Photoelectric Effect

Question 1

What is the photoelectric effect?

Answer 1

• Metal contains conduction electrons, which move about freely inside the metal.
• These electrons collide with each other and with the positive ions of the metal.
• If a light of a high enough frequency is shone onto the surface of the metal.
• Free electrons on the surface of the metal absorb energy from the light
• If an electron absorbs enough energy, the bonds holding it to the meta break and the electron is released as a photo-electron.

Question 2

What are some conclusions drawn about the photoelectric effect?

Answer 2

• Photoelectric emission does not occur if the frequency of the incident electromagnetic radiation is below the threshold frequency, the minimum frequency depends on the the type of metal, this means that the wavelength of the incident light must be less than a maximum value = speed of light/ threshold frequency.
• Number of electrons emitted per second is proportional to the intensity of the incident radiation, only if the frequency of the incident electromagnetic radiation is greater than the threshold frequency, however if the incident radiation’s frequency is less than the threshold frequency, no photoelectric emission from that metal surface can take place, irregardless of the intensity.
• Photoelectric emission occurs without delay as soon as the incident radiation is directed at the surface, provided the incident radiations’ frequency is greater than the threshold frequency.
• Photo-electrons are emitted with a variety of kinetic energies, ranging from zero to a maximum value, the maximum kinetic energy increases with the frequency of radiation, and is unaffected by the intensity.

Question 3

How does wave theory attempt to explain photoelectric emission?

Answer 3

• For a particular frequency of light, the energy carried carried is proportional to the intensity of the beam.
• The energy carried by the light would be spread evenly over a front.
• Each free electron on the surface of the metal would gain a bit of energy from each incoming wave.
• Gradually each electron would gain enough energy to leave the metal.

Question 4

Why can wave theory not be used to explain photoelectric emission?

Answer 4

• Higher the intensity of the wave, the more energy it should transfer to each electron, kinetic energy should increase with intensity, there is no explanation for the kinetic energy being dependent only on the frequency.
• There is no explanation for threshold frequency, according to wave theory the electrons should be emitted eventually, irregardless of the frequency.

Question 5

How did Einstein attempt to explain the photoelectric effect?

Answer 5

• When the light is incident on a metal surface, an electron at the surface absorbs a single photon from the incident light and therefore gains energy equal to hf, where hf is the energy of a light photon.
• An electron can leave the metal surface if the energy gained form a single photon exceeds the work function of the metal.

Question 6

What is the work function?

Answer 6

• The minimum energy needed by an electron to escape from the metal surface.
• Minimum amount of energy electron can lose.

Question 7

What happens to excess energy gained by the photo-electron?

Answer 7

It becomes kinetic energy.

Question 8

What assumption did Einstein make about light?

Answer 8

Composed of wave-packets/photons

Question 9

What is the formula for the energy of a photon?

Answer 9

E = hf 
h = Planck constant
f = frequency of light.

Question 10

What is the formula for the maximum kinetic energy of an emitted electron?

Answer 10

Ekmax = hf - Φ

Question 11

What is the formula for energy of a photon ( using the work function )?

Answer 11

hf = Ekmax + Φ

Question 12

Why is the kinetic energy of the electron independent of the intensity?

Answer 12

Each electron can only absorb one photon at a time, increasing the intensity means more photons per second on an area, each photon still has the same energy.

Question 13

What is the stopping potential?

Answer 13

• The minimum potential needed to stop photoelectric emission, Vs.
• Potential difference needed to stop the fastest moving electron.

Question 14

What happens to the Ekmax at the stopping potential?

Answer 14

• The maximum kinetic energy of the emitted electron is reduced to zero because each emitted electron must do extra work equal to e x Vs to leave the metal surface.
• Ekmax = e x Vs`

Question 15

What is the formula for Ekmax?

Answer 15

1/2 MV ( max ) ^ 2

Question 16

What is a vacuum photocell

Answer 16

A glass tube that contains:
• Metal plate called the photo-cathode
• Smaller metal electrode called the anode.

Question 17

What happens in a photo-cathode?

Answer 17

• When light of a frequency greater than the threshold frequency for the metal is directed at the photo-cathode, electrons are emitted from the cathode and are attracted to the anode.

Question 18

What is the photoelectric current proportional to?

Answer 18

Number of electrons per second that transfer from the cathode to the anode.