Photoelectricity Flashcards
Describe the photoelectric effect
• Shining light of a high enough frequency onto the surface of a metal causes it to emit electrons, for most metals this frequency falls in the UV range but for some it’s visible light. • Free electrons on the surface of the metal can sometimes absorb energy from the light • If an electron absorbs energy, the bonds holding it to the metal can be broken and the electron released • The emitted electrons are called photoelectrons.
What conclusions can be drawn from the photoelectric effect:
• No photoelectrons are emitted if the radiation has a frequency below a certain value (dependent on the type of metal used) • The photoelectrons are emitted with a variety of kinetic energies ranging from zero to some maximum value which increases with the frequency of radiation and varied depending on the type of metal used but was independent of the intensity of the incident light • Photoelectric emission was almost instantaneous once light of a suitably high frequency was incident on the metal surface • The number of photoelectrons emitted per second is directly proportional to the intensity of the radiation • These couldn’t be explained using wave theory
What did wave theory predict regarding photoelectricity:
• Energy carried is proportional to intensity of beam • Energy carried by light would spread evenly over the wavefront • Each free electron on metal surface would gain a bit of energy from each incoming wave • Gradually over time, each electron would gain enough energy to be able to leave the metal
Wave theory problems:
• Incorrectly predicted time lag before electron emission – observed instantaneously • Threshold frequency – at low frequencies, emission should still occur just take longer • If light were a wave, a brighter light should increase the maximum Ek of electrons so a higher intensity = more kinetic energy but not the case
What is a black body
a body that can absorb and emit all wavelengths of EM radiation e.g. a star
The graph of intensity against wavelength (micrometres) for a black body shows
that power radiated varies with wavelength. Increases dramatically at the start to a beam then decreases exponentially whereas the classical prediction was a exponential decrease starting after the peak i.e. at lower wavelengths it was infinitely large. Wave theory could not explain this. Visible wavelengths occur around the peak.
What was the UV catastrophe:
wave theory failed to correctly predict the energy spectrum of a black body at short wavelengths
What was Planck’s solution to the UV catastrophe:
• Energy of EM waves quantised, not continuous • EM waves released in discrete wave-packets of energy the size of quantum, E = hf • Wave packets = photons = quantum = least quantity of EM radiation= massless particle
Einstein’s explanation of photoelectric effect and threshold frequency:
• Photons of light have one-to-one interaction with an electron on a metal’s surface • All of photons energy = hf is transferred to one specific electron • If hf >= work function, electron is emitted otherwise it isn’t as it needs enough energy to break the bonds holding it there. • Threshold frequency is minimum frequency a photon can have and still cause a photoelectron to be emitted • Energy of photon at threshold frequency = work function: phi = h*fmin
What is the work function
the minimum energy for an electron to break the bonds holding it to the surface of a metal and escape. It is dependent on the type of metal just like the threshold frequency
How does the photon model explain the photoelectric effects maximum kinetic energy: (4-5)
• The energy transferred to an electron by a photon depends on the photon’s frequency • The kinetic energy the electron will be carrying when leaving the metal = energy gained from photon – energy lost trying to leave the metal • Electrons from deeper down the metal lose more energy than electrons on the surface so photoelectrons have a range of energies • Maximum kinetic energy occurs when the electrons are on the surface of the metal so no energy is lost doing work getting to the surface of the metal to be emitted. • The intensity of the light affects the number of photoelectrons emitted because if there are more photons incident per second then more one-to-one interactions can occur with electrons so more electrons can absorb the photon’s energy and be emitted however it does not affect the kinetic energy of the photoelectrons because each photon has the same amount of energy and only one can be absorbed by an electron.
Graph of maximum kinetic energy against frequency
Ekmax = hf – phi Gradient = h, x intercept = fmin, y intercept = -phi
Photoelectricity circuit diagram (6-7)
• Photocell (terminal left/anode with negative cathode/metal surface right, incident light on metal surface) • Microammeter to the left • Loop with voltmeter opposite photocell • Second loop with potential divider (resistor) opposite voltmeter • Cell opposite resistor • Loops connected on the right • Arrow from first loop to resistor
Explain photoelectricity circuit: (8-9)
• Cathode placed near emitting metal surface to repel photoelectrons • Light shone on metal surface causing electrons to be emitted and collect at the terminal/anode allowing a current to flow • Increasing pd of electrode by adjusting potential divider makes it more negative to the point photoelectrons cannot reach it • As supply voltage increases, more electrons are repelled back and the ammeter reading falls • Only the fastest electrons reach the collecting electrode as most kinetic energy (0.5mv^2) • When the supply voltage reaches the stopping potential (measured using a potential divider), the electric potential energy of an electron near the electron equals the maximum kinetic energy of the electrons and so no electrons can reach the collecting electrode and the current falls to zero • The electrical energy transferred to an electron crossing electrodes is eV • When the potential reaches the stopping potential, emission stops as hf – phi – eV has been reduced to zero as Ekmax = eV so electrons cannot travel around the circuit meaning the current decreases to zero • So hf – phi = qVs meaning Vs = hf/q – phi/q
Two equations for kinetic energy of electrons in photocell:
• At the metal surface, the electrons have kinetic energy = 0.5mv^2 = hf-phi • At the LHS terminal/negative electrode, they have kinetic energy = hf – phi - eV
Graph of stopping potential against frequency
y intercept = -phi/q, gradient = h/q, x intercept = fmin
