S-Cool Revision Summary

S-Cool Revision Summary

The Photoelectric Effect

The wave model of light can't explain the photoelectric effect i.e.:

- the threshold frequency.

- the maximum Ek of the photoelectrons.

- no. of electrons is proportional to intensity, not frequency.

- the instantaneous production of photoelectrons.

Einstein explained this using Quantum Theory:-

- light comes in packets of energy (photons).

- each photon has energy that depends on the frequency of the light:

E = h f

- each photon interacts with only one electron (and vice versa).

Higher frequency, shorter wavelength radiation is more energetic.

The work function, Φ, is the energy needed for an electron to leave the surface of the material.

h f = Φ + 1/2 m v2max

This is Einstein's photoelectric equation.

Photoelectric Current

Photoelectric current and stopping Potential Diagrams

- The plate is called the "emitter".

- The electrons that cross the gap are collected at the other metal plate - called "the collector"

- The flow of electrons across the gap sets up an emf between the plates that causes a current to flow around the rest of the circuit. That's a photoelectric cell producing a photoelectric current.

Stopping Potential, Vs

Photoelectric current and stopping Potential Diagrams

The emitter gives out electrons. So we call it a cathode.

If you remember, work done moving a charge through a pd is W = QV and in this case charge, Q = e, the charge on an electron,


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Using this information you can calculate the maximum energy of the photoelectrons emitted from the metal.

Rewrite the photoelectric equation (from about 3 screens above this point) as

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E = hf

h f = Φ + 1/2 m v2max


h = Planck's constant, Js

f = frequency, Hz

λ = wavelength, m

m = mass, kg

v = velocity, ms-1

Φ = work function, J

E = energy, J