12.1.3 Observing the Photoelectric Effect

• The photoelectric effect can be observed on a gold leaf electroscope
• A plate of metal, usually zinc, is attached to a gold leaf, which initially has a negative charge, causing it to be repelled by a central negatively charged rod
  • This causes negative charge, or electrons, to build up on the zinc plate

• UV light is shone onto the metal plate, leading to the emission of photoelectrons
• This causes the extra electrons on the central rod and gold leaf to be removed, so, the gold leaf begins to fall back towards the central rod
  • This is because they become less negatively charged, and hence repel less

• Some notable observations:
  • Placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly
  • Using a higher frequency light source does not change the how quickly the gold leaf falls
  • Using a filament light source causes no change in the gold leaf’s position
  • Using a positively charged plate also causes no change in the gold leaf’s position

Typical set-up of the gold leaf electroscope experiment

Laws of Photoelectric Emission

• The photoelectric effect is evidence that light does not purely behave as a wave
• This is demonstrated by the following observations

• Observation 1:

• Explanation 1:
  • Placing the UV source closer to the plate increases the intensity incident on the surface of the metal
  • Increasing the intensity, or brightness, of the incident radiation increases the number of photoelectrons emitted per second
  • Therefore, the gold leaf loses negative charge more rapidly

   

• Observation 2:

• Explanation 2:
  • The maximum kinetic energy of the emitted electrons increases with the frequency of the incident radiation
  • In the case of the photoelectric effect, energy and frequency are independent of the intensity of the radiation
  • So, the intensity of the incident radiation affects how quickly the gold leaf falls, not the frequency

• Observation 3:

• Explanation 3:
  • If the incident frequency is below a certain threshold frequency, no electrons are emitted, no matter the intensity of the radiation
  • A filament light source has a frequency below the threshold frequency of the metal, so, no photoelectrons are released

   

• Observation 4:

• Explanation 4:
  • If the plate is positively charged, that means there is an excess of positive charge on the surface of the metal plate
  • Electrons are negatively charged, so they will not be emitted unless they are on the surface of the metal
  • Any electrons emitted will be attracted back by positive charges on the surface of the metal

   

• Observation 5:

• Explanation 5:
  • A single photon interacts with a single electron
  • If the energy of the photon is equal to the work function of the metal, photoelectrons will be released instantaneously

In the photoelectric effect, a single photon may cause a surface electron to be released if it has sufficient energy

Step 1: Outline what wave theory predicts about the photoelectric effect

• • Wave theory predicts:
    • If radiation strikes a metal surface and ejects an electron, the kinetic energy of the electron should depend on the intensity of the incident wave

Step 2: Outline an observation of the photoelectric effect experiment

• • From observation:
    • There is no electron emitted if the frequency of the radiation is below a certain threshold frequency
    • The intensity of the radiation does not determine the energy of the emitted electron
    • Above the threshold frequency, the maximum kinetic energy of the electrons increases with the frequency of the radiation

Step 3: Suggest how this observation supports the particulate nature of light

• • This suggests:
    • In its interaction with matter, an electromagnetic wave behaves like a stream of particles, or photons
    • These photons carry energy which is proportional to the frequency of the radiation

Stopping Voltage

• Stopping voltage, V_s, is defined as:

The voltage required to stop photoelectron emission from occurring

• The photons arriving at the metal plate cause photoelectrons to be emitted
  • This is called the emitter plate

• The electrons that cross the gap are collected at the other metal plate
  • This is called the collector plate

This set-up can be used to determine the maximum kinetic energy of the emitted photoelectrons

• The flow of electrons across the gap results in an e.m.f. between the plates that causes a current to flow around the rest of the circuit
  • Effectively, it becomes a photoelectric cell producing a photoelectric current

• If the e.m.f. of the variable power supply is initially zero, the circuit operates only on the photoelectric current
• As the supply is turned up, the emitter plate becomes more positive (because it is connected to the positive terminal of the supply)
• As a result, electrons leaving the emitter plate are attracted back towards it
  • This is because the p.d. across the tube opposes the motion of the electrons between the plates

• If any electrons escape with enough kinetic energy, they can overcome this attraction and cross to the collector plate
  • And if they don't have enough energy, they can't cross the gap

• By increasing the e.m.f. of the supply, eventually, a p.d. will be reached at which no electrons are able to cross the gap – this is the stopping voltage, V_s
• At this point, the energy needed to cross the gap is equal to the maximum kinetic energy KE_max of the electrons

KE_max = eV_S