physics chapter 3.1-3.3 quantum phenomena

the photoelectric effect

electrons are emitted from the surface of a metal when electromagnetic radiation above a certain frequency is directed at the metal

the threshold frequency

the minimum frequency of the incident radiation required for photoelectric emission of electrons from a metal surface to take place

what factor determines threshold frequency

the type of the metal

what is the number of electrons emitted per second proportional to

the intensity of the incident radiation, provided the frequency is greater than the threshold frequency

when does photoelectric emission occur once the incident radiation is directed at the surface

without delay, provided the frequency of the radiation exceeds the threshold frequency

wave theory of light

states that light consists of a series of waves rather than individual particles

energy of a photon

the Planck constant, h, x frequency, f,

the work function of the metal

the minimum energy needed by an electron to escape from the metal surface - excess energy gained by photoelectrons becomes its kinetic energy

equation for maximum kinetic energy of an emitted electron

Ek = hf - work function

equation for the threshold frequency of the metal

f min = work function / h

the stopping potential

the minimum potential needed to stop photoelectric emission - at this potential, the maximum kinetic energy of the emitted electron is reduced to zero as each emitted electron must do extra work to leave the metal surface, equal to e x stopping potential

quantised

only certain levels of energy are allowed

what does average kinetic energy of conduction electrons depend on

the temperature of the metal - conduction electrons in a metal move around at random

what happens when a conduction electron absorbs a photon

its kinetic energy increases by an amount equal to the energy of the photon - if energy of photon exceeds work function of metal, the conduction electron can leave the metal

what happens if an electrons that has absorbed a photon doesn't leave the metal

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

equation for the number of photoelectrons per second that transfer from the cathode to the anode in a vacuum photocell

current / charge of the electron ( I / e )

light intensity

a measure of the energy per second carried by the incident light

what is the photoelectric current in a photocell vacuum proportional to

the intensity of the light incident on the cathode - because each photoelectron must have absorbed one photon to escape from the metal surface

how does intensity of incident light affect maximum kinetic energy of a photoelectron

it doesn't affect it - energy gained by a photoelectron is due to the absorption of one photon only

an ion

a charged atom

how is an ion formed

from an uncharged atom by adding or removing electrons from the atom

ionisation

any process in which atoms become charged - any process of creating ions

how can you measure the energy needed to ionise a gas atom

by making electrons collide at increasing speed with the gas atoms in a sealed tube

how can you calculate the ionisation energy of a gas atom in a sealed tube

by measuring the pd between the filament and the anode, + when current starts to increase, the ionisation energy of a gas atom is equal to the work done on each electron from the filament = electron charge x pd (eV)

electron volt

a unit of energy equal to the work done when an electron is moved through a pd of 1V

excitation

when gas atoms absorb energy from colliding electrons without being ionised - happens at certain energies which are characteristic of the atoms of the gas

excitation energies

the energy values at which an atom absorbs energy - can be determined by increasing the pd between the filament + the anode in a gas-filled tube and measure the pd when the anode current falls

what happens when excitation occurs

the colliding electron makes an electron inside the atom move from an inner shell to an outer shell

why is energy needed for excitation to occur

because the atomic electron moves away from the nucleus of the atom