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IIT JEE 2009 Physics Paper2 Code 1 Solutions


1.     Photoelectric effect experiments are performed using three different metal plates p, q and r having work functions fP = 2.0 eV, fq = 2.5 eV, and fr = 3.0 eV, respectively. A light beam containing wavelengths of 550 nm, 450 nm and 350 nm with equal intensities illuminates each of the plates. The correct 1-V graph for the experiment is

  photoelectric effect

Sol.   (A)

        VB = (1/e)[(hc/l) - f]

        VP = (1/e)[(1240/550) - 2]eV  = 0.2545 V

        Vq = (1/e)[(1240/450) - 2.5]eV  = 0.255 V

        Vr = (1/e)[(1240/350) - 3]eV  = 0.5428 V

        If n is the number of photons in unit time then nhc/l = I

        => iP : iq : ir = nP : nq = λP : λq : λT

2.     A uniform rod of length L and mass M is pivoted at the centre. Its two ends are attached to two springs of equal spring constants k. The springs are fixed to rigid supports as shown in the figure, and rod is free to oscillate in the horizontal plane. The rod is gently pushed through a small angle q in one direction and released. The frequency of oscillation is


     (A) 1/2π √(2k/M) 

     (B) 1/2π √(k/M) 

     (C) 1/2π √(6k/M) 

     (D) 1/2π √(24k/M)

Sol.   (C)

       We have ,Restoring torque = –2 × k (l/2 θ ) l/2=(Id2 θ )/ dt2 

       (d2 θ )/ dt2 =(kl2/2 (-θ ))/(Ml2/12) 

       => ω = √(6k/M)

        Hence, frequency of oscillation is C

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