IIT JEE 2009 Physics Paper2 Code 1 Solutions

SINGLE CORRECT CHOICE TYPE

1.     Photoelectric effect experiments are performed using three different metal plates p, q and r having work functions f_P = 2.0 eV, f_q = 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

  

Sol.   (A)

        V_B = (1/e)[(hc/l) - f]

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

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

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

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

        => i_P : i_q : i_r = n_P : n_q = λ_P : λ_q : λ_T

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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