Q43 / 54 · JEE Advanced 2020 (Paper 1) · Physics

A particle of mass m moves in circular orbits with potential energy Ur=Fr, where F is a positive constant and r is its distance from the origin. Its energies are calculated using the Bohr model. If the radius of the particle's orbit is denoted by R and its speed and energy are denoted by v and E respectively, then for the nth orbit (here h is the Planck's constant)

  • A
    R∝n1/3 and v∝n2/3
  • Correct answer
    B
    R∝n2/3 and v∝n1/3
  • Correct answer
    C
    E=32n2h2F24π2m1/3
  • D
    E=2n2h2F24π2m1/3

Solution

U=F·r

Fc=-dUdr=-F

mv2r=F

mur=nh2π

⇒mur2mu4r=n2h24π2·F

⇒mr3=n2h24π2F

⇒r=n2h24π2mF1/3

⇒r∝n2/3

E=U+K=F·r+1/2 mv2

=F·r+1/2F·r

E=3/2F·r

Put value of r,

E=32Frn2h24π2mF1/3

Hence, the correct answer is options: 'B', 'C'.

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