3.2 More about photoelectricity

Question 1

What did Max Plank say?

Answer 1

» That the energy of each vibrating atom is quantised

Question 2

What is the definition of quantised?

Answer 2

» Only certain levels of energy are allowed

Question 3

What did Planck imagine the energy levels to be like?

Answer 3

» Rungs of ladder
» Each atom absorbing or emitting radiation when it moved up or down a level

Question 4

What did Einsteins photon model and Planck’s theory show?

Answer 4

» Showed that the energy is quantised

Question 5

What does the average kinetic energy of a conduction electron depend on?

Answer 5

» The temperature of the metal

Question 6

What is the work function of a metal?

Answer 6

» Minimum energy needed by a conduction electron to escape from the metal surface when the metal is at 0 potential

Question 7

What is the work function measured in?

Answer 7

» eV
» Need to times by 1.60 x 10^-19 to get into joules

Question 8

What happens when a conduction electrons absorbs a photon?

Answer 8

» Kinetic energy increases by an amount equal to the energy of the photon

Question 9

When can the conduction electron leave the metal?

Answer 9

» If the energy of the photon exceeds the work function of the metal

Question 10

What happens if the electron does not leave the metal?

Answer 10

» It collides repeatedly with other electrons and positive ions, and it quickly loses it extra kinetic energy

Question 11

What does the vacuum photocell contain?

Answer 11

» It is a glass tube that contains a metal plate, referred to as the photocathode
» And contains a smaller metal electrode referred to as the anode

Question 12

What happens when light of a frequency greater than the threshold frequency for the metal is directed at the photocathode?

Answer 12

» Electrons are emitted from the cathode and are attracted to the anode
» Microammeter in the circuit can be used to measure the photoelectric current

Question 13

What is the photoelectric current proportional to?

Answer 13

» The number of electrons per second that transfer from the cathode to the anode

Question 14

What is the equation for the number of photoelectrons emitted per second from the cathode to the anode?

Answer 14

» I/e
» Where I is the photoelectric current
» e is the charge of the electron

Question 15

What is the photoelectric current proportional to in terms of light?

Answer 15

» The light incident on the cathode

Question 16

What is the light intensity?

Answer 16

» The measure of the energy per second carried by the incident light
» Which is proportional to the number of photons per second incident on the cathode

Question 17

How is the the number of photoelectrons emitted per second( photoelectric current) proportional to the intensity of light?

Answer 17

» Each photoelectron must have absorbed one photo to escape from the metal surface

Question 18

Why does the intensity of light does not affect the maximum kinetic energy of a photoelectron?

Answer 18

» Energy gained by a photoelectron is due to absorption of one photon only
» Therefore, the maximum kinetic energy is still given by Ek max = hf - work function

Question 19

What is the purpose of a photocell?

Answer 19

» To measure the maximum kinetic energy of a the photoelectrons for a given frequency of light

Question 20

How can you find the value of h?

Answer 20

» If the measurements for different frequencies are plotted as a graph of Ekmax against f, a straight line in the form of y = mx + c is obtained
» Y= Ekmax, x = f, c = negative work function
m = planks constant

Question 21

Describe the process by which mercury atom becomes excited in a mercury atom?

Answer 21

» Mercury atoms collide with the electrons when the power is turned on
» Causes the mercury atoms to be excited as the collisions between the atoms and electrons lead to a transfer of energy
» Therefore the mercury atom becomes excited as the electrons gain energy from the collision and promote to a higher energy level