Alarm systems use the photoelectric effect. A beam of light strikes a piece of metal in the photocell, ejecting electrons continuously and causing a small electric current to flow. When someone steps into the light beam, the current is interrupted, and the alarm is triggered. An alarm system uses 100. g of a metal in the photocell. You know that the metal is one of 3 possible metals whose work functions are known. If you shine a violet laser (405 nm) on the photocell, you detect a current. If you shine a green laser (532 nm) on the photocell, you do not detect a current. Metal                              Work function (J) Potassium                      3.69 x 10-19 Sodium                           4.19 x 10-19 Aluminum                       6.54 x 10-19 (a) Which metal is being used in the photocell? How do you know? (b) What is the velocity of the detected electrons? (c) Do you expect to detect a current if you increase the intensity of the green laser? Why?

Alarm systems use the photoelectric effect. A beam of light strikes a piece of metal in the photocell,
ejecting electrons continuously and causing a small electric current to flow. When someone steps into the
light beam, the current is interrupted, and the alarm is triggered.
An alarm system uses 100. g of a metal in the photocell. You know that the metal is one of 3 possible
metals whose work functions are known. If you shine a violet laser (405 nm) on the photocell, you detect a
current. If you shine a green laser (532 nm) on the photocell, you do not detect a current.
Metal                              Work function (J)
Potassium                      3.69 x 10-19
Sodium                           4.19 x 10-19
Aluminum                       6.54 x 10-19

(a) Which metal is being used in the photocell? How do you know?

(b) What is the velocity of the detected electrons?

(c) Do you expect to detect a current if you increase the intensity of the green laser? Why?

(d) Draw an energy level diagram (Energy on the y-axis) which shows the energy level of
i. the electron while in the metal
ii. the metal + removed electron (make this your zero of energy)
iii. the metal + removed electron with a kinetic energy equal to that of part (b)
iv. indicate what transition represents Φo