Quantum Physics

Question 1

describe what would happen to an uncharged gold-leaf electroscope if its top surface were to come into contact with a positive electrode [2]

Answer 1

it would gain a positive charge [1] and the gold leaf would rise [1]

Question 2

if a particular metal had a threshold frequency in the red part of the visible spectrum, explain what would happen to the metal if radiation was incident on its surface from the INFRARED part of the spectrum [2]

Answer 2

no emission [1]
infrared protons are below threshold frequency and have insufficient energy [1]

Question 3

if a particular metal had a threshold frequency in the red part of the visible spectrum, explain what would happen to the metal if radiation was incident on its surface from the BLUE part of the visible spectrum [1]

Answer 3

emission of photoelectrons because blue photos are above threshold frequency [1]

Question 4

explain why the maximum kinetic energy of photoelectrons emitted during the photoelectric effect depends on the frequency of the incident radiation [3]

Answer 4

• energy transferred to each electron comes from a single photon in a one-to-one interaction [1]
• energy of each photon depends on frequency (E = hf) [1]
• greater frequency results in higher energy of the photon and so the greater maximum kinetic energy of the electron [1]

Question 5

the threshold wavelength is the longest wavelength that will give rise to the photoelectric effect. derive an expression for threshold wavelength in terms of threshold frequency f0

Answer 5

maximum wavelength that would cause photoelectric emission from the surface of the metal

Question 6

state and explain the effect of quadrupling the intensity of incident radiation (keeping the frequency constant) on a metal surface emitting photoelectrons

Answer 6

• increased emission [1]
• number of emitted electrons per second quadruples [1]
• quadrupling the intensity results in 4x number of photons, therefore 4x the number of electrons emitted [1]

Question 7

describe the photoelectric effect in terms of a gold-leaf electroscope

Answer 7

• the electroscope has a negative charge
• this causes the gold leaf to rise as its repelled by negative stem
• UV light is shone on metal top plate, the gold leaf falls (shows electroscope loses charge)
• electrons (now called photoelectrons) have been emitted from the top plate