \) is \( V = \frac{\sigma R}{\varepsilon_0} \). Since \( R_1 > R_2 \), \( V_1 > V_2 \). Charge flows from higher potential (larger sphere) to lower potential (smaller sphere).