Physics 4.5

Quantum physics

How does electromagnetic radiation travel through space?

As a continuous wave

What are photons?

Discrete energy packets

When does electromagnetic radiation act as photons?

When it interacts with matter

What is the equation linking energy of a photon to frequency of the electromagnetic radiation?

E = hf = hc / lamda

What is h?

• Planck constant

• 6.63×10^-34

What is 1 electronvolt?

The energy transferred when an electron travels through a potential difference of 1 volt

What is 1 electronvolt in joules?

1eV = 1.60×10^-19J

What is the photoelectric effect?

When electromagnetic radiation is shone onto a metal electrons are released from the surface

What happens if you try to use visible light for the photoelectric effect?

No electrons will be removed from the plate

What is the work function of a metal?

Minimum energy required to free an electron from the surface of the metal

What must a photon have in order to free an electron from a metal surface?

Energy at least as great as the work function

What is threshold frequency?

Minimum frequency required of the electromagnetic radiation to free electrons from the surface of the metal

Does visible light produce the photoelectric effect?

No as the frequency is too low

What is the photoelectric equation?

hf = phi + KEmax

• phi = work function of the metal

• KEmax = max ke of released electron

What determines the KE of a released electron?

Amount of energy left after it is released (after using up work function amount)

How does intensity of radiation relate to electron emission if frequency < threshold frequency?

no electrons will be released regardless

How does intensity of radiation relate to electron emission if frequency > threshold frequency?

increasing intensity increases rate of electron emission as it increases the number of photons available to interact with the electrons

How can you increase the kinetic energy of the released electrons?

Increase the frequency of radiation further above the threshold so there's more energy left over

What is wave particle duality?

• diffraction and superposition of light relies on the radiation acting as a wave

• Photoelectric effect relies on it acting as discrete photons

• Therefore electromagnetic waves have a dual nature

What is the de broglie equation?

Lamda = h/p = h/mv

What can the de Broglie equation be applied to?

All particles with mass

How can electrons be used as evidence for wave-particle duality?

• electrons are particles with mass and charge, they can be accelerated and deflected by magnetic and electric fields

• However they can also be made to diffract when fired at thin polycrystalline graphite which is a property of waves