Convert between energy units:

a. 231 cal to kJ

b. 132 × 104 kJ to kcal

c. 4.99 × 103 kJ to kWh

d. 2.88 × 104 J to Cal

Solution: The phenomenon of ejection of electrons from the surface of a metal when light of suitable frequency strikes on it, is called photoelectric effect. Step1: a) According to Planck’s quantum theory of radiation, the amount of energy associated with a quantum(in case of light, the quantum of energy is often called photon) of radiation is proportional to the frequency of light, E = h ---(1), where h is a universal constant called Planck’s constant. is the frequency and E is the energy of a photon. Or, E = hc/ -------(2), where, is the wavelength of light and c is the velocity. Step2: Here, given minimum energy required = 6.94 x 10 -19J Planck's constant, h = 6.626 x 10 J.s -34 Substituting the values in equation (1), we get, 6.94 x 10 -19J = (6.626 x 10 J.s) x -19 -34 = (6.94 x 10 J) / (6.626 x 10 J.s) = 1.05 x 10 s . -1 Thus, minimum frequency required to eject an electron is 1.05 x 10 s . 15 -1 Step3: b) From equation (2), = hc/E Here, Velocity of light, c = 3 x 10 m/sec 8 Energy, E = 6.94 x 10 -19J Substituting the values, we get, = [(6.626 x 10 J.s) x (3 x 10 m/sec)] / (6.94 x 10 -19J) -7 = 2.86 x 10 m. Thus, wavelength of the light = 2.86 x 10 m. -7 Step4: c) For each metal a minimum frequency of light called threshold frequency is required below which there is no ejection of electrons...