Photoelectric Effect

Question 1

what is an electron volt?

Answer 1

• energy gained by an electron travelling through a potential difference of 1 volt

eV = 1.6 x 10^-19J

Question 2

give the relation of the electron volt with kinetic energy

Answer 2

eV = 1/2 mv^2

• electron accelerates from rest, electron volt is equal to the KE gained

Question 3

what is the photoelectric effect?

Answer 3

• phenomenon in which photoelectrons are emitted from the surface of a metal when electromagnetic radation is shone on it

Question 4

what is the threshold frequency?

Answer 4

• minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal

Question 5

what is the work function of a metal?

Answer 5

• the minimum energy required to release a photoelectron from the surface of a metal

Question 6

what is stopping potential?

Answer 6

• the potential difference required to stop a photoelectron emission from occuring

Question 7

explain the concept of stopping potential

Answer 7

• photons transfer energy to electrons, any energy above work function gets converted into kinetic energy
• applying a negative voltage repels electrons, and at Vs even most energetic electrons are stopped
• eVs = Ek max
• provides evidence that stopping potential depends on frequency so light behaves as quantised particles.

Question 8

what happens if you increase the intensity of the electromagnetic radiation?

Answer 8

• number of photons incident on the metal plate increases
• mumber of photoelectrons emitted from the plate increases ( photoelectric current )

Question 9

why does increasing the intensity not affect the kinetic energy of the photoelectrons?

Answer 9

• increasing the intensity increases the number of photons, not their energy
• since each electron absorbs one photon, maximum kinetic energy stays the same

Question 10

what is the photoelectric equation?

Answer 10

hf = ø + Ekmax

Question 11

how can we plot a graph of the photoelectric equation?

Answer 11

Ekmax = hf - ø

plot Ekmax against frequency

x intercept is threshold frequency

y intercept is work function

gradient is h

Question 12

why is kinetic energy a maximum?

Answer 12

• each electron in the metal requires the same energy from the photons for any given frequency
• however, energy required to remove an electron from the metal varies because some electrons are on the surface and some are deeper in the metal

Question 13

what is photoelectric current?

Answer 13

• measure of the number of photoelectrons emitted per second

I = qn

Question 14

give some useful equations for quantum?

Answer 14

current = ne

I = P / A