Quantum

Question 1

Gold leaf electroscope

Answer 1

• gold leaf stands up because it is negatively charged
• charge is lost due to photoelectric effect of UV on zinc
• gold leaf droops back down (showing that charge is lost)

Question 2

Observations of the photoelectric effect

Answer 2

• EM radiation causes metal to give off electrons
• only works above a threshold frequency (f0) for each metal
• electron energy increases with frequency
• amount of electrons increases with intensity
• can only be explained with the photon model

Question 3

Find energy of a photon

Answer 3

Energy = planck constant*frequency
(E = hf
or
E = hc/λ)

Question 4

Explain the photoelectric effect

Answer 4

• one photon hits one electron, giving all its energy
• electron leaves the surface if energy is at least at the work function
• if not, energy is quickly lost to surroundings

Question 5

Work function

Answer 5

Minimum photon energy needed to remove an electron from the surface of a specific metal

Question 6

Find work function

Answer 6

Work function = planck constant*threshold frequency

ϕ = hf0

Question 7

Find maximum kinetic energy of a photoelectron

Answer 7

Max kinetic energy = photon energy - work function

0.5mv² = hf - ϕ

Question 8

Find threshold frequency

Answer 8

Threshold frequency = work function/planck constant

f0 = ϕ/h

Question 9

Planck constant (or h)

Answer 9

6.63 x10^-34

Question 10

Find wavelength of a particle

Answer 10

Wavelength = planck constant/momentum
(λ = h/mv)
Found using E = hc/λ and E = mc²
Units work out too (h is kgm²/s)

Question 11

Photon

Answer 11

A quantum of EM energy

Question 12

Find energy of a photon from an LED

Answer 12

Photon energy = energy transferred by electron
(hf = eV
or
hc/λ = eV)

Question 13

Explain how to find h using LEDs

Answer 13

• find the threshold voltages of different coloured LEDs
• plot electron energy (eV) against light frequency (c/λ)
• the gradient is the planck constant

Question 14

Threshold frequency

Answer 14

Minimum photon frequency needed to remove an electron from the surface of a specific metal

Question 15

Effect of increased light intensity on photoelectrons

Answer 15

• kinetic energy stays the same

- rate of emission increases (directly proportional)

Question 16

Effect of light frequency on photoelectrons

Answer 16

• kinetic energy increases

- rate of emission stays the same

Question 17

Units of the planck constant

Answer 17

h = E/f
so Js
or kgm²/s

Question 18

Diffraction of electrons

Answer 18

• electrons are shot through a thin sheet of polycrystalline graphite by an electron gun
• gaps between the carbon atoms are similar in size to electron wavelengths
• the electrons diffract as waves