Download Photoelectric Effect

Photoelectric Effect
Absorption
 Charges can absorb electromagnetic waves.
• Can cause charge to accelerate
absorbed photon
Moving charge
Ejected Electrons
 Electromagnetic energy
I
could eject electrons from
the surface.
• Accelerate with voltage
• Measure current
V
e
 Classical expectation is that
the current varies with
intensity.
• May also vary with
frequency.
I  intensity
Stopping Potential
 As the voltage is decreased
I
the photocurrent decreases.
• Matches classical view
V
e
 The voltage can be reversed
and some current flows.
• Stopping point independent
of intensity
• Characteristic of metal
KEmax  eV
Atomic View
 A photon can eject an electron
from an atom.
• Photon absorbed
• Electron released
g
e
Z
 Energy within the atom is
quantized.
eVS  hf  f
• Minimum energy f needed for
interaction
• Einstein’s photoelectric
equation
Work Function
 The minimum energy is the
work function of the metal.
 Below a certain frequency no
current can flow.
• f0 = f/h
 Work functions for some
metals:
•
•
•
•
•
•
•
•
•
Na: 2.28 eV
Co: 3.90 eV
Al: 4.08 eV
Pb: 4.14 eV
Zn: 4.31 eV
Fe: 4.50 eV
Cu: 4.70 eV
Ag: 4.73 eV
Pt: 6.35 eV
Photocell
 The photoelectric effect is commonly used to measure
light.
• Camera light meter
 It can also generate electricity.
• Photovoltaic cell
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