Quantum Mechanics

Question 1

Define a photon

Answer 1

a packet (quanta) of energy

Question 2

State the order of the electromagnetic spectrum, in terms of increasing frequency

Answer 2

Radiowaves

Microwaves

Infrared

Visible

Ultraviolet

X-rays

Gamma rays

Question 3

What do all electromagnetic waves have in common?

Answer 3

1. They all travel at the speed of light in a vacuum
2. They are transverse waves.

Question 4

How are electromagnetic waves emitted?

Answer 4

From a charged particle when it loses energy, this can happen when:

An electrom moves from a higher to a lower energy state

A fast moving electron is stopped e.g. x-rays

Question 5

What is the relationship between photon energy and frequency?

Answer 5

Directly proportional

Question 6

What is the relationship between photon energy and wavelength?

Answer 6

inversely proportional

Question 7

Define an eV

Answer 7

1 eV is the energy gained by an electron passing through a potential difference of 1 V

Question 8

Describe the photoelectric effect experiment?

Answer 8

1. An insulating rod is given a negative charge.
2. The negative charge is passed onto a zinc plate.
3. The charge moves down onto the gold leaf
4. The gold leaf repels and deflects away from the center
5. Various light source are shone onto the zinc plate

Question 9

Describe the observations of the photoelectric experiment

Answer 9

1. When UV light (high frequency) light is shown on the zinc plate the leaf falls immediately so electrons are being lost from the plate
2. If a filament lamp (lower frequency) is used the electroscope doesn’t discharge (gold leaf doesn’t move)
3. When very intense visible light is used, no electrons are emitted (gold leaf doesn’t move)
4. If very low intensity UV light is used the discharge starts immediately

Question 10

Describe the conclusions of the photoelectric experiment

Answer 10

1. There is a threshold frequency (minimum frequency) below which no photoelectrons are released
2. The light is not interacting as a wave as enough energy would be eventually be transferred and photoelectrons would be released
3. light is interacting as single photons** with the **surface electrons in an one to one interaction
4. If the photons have enough energy/frequency (E=hf) then a photoelectron is released.
5. If the frequency of the wave is to small it won’t have enough energy to release a photoelectron with a one to one interaction

Question 11

Define threshold frequency

Answer 11

Threshold frequency – minimum frequency of radiation needed to release photoelectrons

Question 12

Define work function

Answer 12

Work function – energy an electron must gain to be released from the metal

Question 13

Define intensity

Answer 13

The amount of energy arriving per second per m²

Question 14

In the photoelectric effect what is the affect of increasing the intensity of visible light?

Answer 14

No effect, the frequency will be below the threshold frequency so no electrons will be released

Question 15

In the photoelectric effect , if UV radiation is used with a constant frequency but the intensity is increased, what is the effect?

Answer 15

More photoelectrons are released each second - If intensity is increasing but each photon has a constant energy (E = hf) then more photons must be arrving and interacting with electrons per second.

Each photoelectron will have a constant kinetic energy as the energy of the incoming photons is constant (E=hf)

Question 16

In the photoelectric effect, if UV radiation of constant intensity was used but its frequency was increased what would be the effects?

Answer 16

Less electrons would be released each second - the same energy must be arriving per second but each photon has more energy - so less photons must be arriving per second

Each released photoelectron will have more kinetic energy as the incoming photons have more energy (E = hf)

Question 17

Describe the stopping potential experiment

Answer 17

1. Photoelectrons are emitted from a metal surface.
2. They then have to travel towards the negative terminal of a battery.
3. This means they have to do work against the potential difference.
4. The stopping potential is the p.d. needed to stop the fastest electrons with the maximum kinetic energy.
5. The work done by the p.d. is equal to the initial energy of the electrons.
6. W = VQ

Question 18

From a graph of kinetic energy against frequency what can be found out?

Answer 18

Gradient = h

x intercept = threshold frequency

y intercept = - work function

Question 19

Describe excitation

Answer 19

An electron can move up an energy level by gaining energy (e.g. via heating or collisions with other particles). This is called excitation.

Question 20

Describe how excitation happens via collision with electrons

Answer 20

Electrons collide with atom transfering some or all of its energy.

