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(i) Define the terms : ‘impact parameter’ and ‘distance of closest approach’ for an α-particle in Geiger-Marsden scattering experiment.

(ii) What will be the value of the impact parameter for scattering angle (I) θ = 0° and (II) θ = 180°?

Answers (1)

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(i) Definitions:

1. Impact Parameter: It is the perpendicular distance between the initial direction of motion of the α-particle and the central line passing through the nucleus of the atom.

2. Distance of Closest Approach: It is the minimum distance between the α-particle and the nucleus during the scattering process, where the entire kinetic energy of the α-particle is converted into electrostatic potential energy.

$\frac{1}{4\pi \varepsilon_0} \frac{2Ze^2}{r_{\min}} = \frac{1}{2} m v^2$

(ii) Values of Impact Parameter:

Scattering Angle (θ)

Value of Impact Parameter (b)

Explanation

θ = 0°

b → ∞

The α-particle passes far away from the nucleus, so it experiences no deflection.

θ = 180°

b = 0

The α-particle moves directly towards the nucleus and is completely repelled backward.


Final Answer:

  • Impact parameter: shortest perpendicular distance from initial α-particle path to nucleus.

  • Distance of closest approach: minimum separation between α-particle and nucleus.

  • For θ = 0°, b = ∞; for θ = 180°, b = 0.

Posted by

Saumya Singh

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