Page 1 Introductory Exercise 30.2 Q 1. A silver ball is suspended by a string in a vacuum chamber and ultraviolet light of wavelength 2000 Å is directed at it. What electrical potential will the ball acquire as a result? Work function of silver is 4.3 eV. Q 2. 1.5 mW of 400 nm light is directed at a photoelectric cell. If 0.1 % of the incident photons produce photo electrons, find the current in the cell. Q 3. Is it correct to say that K max is proportional to f ? If not, what would a correct statement of the relationship between K max and f ? Q 4. Light of wavelength 2000 Å is incident on a metal surface of work function 3.0 eV. Find the minimum and maximum kinetic energy of the photoelectrons. Q 5. When a metal is illuminated with light of frequency f the maximum kinetic energy of the photoelectrons is 1.2 eV. When the frequency is increased by 50% the maximum kinetic energy increases to 4.2 eV. What is the threshold frequency for this metal. Solutions 1. K max (in eV) = E - W = 1.9 eV Therefore stopping potential is 1.9 V. 2. Number of photons incident per second. Number of electrons emitted per second = 0.1% of ? Current = Charge (on photoelectrons per second) = 0.48 × 10 -6 A = 0.48 ?A 3. K max = E - W = hf - hf 0 = h(f - f 0 ) K max ? (f - f 0 ) 4. K min =0 and K max = E - W Page 2 Introductory Exercise 30.2 Q 1. A silver ball is suspended by a string in a vacuum chamber and ultraviolet light of wavelength 2000 Å is directed at it. What electrical potential will the ball acquire as a result? Work function of silver is 4.3 eV. Q 2. 1.5 mW of 400 nm light is directed at a photoelectric cell. If 0.1 % of the incident photons produce photo electrons, find the current in the cell. Q 3. Is it correct to say that K max is proportional to f ? If not, what would a correct statement of the relationship between K max and f ? Q 4. Light of wavelength 2000 Å is incident on a metal surface of work function 3.0 eV. Find the minimum and maximum kinetic energy of the photoelectrons. Q 5. When a metal is illuminated with light of frequency f the maximum kinetic energy of the photoelectrons is 1.2 eV. When the frequency is increased by 50% the maximum kinetic energy increases to 4.2 eV. What is the threshold frequency for this metal. Solutions 1. K max (in eV) = E - W = 1.9 eV Therefore stopping potential is 1.9 V. 2. Number of photons incident per second. Number of electrons emitted per second = 0.1% of ? Current = Charge (on photoelectrons per second) = 0.48 × 10 -6 A = 0.48 ?A 3. K max = E - W = hf - hf 0 = h(f - f 0 ) K max ? (f - f 0 ) 4. K min =0 and K max = E - W -3.0 ? 3.19 eV 5. K max =E - W 1.2 = E - W ...(i) 4.2 = 1.5 E - W ...(ii) Solving this equation, we get W = 4.8 eV = hf 0 = 1.16 × 10 15 HzRead More

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