38.38 CP The work function of a material is examined using the apparatus shown schematically in Fig. P38.38. The ammeter measures the photocurrent. The voltage of the anode relative to the cathode is VAC. (a) The switch is in position a while radiation with wavelength 140 nm is incident on the cathode. The potentiometer is set at R = 3.20 kΩ, and there is no photocurrent. What is the potential difference VAC? (b) The resistance R is slowly adjusted to lower values until the first hint of a photocurrent is detected in the ammeter. That happens when R = 334 Ω. What is the value of VAC at that point? (c) What is the work function ϕ of the cathode material? (d) The switch is then thrown to position b and the ammeter measures a photocurrent i = 3.71 mA. What is the current I in this case? (e) What is the value of VAC? (f) The switch is returned to position a, and the wavelength of the light is reduced to 65.0 nm. The photocurrent increases. What is the minimum value of R for which the photocurrent will cease?