Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 11
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Suppose, the Electron in a Hydrogen Atom Makes Transition from N = 3 to N = 2 in 10−8 S. the Order of the Torque Acting on the Electron in this Period, Using the Relation Between - Chemistry


Suppose, the electron in a hydrogen atom makes transition from n = 3 to n = 2 in 10−8 s. The order of the torque acting on the electron in this period, using the relation between torque and angular momentum as discussed in the chapter on rotational mechanics is


  • 10−34 N m

  • 10−24 N m

  • 10−42 N m

  • 10−8 N m

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10−42 N-m

The angular momentum of the electron for the nth state is given by

`L_n = (nh)/(2pi)`

Angular momentum of the electron for n = 3,
 n = 3 , `L_i = (3h)/(2pi)`

Angular momentum of the electron for n = 2, `L_f= (2h)/(2pi)`

The torque is the time rate of change of the angular momentum.

Torque `tau = (L_f - L_i)/t`

 = `((2h//2pi)-(3h//2pi))/10^-8`

= `-(h//2pi)/((10^-8)`

= `(-10^-34)/10^-8   ..............[∴ h/(2pi) ≈ 10^-34 J -s ]`

= -10-42 N - m

The magnitude of the torque is `10^-42 N.m`

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HC Verma Class 11, Class 12 Concepts of Physics Vol. 2
Chapter 21 Bohr’s Model and Physics of Atom
MCQ | Q 3 | Page 383
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