Quantam Physics

Question 1

What is the photoelectric effect?

Answer 1

It is the emission of photoelectrons from a metal’s surface after electromagnetic radiation of frequency above or equal to the threshold frequency is shone on the metal.

Question 2

What is the threshold frequency?

Answer 2

It is the minimum frequency of the em radiation required for electrons to be emitted from the metals surface.

Question 3

What is the work function of a metal?

Answer 3

It is the minimum energy required for an electron to leave the surface of the metal.Units:Joules.

Question 4

What are the 4 main conclusions from the photoelectric effect?

Answer 4

1. Photelectrons will only be emitted if the frequency of the light is >= threshold frequency.
2. The electrons are emitted with a variety of kinetic energies varying from 0 to a maximum.Increasing the frequency of the radiation increases the max ke.
3. Increasing the intensity of the radiation increases the number of electrons emitted per second.Number of electrons emitted is proportional to intensity

Question 5

How did the photoelectric effect demonstrate that light is made up of photons?

Answer 5

It was hypothesised that em waves existed in discrete packets of energy called photons.These photons had a one on one interaction with the electrons at the surface of the metal.Transferring all their energy proving that intensity does not have an effect on the max ke.

Question 6

How to calculate threshold frequency?

Answer 6

It equals the work function divided by plancks constant.

Question 7

How to calculate the max ke of the emitted electrons?

Answer 7

Kemax=hf-workfunction.

Question 8

What is the definition of the intensity of the radiation?

Answer 8

The number of photons per second incident on the metal per unit area.

Question 9

What is the stopping potential?

Answer 9

It is the potential required to bring a photoelectron emitted with max ke back to the surface of the metal.

Question 10

What is the relationship between stopping potential and max ke?

Answer 10

Kemax=eVs.

Question 11

What is an electron volt?

Answer 11

It is the ke gained by an electron after being accelerated through a pd of 1v.

Question 12

What is the ground state?

Answer 12

It is the lowest energy state in which an an atom can be in.

Question 13

How can an electron become excited?

Answer 13

It can become excited by absorbing a photon with exactly the energy difference between the two levels.The electron moves to a higher energy level,

Question 14

How can an electron become dexcited?

Answer 14

By emitting a photon with exactly the energy difference of the two levels.The electron can now move down energy level.

Question 15

What is ionisation?

Answer 15

It is when an electron is completely removed out of the atom from the ground state of the atom.

Question 16

What happens to the stability of the atom when it becomes excited?

Answer 16

It becomes unstable as there is now a vacancy in a lower energy level.Eventually an electron from a higher energy level will move down to fill this vacancy and emit a photon in the process.

Question 17

What is ionisation energy?

Answer 17

It is the energy required to completely remove an electron from the ground state of an atom to the outside of the atom.

Question 18

What can an excited atom do to go back to ground state?

Answer 18

It can dexcite directly or indirectly back to ground state.

Question 19

How does a fluorescent tube work?

Answer 19

It is a glass tube with a phosphor coating that contains low pressure mercury gas.When a voltage is applied across the tube free electrons are accelerated and collide with the mercury gas atoms exciting them.When they dexcite they emit UV photons.These photons are absorbed by the coating exciting the atoms.When they dexcite they emit visible light photons.

Question 20

What is a line emission spectra?

Answer 20

It is series of bright lines against a black background.The lines represent the wavelengths of lights emitted by the gas.These represent the photons of different energy emitted when an electron is dexcited.Each element has its own unique line emission spectra as each element has unique discrete energy levels.

Question 21

What type of spectrum does white light produce?

Answer 21

It produces a continuous spectrum of light.With decreasing wavelength to the left.

Question 22

What is a line absorption spectra?

Answer 22

It occurs when you pass white light (a light with continuous spectrum) through a cool gas and it absorbs photons with energy exactly equal to differences in the shells to excite its electrons.At a low temperature most electrons are at their ground state.

Question 23

What happens when light is passed through a narrow gap?

Answer 23

It diffracts like a wave, so it spreads out.

Question 24

How did the photoelectric effect show light was made up of particles?

Answer 24

Due to the fact light exists in discrete bundles of energy and they have a one on one interaction with an electron and transfer all its energy to it.

Question 25

What is the formula for the debroglue wavelength?

Answer 25

lambda=h/mv.

Question 26

How does electron diffraction work?

Answer 26

Electrons are accelerated at high velocities in a vacuum and then pass through the atoms of a graphite crystal sheet and diffract onto a screen.

Question 27

Describe the electron diffraction pattern?

Answer 27

Concentric circles are produced when an electron is diffracted.

Question 28

What happens to the diffraction pattern when the momentum changes?

Answer 28

When the momentum increases the debroglie wavelength reduces so the rings become more tightly packed together.When the momentum reduces the wavelength becomes larger so the diffraction pattern becomes wider.