7) The Photoelectric Effect

Question 1

Explain in terms of photons why electrons are released from the metal surface.

Answer 1

• Light consists of photons, each of energy E = hf, one photon is absorbed by one electron.
• An electron can escape from the surface if hf > ϕ.

Question 2

State what is represented by the gradient of a graph which shows how the maximum kinetic energy of the electrons varies with the frequency of the light shining on the metal surface.

Answer 2

Planck’s constant.

Question 3

When monochromatic light is shone on a clean metal surface, electrons are emitted from the surface due to the photoelectric effect. State and explain the effect on the emitted electrons of increasing the frequency of the light.

Answer 3

• The maximum kinetic energy of released electrons increases.
• This is because increasing the frequency of the photons increases their energy.

Question 4

When monochromatic light is shone on a clean metal surface, electrons are emitted from the surface due to the photoelectric effect. State and explain the effect on the emitted electrons of increasing the intensity of the light.

Answer 4

• The number of electrons emitted per second increases.

- Because there are now more photons striking the metal surface per second.

Question 5

State what is meant by the threshold frequency of the incident light.

Answer 5

The minimum frequency for electrons to overcome work function.

Question 6

Explain why the kinetic energy of the emitted electrons has a maximum value.

Answer 6

• Photons have energy dependent on frequency.
• One to one interaction between photon and electron.
• Max KE = photon energy – work function.
• More energy required to remove deeper electrons.

Question 7

Explain with reference to the work function why, if the frequency of the radiation is below a certain value, electrons are not emitted.

Answer 7

• The work function is the minimum energy needed to release an electron.
• Below a certain frequency energy of the photon is less than work function.

Question 8

State a unit for work function.

Answer 8

Joule.

Question 9

State what is meant by work function.

Answer 9

The minimum energy required to remove an electron from a metal surface.

Question 10

Explain why, when ultraviolet light is shone on a positively charged plate, no charge is lost by the plate.

Answer 10

-The process involves the ejection of electrons which are
negatively charged.
-Any electrons ejected will only make the positive charge
greater.