Quantum Mechanics

Question 1

Define a photon

Answer 1

a packet (quanta) of energy

Question 2

What is the relationship between photon energy and frequency?

Answer 2

Directly proportional

Question 3

What is the relationship between photon energy and wavelength?

Answer 3

inversely proportional

Question 4

Define an eV

Answer 4

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

Question 5

Describe the photoelectric effect experiment?

Answer 5

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

Question 6

Describe the observations of the photoelectric experiment

Answer 6

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

Question 7

Describe the conclusions of the photoelectric experiment

Answer 7

there is a threshold frequency (minimum frequency) below which no photoelectrons are released
the light is not interacting as a wave as enough energy would be eventually be transferred and photoelectrons would be released
light is interacting as single photons with the surface electrons in an one to one interaction
If the photons have enough energy/frequency (E=hf) then a photoelectron is released.
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 8

Define threshold frequency

Answer 8

Minimum frequency of electromagnetic radiation to cause electron emission from a metal surface.

Question 9

Define work function

Answer 9

Minimum energy needed to liberate an electron from a metal surface.

Question 10

Define intensity

Answer 10

The amount of energy arriving per second per m2

Question 11

Describe the stopping potential experiment

Answer 11

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

Question 12

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

Answer 12

Gradient = h
x intercept = threshold frequency
y intercept = - work function

Question 13

Describe excitation

Answer 13

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 14

Describe relaxation

Answer 14

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 15

Define ionisation

Answer 15

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

Question 16

Why do energy levels have negative values?

Answer 16

For an electron that is bound to the atom, energy has to be added to release it. Work has to be done to move an electron to a higher energy level
So we say that the electron has negative energy.
An electron that has just been removed (‘IONIZED’) is defined as having zero energy.

Question 17

Explain how fluorescent tubes work

Answer 17

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

Question 18

What is a line emission spectrum?

Answer 18

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 19

What is a continuous spectrum?

Answer 19

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 20

What is an absorption spectrum?

Answer 20

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

Question 21

State some wave properties

Answer 21

Refraction
Diffraction
Interference
Polarisation

Question 22

State some particle properties

Answer 22

Light travels in straight lines
Photoelectric effect
Carrying a charge/mass
Deflection by an electric or magnetic field

Question 23

Describe an experiment that showed electrons to behave like a wave

Answer 23

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 24

State the evidence that electrons can act like waves

Answer 24

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 25

What is the condition required for noticeable diffraction?

Answer 25

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