If the electron has more energy than the energy difference between the energy levels, then the eletron will take the excess energy away as kinetic energy.

If the electron has the same energy as the difference between energy levels then it will transfer all of its energy and the electron will stop.

If the electron has less energy than then difference between the energy levels then it will be deflected by the atom with no loss in kinetic energy or excitation.

Question 21

Describe excitation by using photons

Answer 21

Electrons in an atom absorb a photon and all of its energy. This can only happen if the energy of the photon is exactly equal to the energy the electron would need to gain to move to a higher energy level.

If the photons energy is smaller of larger than the difference between the two energy levels then it wont be absorbed.

Question 22

Describe relaxation

Answer 22

An electron can move down to a lower energy level by losing energy. This is done by releasing a photon. This is called relaxation.

Question 23

Define ionisation

Answer 23

If an electron gains enough energy it can escape the atom. This is called ionisation.

Question 24

Why do energy levels have negative values?

Answer 24

1. For an electron that is bound to the atom, energy has to be added to release it.
2. Work has to be done to move an electron to a higher energy level
3. But we define an electron as just being ionized as having zero energy.
4. So we say that the electron in an energy level has negative energy.

Question 25

Explain how fluorescent tubes work

Answer 25

1. Fluorescent tubes contain mercury vapour across which a high p.d. is applied. 2. The high voltage accelerates free electrons and ionise some mercury atoms 3. The free electrons collide with electrons in the mercury atoms the electrons are excited to a higher energy level. 4. When these electrons relax they lose high energy photons in the UV range. 5. A phosphorous coating on the inside of the tube absorbs these photons, which excites the electrons to a higher energy level. 6. These electrons then relax and release lower energy photons of visible light

Question 26

How is a line emission spectrum produced?

Answer 26

Given out by gas atoms which have been excited through discharge of electricity or by heating to high temps. Seen as discrete lines as they contains only specific energies, frequencies and wavelengths The electrons in the atom can only orbit at certain distances (shells/levels) from the nucleus, i.e. the radius of the orbit is “quantized” An excited electrons will jump down (relax) to a lower energy level. In jumping down the electrons emit a photon of energy equal to the difference in the energy between the two levels. (E= hf)

Question 27

Explain why line spectra are seen as discrete lines

Answer 27

Line spectra are seen as discrete lines as they contains only specific energies, frequencies and wavelengths. The electrons in the atom can only orbit at certain distances (shells/levels) from the nucleus, i.e. the radius of the orbit is “quantized” An excited electrons will jump down (relax) to a lower energy level. In jumping down the electrons emit a photon of energy equal to the difference in the energy between the two levels. (E= hf)

Question 28

What is a continuous spectrum?

Answer 28

A continuous spectrum is a spectrum in which all wavelengths/frequencies are present between certain limits. It is given out by hot glowing objects. All wavelengths are present because the electrons are not bound to atoms and are free so they are not confined to specific energy levels.

Question 29

What is an absorption spectrum?

Answer 29

Caused by atoms of hot gas absorbing particular frequencies or wavelength as white light passes through. seen as continuous spectra with dark bands

Question 30

State some wave properties

Answer 30

1. Refraction 2. Diffraction 3. Interference 4. Polarisation

Question 31

State some particle properties

Answer 31

1. Light travels in straight lines 2. Photoelectric effect 3. Carrying a charge/mass 4. Deflection by an electric or magnetic field

Question 32

Describe an experiment that showed electrons to behave like a wave

Answer 32

Electron diffraction through graphite - Electrons fired from a cathode so they pass through a graphite target. A series of bright rings can be seen on a phosphorescent screen.

Question 33

State the evidence that electrons can act like waves

Answer 33

Electrons are being **_diffracted**_ as they pass through the graphite. Electrons are _**constructively interfering**_ to produce bright rings. Electrons are _**destructively interfering_** to produce dark rings. The same pattern build up with just one electron at a time.

Question 34

What is the condition required for noticeable diffraction?

Answer 34

For diffraction to be noticeable the wavelength must be similar to the size of the gap it is passing through

Question 35

Describe the two main methods that cause excitation

Answer 35

1. Collision with electrons 2. Absorbing a photon