Quantum - Photoelectric effect

• What is the photoelectric effect?

  A famous experiment that demonstrates light having particle like behaviour.
• What happens in the photoelectric effect?

  Electrons are ejected from metal surfaces when electromagnetic radiation of a high enough frequency falls on them.
• Explain the practical use of a photoelectric cell

  When em radiation, of frequency above the threshold frequency, falls on the photoelectric cell then the photo-electrons are released. These flow due to the p.d. of the supply cell and a current is registered in the micro-ammeter. If the intensity is doubled then twice as many photons arrive per second, so twice as many electrons are released per second. As current is the rate of flow of charge, the current doubles. This is assuming that one photon can only release one electron, and also that all photo-electrons emitted are collected.
• Where can the photoelectric effect be observed?

  On a gold leaf electroscope.
• Describe the gold leaf experiment.

  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 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.
• What will happen if we place the UV light source closer to the metal plate?

  Placing the UV light source closer to the metal plate causes the gold leaf to fall more quickly
• What will happen if we use a higher frequency light source in the gold leaf experiment?

  Using a higher frequency light source does not change how quickly the gold leaf falls.
• What will happen if we use a filament light source in the gold leaf experiment?

  Using a fiament lamp causes no change in the gold leaf's position.
• What will happen if we use a positively charged plate in the gold leaf experiment?

  Using a positively charged plate causes no change in the gold leaf's position.
• When does the emission of photoelectrons start in the gold leaf experiment?

  Emission of photoelectrons happens as soon as the radiation is incident on the surface of the metal.
• Why does placing the UV light source closer to the metal plate cause the gold leaf to fall more quickly?

  Placing the UV light 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.
• Why does using a higher frequency of light not change how quickly the gold leaf falls?

  The maximum kinetic energy of the emitted electrons increases with the frequency of incident radiation.

  In the case of the photoelectric effect, energy and frequency are independent of the intensity and radiation.

  So, the intensity of the incident radiation affects how quickly the gold leaf falls, not the frequency.
• Why does using a filament light source cause no change in the gold leaf's position?

  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.
• Why does using a positively charged plate cause no change in the gold leaf's position?

  If the plate is positively charged, that means there is an excess of 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.
• Why does the emission of photoelectrons happen as soon as the radiation is incident on the surface of the metal?

  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.