M7 Topic 3 Part 2: The photoelectric effect

Question 1

what is the photoelectric effect

Answer 1

the spontaneous emission of electrons from a metallic surface when EM Radiation is incident upon it

Question 2

when and who discovered the photoelectric effect

Answer 2

Hertz fist observed the effect in 1887: spark gap in his radio wave experiment

Question 3

how many electrons does 1 photon liberate (if it has enough energy)

Answer 3

1 electron

Question 4

what happens during the photoelectric effect (2)

Answer 4

• photons with enough energy to overcome the work function are incident on the metal surface releasing a electron
• The electron absorbs the photons energy, allowing it to overcome the attraction holding them to the metallic structure

Question 5

what is the work function

Answer 5

the minimum amount of energy required to liberate an electron from the metals structure

Question 6

what determines the size of the work function

Answer 6

how closely bound the electrons are to the different metals structures

Question 7

what is the photoelectric equation

Answer 7

Φ = hf₀

Question 8

what does each symbol in the photoelectric effect represent (3)

Answer 8

Φ = the work function of the metal (Joules or electron volt (J or eV))
h = Plank’s constant (6.63 x 10⁻³⁴J or 4.14 x 10⁻¹⁵eV)
f₀ =the threshold frequency for the metal (hertz (Hz))

Question 9

what happens if the photon has less energy than the work function

Answer 9

no electrons will be emitted

Question 10

what happens if the photon has energy equal to the work function

Answer 10

electrons will be emitted but have no kinetic energy and remain on the metal surface

Question 11

what happens if the photon has more energy than the work function

Answer 11

the electron will be emitted and the additional energy will transform into kinetic energy for the electron

Question 12

what is the equation to calculate the kinetic energy of the electron

Answer 12

kₘₐₓ = hf - Φ

Question 13

what do the symbols represent in the kinetic energy of a photon equation, and what units are they measured in (4)

Answer 13

kₘₐₓ = the maximum kinetic energy of an emitted photoelectron (Joules or electron volts J or eV))
h = Plank's constant (6.63 x 10³⁴J or 4.14 x 10⁻¹⁵eV)
f = the frequency of the incident photon (hertz (Hz))
Φ = the work function of the metal ( Joules or electron volts J or eV))

Question 14

How do you calculate kinetic energy in the photoelectric effect in a electric field experiment

Answer 14

by measuring the voltage generating the electric field, as the work done by the electric field must be equal to the kinetic energy

Question 15

what does the y intercept represent in the photoelectric effect in a electric field experiment graph

Answer 15

the work function (Φ)

Question 16

what does the x intercept represent in the photoelectric effect in a electric field experiment graph

Answer 16

the threshold frequency (f₀)

Question 17

what does the gradient represent in the photoelectric effect in a electric field experiment graph

Answer 17

Planck’s constant (h)

Question 18

what observation where made about the photoelectric effect (2)

Answer 18

• Intensity only increases the number of electrons not their energy
• Electrons are emitted instantaneously

Question 19

what predictions did classical physicist make about the photoelectric effect (3)

Answer 19

• The higher the brightness (intensity) of the incident light, the greater the energy of electrons, emitted from the surface.
• light waves of any frequency should be able to liberate electrons, provided a reasonable intensity is maintained.
• If the incident light is of low intensity, the metal surface must be continuously exposed for some time until enough waves strike the surface to knock off electrons.

Question 20

What were the experimental results of the photoelectric effect (3)

Answer 20

• The energy of the knocked-off electrons does not depend on the intensity of incident light.
• Electrons are not knocked off the surface unless the frequency of the incident light wave is more than a critical value (threshold frequency).
• Electrons appear as soon as light falls on the metal surface.

Question 21

How did einstein explain this the experimental results of the photoelectric effect

Answer 21

he used Planck’s quanta model for light which showed light as particles called quanta or photons which carried energy that could be transferred to matter

Question 22

How did Einstein use Planck’s quanta model for light (5)

Answer 22

he proposed the following assumptions:

1. As light existed as photons. The energy of each photon represented as E=hf;
2. Light intensity depends on the number of photons (the more photons, the greater the intensity of light);
3. Photons with the highest energy correspond to light of the highest frequency;
   
   4. To produce the photoelectric effect, when an electron is liberated from a metal, energy is required to overcome the forces holding the electron to the surface, the work function;
4. If the energy of the photon is greater than the work function, the additional energy in the photon will provide kinetic energy to the photoelectrons.