2.4 - Particles and Radiation - The Photoelectric Effect

Question 1

What is an electron volt?

Answer 1

unit commonly used to express very small energies
- because quantum energies tend to be much smaller than 1 joule

Question 2

What is the potential difference?

Answer 2

the energy transferred per unit of charge flowing from one point to another
V= E/Q

Question 3

How is the electron volt derived from the definition of potential difference?

Answer 3

• when an electron travels through a potential difference, energy is transferred between two points in a circuit, or electric field
• if an electron with a charge of 1.6 x 10^-19 C, travels through a potential difference of 1 V, the energy transferred is equal to 1.6 x 10^-19

Question 4

What is the definition of an electron volt?

Answer 4

the energy gained by an electron travelling through a potential difference of one volt
1eV=1.6 x 10^-19

Question 5

What is the electron volts relation to kinetic energy?

Answer 5

• when a charged particle is accelerated through a potential difference, it gains kinetic energy
• if an electron accelerates from rest, an electron volt is equal to the kinetic energy gained: ev = 0.5mv^2
• this can be rearranged to give the speed of the electron

Question 6

What is the Photoelectric effect?

Answer 6

when electrons are emitted from the surface of a metal upon the absorption of electromagnetic radiation

Question 7

What are photoelectrons?

Answer 7

the electrons removed from metal using the photo electric effect

Question 8

What does the photoelectric effect provide?

Answer 8

• important evidence that light is quantised or carried in discrete packts
• shown by the fact each electron Cana absorb only a single photon
• this means the only frequencies of light above a threshold frequency will emit a photoelectron

Question 9

What is a definition of the threshold frequency?

Answer 9

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

Question 10

What is the definition of the threshold wavelength, related to threshold frequency by the wave equation?

Answer 10

the longest wavelength of incident electromagnetic radiation that would remove a photoelectron from the surface of a metal

Question 11

What is another word for the threshold energy?

Answer 11

work function (Φ)

Question 12

What is a definition of the work function?

Answer 12

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

Question 13

if the electron in a metal as trapped inside an ‘energy well’ where the energy between the surface and the top of the well is equal to the work function Φ, a single electron absorbs one photon, why is this?

Answer 13

an electron can only escape from the surface of the metal if it absorbs a photon which has an energy equal to Φ or higher

Question 14

What is the stopping potential of photoelectrons defined as?

Answer 14

the potential difference required to stop photoelectron emission from occurring

Question 15

What is the emitter plate?

Answer 15

• a plate which photons arrive at that causes photoelectrons to be emitted

Question 16

What is a collector plate?

Answer 16

electrons that travel from the emitter plate through a gap between the two metal plates meet a collector plate

Question 17

What equation is used to determine the energy pf a photon?

Answer 17

E=hf = hc/lamda

energy = plancks constant x frequency = (PC x 3x10^8)/wavelength

Question 18

why does a photon need to be of a minimum frequency to free an electron?

Answer 18

• the energy of the photon is determined by its frequency
• photons energy must be greater than the work function for an electron to be emitted

Question 19

what happens if a photon has a frequency higher than the threshold frequency, what would occur?

Answer 19

the electron will be liberated and the remaining energy is the kinetic energy of the electron

Question 20

if light is incident on a metal and photoelectric emission does not occur, what is the effect of increasing light intensity

Answer 20

• if ti is more intense more photons incident on the metal each second
• each phhoton carries the same amount of energy as before
• thus there is not enough energy to free an electron
• there is no effect

Question 21

What is the KE of a photoelectron and what is it dependent on?

Answer 21

• the intensity of the incident radiation
• each electron can only absorb one photon
• KE is only dependent on the frequency of the indident radiation
• increasing the number of photons striking the metal will not increase the KE of electrons, it will increase the number of photoelectrons emitted

Question 22

What is the intensity of a photoelectron and what is it dependent on

Answer 22

the rate of energy transferred per nit area and is related to the number of photons striking th metal plate

Question 23

Why is KE a maximum?

Answer 23

• each electron in the metal squires the same amount of energy from the photons in the incident radiation for any given frequency
• the energy required to remove an electron from the metal varies because some electrons are not he surface whilst others are deeper in the metal

Question 24

What is the photoelectric current?

Answer 24

• a measure of the number of photoelectrons emitted per second

Question 25

How is the value of the photoelectric current calculated?

Answer 25

• by the number of electrons emitted multiplied by the charge on one electron

Question 26

What is the photoelectric current proportional to and why?

Answer 26

• proportional to the intensity of the radiation incident on the surface of the metal
• because intensity is proportional to the number of photons striking the metal per second