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?
Compton effect
The incoming photons' energy must be in the range of an X-ray frequency to generate the Compton effect. The electron does not lose enough energy that reduces the wavelength of scattered photons towards the visible spectrum. As a result, with visible lights, the Compton effect is missing.
Recoil Velocity
The amount of backward thrust or force experienced by a person when he/she shoots a gun in the forward direction is called recoil velocity. This phenomenon always follows the law of conservation of linear momentum.
Alarm systems use the
ejecting electrons continuously and causing a small
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
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