Quantum Physics

Question 1

State the unit of work function

Answer 1

Joules

Question 2

Explain why the kinetic energy of emitted electrons has a maximum value

Answer 2

Because the energy available depends on the frequency of the radiation and can be no more than the (hf)

Question 3

Explain with reference to the work function why, if the frequency of the radiation is below a certain value ,electrons are no emitted

Answer 3

In order for electrons to be emitted from an electron well. A minimum amount of energy is needed , this is called the work function

Question 4

State what is meant by work function

Answer 4

Minimum energy required to remove electrons from a metal

Question 5

In fluorescent tune ,explain how the mercury vapour and the coating of its inner surface contribute to the production of visible light

Answer 5

mercury vapour at low pressure is conducting (1) atoms of mercury are excited by electron impact (1) producing (mainly) ultra violet radiation (1)
which is absorbed/ excites the coating (1)
which, upon relaxing, produces visible light

Question 6

State what is meant by ground state

Answer 6

when electrons/atoms are in their lowest/minimum energy

Question 7

An atom can also become exited by absorption of photons. Explain why only photons of certain frequencies cause excitation in a particular atom

Answer 7

electrons occupy discrete energy levels
and need to absorb an exact amount of/enough energy to move to a higher level photons need to have certain frequency to provide this energy or e = hf
energy required is the same for a particular atom or have different energy levels all energy of photon absorbed

Question 8

What is meant by an exited atom

Answer 8

An electron has moved to a higher energy level

Question 9

Describe the process by which mercury atoms become exited in a fluorescent tube

Answer 9

Current flows through the tube
The electrons collide with mercury atoms
Raising the electrons to a higher energy level in the mercury atoms

Question 10

What is the purpose of the coating in a fluorescent tube

Answer 10

High energy photons are absorbed by the coating powder, this powder then emits photons in the visible spectrum
Incident photons have a variety of different wavelengths

Question 11

Explain why fluorescent tube contains vapour at a low pressure

Answer 11

Mercury is a liquid at room temperature - in order for it to turn to a gaseous stable , the pressure must be kept low

Question 12

6 marker - the photoelectric effect suggests that electromagnetic waves exhibit particle behaviour. Explain what the threshold frequency is and why it’s existence supports particle nature of electromagnetic waves

Answer 12

Threshold frequency - minimum frequency for emission of electrons

• if frequency below threshold frequency, no emission of electrons, even if intensity increases
• as energy of photon is less than work function
• wave theory cannot explain this as energy of wave increases with intensity

Question 13

Ionisation may be caused by electron impact.

Explain how else might an atom be ionised ?

Answer 13

Impact by ionising radiation

Question 14

What is ionisation energy

Answer 14

The minimum energy needed to move the electron from the atom completely from the ground state

Question 15

Why do exited mercury atoms emit photons of characteristic frequencies

Answer 15

Mercury atoms have fixed energy levels
When electrons change levels they lose a specific amount of energy
Leading to photons of specific frequencies

Question 16

What does the gradient of E=hd graph represent

Answer 16

Place constant

Question 17

Explain the meaning of threshold frequency

Answer 17

Minimum photon frequency that will emit electrons from the metal surface

Question 18

At a fixed frequency of incident radiation, the emitted electrons are collected and a current recorded - how can you increase this current ?

Answer 18

Increase brightness
Increases frequency of photons being emitted by light source
More photons being available to liberate electrons

Question 19

Explain why no photoelectrons are emitted when a blue filter is replaced by a red filter

Answer 19

The photons energy for the red filter is lower than the work function of the metal so no electrons can be emitted

Question 20

How does a fluorescent tube work

Answer 20

1) Contain mercury vapour, across which a high voltage is applied
2) this voltage accelerates fast moving free electrons that ionise some of the mercury atoms, producing more free electrons
3) when these collide with mercury atoms they are exited to higher energy levels
4) when these electrons return to their ground state they lose energy by emitting high-energy photons
5) phosphorus coating absorbs them and causes its electrons to emit visible light photons

Question 21

Explain how the threshold frequency provides evidence against the wave theory of light

Answer 21

If light only acted as a wave , the electrons in the metal would slowly absorb incident light waves until they had enough energy to escape the metal - otherwise there would be no threshold frequency

Question 22

The atom in ground state is given energy - state what happens to this energy

Answer 22

Electron in ground state will leave atom with the remaining energy

Question 23

State what will happen to subsequent electrons when one leaves

Answer 23

Electrons in higher energy levels may fall down to replace vacancy left by emitted electrons

Question 24

Give one example of particles acting as a wave

Answer 24

Diffraction - it can only be explained using waves. If light was acting as a particle , the light particles in the beam would either go through the gap

Question 25

Explain why different frequencies are possible when electrons move back down to lower energy levels

Answer 25

Electrons may cascade down to lower levels through different pathways

Question 26

Give an example of a use of a superconductor

Answer 26

bullet trains/powerful magnets

Question 27

Why are energy levels always negative

Answer 27

It’s because the electrons are “bound” to the nucleus, and so you have to put energy in to move it up energy levels