Two metal plates (A, B) are kept horizontally with separation of $\left(\frac{12}{\pi}\right) \mathrm{cm}$, with plate A on the top. An atomizer jet sprays oil (density $1.5 \mathrm{~g} / \mathrm{cm}^{3}$) droplets of radius 1 mm horizontally. All oil droplets carry a charge 5 nC. The potentials $\mathrm{V}_{\mathrm{A}}$ and $\mathrm{V}_{\mathrm{B}}$ are required on plates A and B respectively in order to ensure the droplets do not descend. The values of $\mathrm{V}_{\mathrm{A}}$ and $\mathrm{V}_{\mathrm{B}}$ are $\_\_\_\_$. (Neglect the air resistance to the droplets and take $\mathrm{g}=10 \mathrm{~m} / \mathrm{s}^{2}$)

JEE Main · 2026 · 02 Apr 2026 · Shift 2 · Electrostatics

Two metal plates (A, B) are kept horizontally with separation of

(\frac{12}{π}) cm

, with plate A on the top. An atomizer jet sprays oil (density

1.5 g / cm^{3}

) droplets of radius 1 mm horizontally. All oil droplets carry a charge 5 nC. The potentials

V_{A}

and

V_{B}

are required on plates A and B respectively in order to ensure the droplets do not descend. The values of

V_{A}

and

V_{B}

are

____

.
(Neglect the air resistance to the droplets and take

g = 10 m / s^{2}

)

(A)100 V and 580 V✓ Correct
(B)580 V and 100 V
(C)60 V and 400 V
(D)0 V and -200 V

Answer: (A)

Solution

A droplet stays suspended when the upward electric force balances its weight,

q E = m g

, with

m = \frac{4}{3} π r^{3} ρ

and

E = \frac{Δ V}{d}

. Hence

Δ V = \frac{4 π r ^{3} ρ g d}{3 q}

. Substituting

r = 1 0^{- 3} m

ρ = 1500 kg/m^{3}

d = \frac{12}{π} \times 1 0^{- 2} m

and

q = 5 \times 1 0^{- 9} C

gives

Δ V = \frac{4 π ( 1 0 ^{- 3} ) ^{3} \times 1500 \times 10 \times \frac{12}{π} \times 1 0 ^{- 2}}{3 \times 5 \times 1 0 ^{- 9}} = 480 V

. For the force on the positive droplets to point upward the field must point up, so the lower plate B is at the higher potential; the pair differing by

480

V with

V_{B} > V_{A}

V_{A} = 100 V

and

V_{B} = 580 V

JEE Main · 2026 · 02 Apr 2026 · Shift 2 · Single correct option

Practise more: all JEE Main PYQs on Electrostatics →

JEE Main · 2026 · 02 Apr 2026 · Shift 2 · Electrostatics

Solution

More JEE Main questions on Electrostatics