JEE Main · 2026 · 21 Jan 2026 · Shift 1 · Electrostatics

WhatsApp Email

If an alpha particle with energy 7.7 MeV is bombarded on a thin gold foil, the closest distance from nucleus it can reach is ______ m.
(Atomic number of gold

= 79

and

\frac{1}{4 π ϵ _{0}} = 9 \times 1 0^{9}

in SI units)

(A) $2.95 \times 1 0^{- 14}$ ✓ Correct
(B) $2.95 \times 1 0^{- 16}$
(C) $3.85 \times 1 0^{- 16}$
(D) $3.85 \times 1 0^{- 14}$

Answer: (A)

Solution

At the closest approach all the kinetic energy has become electrostatic potential energy,

K = \frac{1}{4 π ϵ _{0}} \frac{( 2 e ) ( 79 e )}{r}

. With

K = 7.7 \times 1 0^{6} \times 1.6 \times 1 0^{- 19} J

r = \frac{9 \times 1 0 ^{9} \times 2 \times 79 \times ( 1.6 \times 1 0 ^{- 19} ) ^{2}}{7.7 \times 1 0 ^{6} \times 1.6 \times 1 0 ^{- 19}} = 2.95 \times 1 0^{- 14} m

JEE Main · 2026 · 21 Jan 2026 · Shift 1 · Single correct option

Practise more: all JEE Main PYQs on Electrostatics →

More JEE Main questions on Electrostatics

Six point charges are kept 60° apart from each other on the circumference of a circle of radius R as shown in figure. The net electric field at the centre of the circle is ______.
( $ϵ_{0}$ is permittivity of free space)
JEE Main 2026View solution →
Electric field in a region is given by $E = A x \hat{i} + B y \hat{j}$ , where $A = 10 V / m^{2}$ and $B = 5 V / m^{2}$ . If the electric potential at a point $(10, 20)$ is 500 V , then the electric potential at origin is ____ V.
JEE Main 2026View solution →
Three small identical bubbles of water having same charge on each coalesce to form a bigger bubble. Then the ratio of the potentials on one initial bubble and that on the resultant bigger bubble is :
JEE Main 2026View solution →
Five positive charges each having charge $q$ are placed at the vertices of a pentagon as shown in the figure. The electric potential ( $V$ ) and the electric field ( $E$ ) at the center O of the pentagon due to these five positive charges are:
JEE Main 2026View solution →
Two point charges 2q and q are placed at vertex A and centre of face CDEF of the cube as shown in figure. The electric flux passing through the cube is :
JEE Main 2026View solution →
Two shorts dipoles $(A, B)$ , A having charges $\pm 2 μ C$ and length 1 cm and $B$ having charges $\pm 4 μ C$ and length 1 cm are placed with their centres 80 cm apart as shown in the figure. The electric field at a point $P$ , equi-distant from the …
JEE Main 2026View solution →