The Photoelectric Effect

Question 1

What is the photoelectric effect?

Answer 1

• If you shine radiation of a high enough frequency onto the surface of a metal, it will instantly emit electrons
• Metals have delocalised (free) electrons, if they absorb the radiation they will vibrate
• If the electron absorbs enough energy, the bond holding the electron breaks and the electron is released

Question 2

What is the frequency needed for most metals?

Answer 2

The ultraviolet range

Question 3

What are the electrons emitted from the photoelectric effect called?

Answer 3

Photoelectrons

Question 4

What happens if the frequency is too low?

Answer 4

No photoelectrons will be emitted if the radiation is below the threshold frequency

Question 5

What conclusions do you get from the photoelectric experiment?

Answer 5

• The frequency of the light needs to be above the threshold frequency
• The photoelectrons are emitted with different kinetic energies
• The kinetic energy of the photons is not affected by the intensity of the light
• The number of photoelectrons emitted per second is proportional to the intensity of the radiation

Question 6

How do you increase the kinetic energy of the photoelectrons?

Answer 6

Increase the frequency of the light

Question 7

Why does the wave theory not explain the threshold frequency?

Answer 7

Wave theory says:
- The energy carried by a wave is proportional to the intensity of the beam
- The energy carried by the wave spreads evenly over the wavefront
- If the wave was shone on a metal, each free electron would gain a little energy from each wavefront, eventually each electron would have enough energy to break free
- If a wave with a lower frequency (less energy) shone on the metal, it would take longer for the electron to break free but it would happen eventually, but electrons are never emitted unless the frequency is above the threshold frequency so the wave theory can’t explain the threshold frequency

Question 8

Who first suggested that waves are made of packets? What’s another name for packets

Answer 8

Max Planck
Quanta

Question 9

What’s the equations for the energy of photons?

Answer 9

E = hf = hc/λ

E - energy
h - planck’s constant (6.63x10^-34)
f - frequency

Question 10

What is the work function?

Answer 10

The minimum amount of energy needed for an electron to break free

Question 11

What is the condition for electrons to be released?

Answer 11

hf ≥ Φ

h - planck’s constant
f - frequency
Φ - “phi”, work function

Question 12

What is the formula for threshold frequency?

Answer 12

f₀ = Φ/h

f₀ - Threshold frequency
Φ - Work function
h - Planck’s constant

Question 13

What is the equation for the kinetic energy of a photoelectron?

Answer 13

Eₖ = hf - Φ

Question 14

What’s the photoelectric equation?

Answer 14

Hf = Φ + Eₖ(max)

Question 15

What is the stopping potential equation?

Answer 15

eVₛ = Eₖ

e - charge on the electron (1.6x10^-19)
Vₛ - stopping potential
Eₖ - Maximum kinetic energy

Question 16

What is an electron volt?

Answer 16

The kinetic energy gained by an electron when it is accelerated from rest through a potential difference of 1 V