Photoelectric Effect Flashcards
(32 cards)
Describe the photoelectric effect.
Photons of EM radiation are incident on a metal surface. 1 photon transfers all its energy to 1 electron near the surface. Photoelectrons emitted.
Give details about the kinetic energy of the photoelectrons emitted.
Photoelectrons have a range of kinetic energies, from 0 up to a maximum value (determined by frequency of radiation).
Define work function (φ).
The minimum photon energy required to cause the emission of an electron from the surface of a metal.
Define threshold frequency (fo).
The minimum frequency of EM radiation required to cause the emission of an electron from the surface of a metal.
Give the equation linking work function and threshold frequency.
φ = hfo
Give the equation linking frequency, work function and max kinetic energy of electrons.
hf = φ + Ek(max)
Explain how increasing frequency (or decreasing wavelength) of EM radiation affects the photoelectric effect.
If frequency (f) increases or wavelength (ʎ) decreases, photon energy (E) increases, work function (φ) is constant, maximum kinetic energy of electrons increases as Ek(max) = hf – φ.
Explain how increasing intensity (power per unit area) of EM radiation affects the photoelectric effect.
If intensity increases, the number of photons incident per second increases. Number of photoelectrons emitted per second increases.
Define stopping potential.
The potential difference required to stop the emission of electrons from the surface of a metal.
Give the equation for stopping potential.
Vs = Ek(max) / e
Define ground state.
Lowest energy level (n=1). Closest to the nucleus.
Define excitation in terms of energy levels and energy.
An electron moves to a higher energy level when it gains energy equal to the difference between the two levels.
Describe two ways that electrons can gain energy to become excited.
- 1 photon transfers all its energy to 1 electron. * A free electron collides with the electron and transfers some energy from its kinetic store.
Define de-excitation in terms of energy levels, energy and photons.
An electron moves to a lower energy level when it loses energy by emitting a photon, with energy equal to the difference between the two levels.
Define ionisation.
An electron gains enough energy to be removed from an atom.
Define ionisation energy.
The amount of energy required to completely remove an electron from an atom, from the ground state.
Why are energy levels defined as having negative energy?
A free electron removed from an atom is defined as 0 J. All energy levels are negative relative to this.
Describe the structure of the discharge tube used for fluorescent lights.
Glass tube filled with mercury vapour. Beam of free electrons accelerated by a p.d. Phosphor coating.
Describe what happens to the electrons in the mercury vapour.
Free electrons collide with electrons in mercury atoms and transfer energy. Mercury electrons excite then immediately de-excite, emitting UV photons.
Describe what happens to the electrons in the phosphor coating.
UV photons are absorbed by electrons in phosphor coating. Phosphor electrons are excited, then de-excite in stages, emitting visible light photons.
What do line emission spectra look like?
Black background. Coloured lines at certain wavelengths.
How are line emission spectra formed?
Discharge tube filled with gas. Electrons in the gas are excited when free electrons collide and transfer energy.
What do line absorption spectra look like?
Colour spectrum background. Black lines at certain wavelengths.
How are line absorption spectra formed?
White light is shone through a cold gas. Electrons are excited when they absorb photons with certain energies.
