The Professional Development Service for Teachers is funded by the
Department of Education and Skills under the National Development Plan
T.T.

Photoelectric Effect
UV
- - -
-
-
-
-
-
-
-
When UV shines on the cap…
…the leaves collapse
…showing the charge is gone
Negatively Charged Electroscope
PDST Resources for Leaving Certificate Physics
2

Photoelectric Effect
UV
+ + + + +
+ +
+ +
+ + +
+ + +
+ + +
When UV shines on the cap…
…nothing happens
Positively Charged Electroscope
PDST Resources for Leaving Certificate Physics
3

Photoelectric Effect
Electromagnetic radiation (of varying frequency and intensity)
Spectrum
Low freq. High freq.
Radio / μ-wave / IR / Visible / UV / X-ray / γ-ray
No current Current flows
A
Circuit used to analyse circuit:
Increase f
Increase intensity
PDST Resources for Leaving Certificate Physics
Under f o
(No current)
No effect
No effect threshold frequency f o
Over f o
(Current)
No effect
Increases current
4

Photoelectric Effect: Einstein’s explanation
Electromagnetic radiation
-
-
-
-
Circuit used to analyse circuit:
PDST Resources for Leaving Certificate Physics
A
Light is a beam of particles (photons)
Each photon hits one electron
If the electron gains enough energy, it escapes electrons travel to the anode
The more electrons, the bigger the current
5

Photoelectric Effect: Einstein’s Photoelectric Law
Electromagnetic radiation
• Photon (containing energy, E in
)
Energy in each photon
• hits electron
E in
  
E k • electron uses energy, Φ, to escape
• electron crosses gap with energy, E
A k
Kinetic Energy
Circuit used to analyse circuit:
PDST Resources for Leaving Certificate Physics
Energy electrons need to escape
(called the work function)
6

Electromagnetic radiation
Energy in each photon
E
 hf
E in
  
E k
A
Kinetic Energy
E
 1
2 mv
2
Circuit used to analyse circuit: Energy electrons need to escape
PDST Resources for Leaving Certificate Physics
(called the work function)
  hf o 7