APPLICATIONS AND GRAPHICAL ANALYSIS OF PHOTOELECTRIC EFFECT

EMR

http://mutuslab.cs.uwindsor.ca/schurko/molspec/animations/shockw

ave_animations/peeffect5.swf

when a photoelectron is released from a cathode and travels to the anode, the voltage can be reversed to the cathode tube

Stopping Voltage and Kinetic Energy of Electrons

G

e

eee e

+++

Anode (+) Cathode (-)

stop

New cathode (-)

New anode (+)

voltage that stops all electrons from reaching a new cathode

If the stopping voltage is given, the max kinetic energy of the emitted photoelectron can be determined

Stopping Voltage(Vstop)

Eg) Determine the maximum kinetic energy of an electron stopped in a cathode ray tube by 36.0V.

(max) (max)

19

(max) (max)

18

(max) (max)

1 (36.0 ) 1.60 10 (36.0 / )

36.0 5.76 10

k e stop k e stop

k k

k k

E q V E q V

E e V E C J C

E eV E J

Graphical Analysis

Photoelectric Current

Number of electrons that move from a cathode to an anode in some time interval

Current equation: I = q/t (charge is dependent upon the number of electrons being emitted)

Intensity Matters

- the greater the intensity/brightness, the greater the photoelectric current

Because . . . . One photon emits one electron

Dim light – 3 photons hit metal and emit 3 electrons

Bright light – 5 photons hit metal and emit 5 electrons so . . . there is a greater photoelectric current

Frequency Matters

Frequency of EMR affects the energy of the electrons

The greater the frequency of the EMR above the threshold frequency of the metal, the greater the energy of the photoelectrons

Lines are all parallel, with equal slopes:

Slopes = Planck’s constant

X-intercept = threshold frequency of metal

Y-intercept = work function of metal

Graphing Results

Graphs

1. Energy vs frequency 2. Current vs frequency

fo

W

Slope = h

14( 10 )f Hz

1910

kE

J

of f

Graphs

3. Current vs Intensity 4. Current vs Stopping Voltage

Summary:

Applications of Photoelectric Effect practical applications include the photocell,

photoconductive devices and solar cells. A photocell is usually a vacuum tube with two electrodes.

One is a photosensitive cathode which emits electrons when exposed to light and the other is an anode which is maintained at a positive voltage with respect to the cathode. Thus when light shines on the cathode, electrons are attracted to the anode.

An electron current flows in the tube from cathode to anode. The current can be used to operate a relay, which might turn a motor on to open a door or ring a bell in an alarm system.

The system can be made to be responsive to light, as above, or sensitive to the removal of light as when a beam of light incident on the cathode is interrupted, causing the current to stop.

Photocell

Photocells in Medicine

Absorption of light by a bacteria cell causes a drop in the number of photons absorbed by the photocell and a drop in the current

Photocells in Garage Door Openers

Light to photocell is interrupted, and the corresponding drop in photocurrent signals the motor to reverse.

Photocells in Movie Film

Optical sound track is like a bar-code, but much more detailed. The track modulates the intensity of the light at a frequency which is the same as the sound which was used to produced the track.

Assignment:

P. 259 #7 – 15, 27, 29, 31