2.5 Quantum Physics

Question 1

Define the photoelectric effect

Answer 1

occurs when electrons are ejected from a metal when electromagnetic radiation of sufficiently high frequency falls on it

Question 2

What is Planck’s constant

Answer 2

6.63 x10-32 Js

Question 3

What is the work function

Answer 3

the minimum amount of energy needed to liberate electrons from the surface of a metal and to just allow it to escape to an infinite distance from the metal

Question 4

Define the electron-volt (eV)

Answer 4

the energy an electron gains or loses when it moves thru a potential difference of 1V

Question 5

How many joules is 1eV

Answer 5

1eV = 1.6x10-19 J

Question 6

Electrons closer to the nucleus have more/less energy

Answer 6

less energy

Question 7

When an electron in an atom has the lowest possible energy, it is in

Answer 7

ground state

Question 8

What is excitation

Answer 8

an electron can move from one energy level to another higher energy level by absorbing a photon of energy equal to the energy difference between the two states

Question 9

What is relaxation

Answer 9

if an electron in an excited state moves from a high energy level to a vacant lower energy level, a photon of light of energy exactly equal to the energy difference between the two states will be emitted

Question 10

Photons emitted by a laser are… (3)

Answer 10

coherent, have the same wavelength and have a constant phase difference

Question 11

How long do electrons typically spend in the excited state

Answer 11

10^-8 seconds

Question 12

What is population inversion

Answer 12

When there are more electrons in the excited state than the ground state

Question 13

How long can electrons exist in metastable states

Answer 13

10^-3

Question 14

What is stimulated emission

Answer 14

the induction of an electron in a metastable state to be excited into a transition by the presence of a photon of energy equal to the energy difference between the levels

Question 15

What does the acronym LASER stand for?

Answer 15

Light Amplification By Stimulated Emission of Radiation

Question 16

What are electrons in an X-ray emitted by

Answer 16

Cathode which is heated white hot by electric current passing thru it

Question 17

What are the electrons in an X-ray accelerated toward

Answer 17

metal anode

Question 18

Two properties of the metal target in the metal anode

Answer 18

high melting point and high atomic no.

Question 19

Why is an element w a high atomic no. used in X-ray tubes?

Answer 19

As have high atomic no., electron shells generally filled w total complement of electrons so incoming electron knocks out one of these electrons, and this vacancy is immediately replaced by an electron from a higher energy shell coming down. It loses its energy as an X-ray of v specific energy or wavelength

Question 20

In atoms of low atomic no., transitions to the inner shell give rise to the emission of —– light

Answer 20

ultra violet

Question 21

Why are metals like aluminium unsuitable as X-ray tube targets?

Answer 21

Low atomic no. so emission spectra have wavelengths higher than X-rays

Question 22

What does CT stand for in CT Scan?

Answer 22

Computed Tomography

Question 23

What does CAT in CAT Scanning stand for

Answer 23

Computed Axial Tomography

Question 24

What do CT Scans image (broad)(3)

Answer 24

soft tissue, bone and blood vessels

Question 25

How do CT Scans work (brief)

Answer 25

combines use of a digital computer with a rotating X-ray device to create detailed cross-sectional images of different images and body scans

Question 26

What is an advantage of the CT Scans short scan time?

Answer 26

can be used when the body’s in motion (eg. breathing)

Question 27

CT Scans are especially good fro diagnosing what cancers? (3)

Answer 27

lung
liver
pancreatic

Question 28

How do CT Scans detect osteoporosis

Answer 28

measuring bone mineral density

Question 29

Why are CT Scans good for diagnosing inner ear and sinuses problems

Answer 29

can generate v high resolution images and these areas made up of delicate soft tissue structures and fine bones

Question 30

What can CT Scans detect in the sinuses?

Answer 30

Polyps or tumours and diseases causing degeneration of small bones in the inner ear

Question 31

Why are other methods considered before a CT Scan is carried out on children?

Answer 31

X-Ray dose from CT much larger than from conventional radiography and children are particularly sensitive to radiation

Question 32

What do patients have to do before getting a CT scan of their abdomen and why

Answer 32

must drink several cups of contrast agent about an hour before scanning because the agent has high density so bowel highlighted in white and easily distinguished from. intra-abdominal masses

Question 33

What do conventional X-rays do?

Answer 33

Produces a 2D picture that can help find fractures, tumours and foreign objects

Question 34

How are bone and muscle distinguished in X-rays

Answer 34

Denser objects such as bones absorb more radiation than less dense material such as muscle

Question 35

Why do bones appear white on X-Rays?

Answer 35

they absorb more X-Rays and their ‘shadow’ is cast on X’Ray film

Question 36

Radiographers hide behind a —- screen during an X-Ray

Answer 36

lead screen

Question 37

Why do modern hospitals use digital detectors instead of sensitive photographic material?

Answer 37

digital detectors are faster, cheaper, more renewably made and produce no waste

Question 38

Light is said to be “quantized”. What does this mean

Answer 38

Light only exists in packets of discrete energy which depend on its frequency

Question 39

What is meant by ‘optical pumping’?

Answer 39

The process by which a population inversion is achieved

Question 40

What is a meta stable state

Answer 40

One in which the electrons can stay much longer than in an excited state

Question 41

Why are all the energy levels of an atom negative?

Answer 41

Energy must be supplied to electrons in any of these states to ionize the atom and give the electron 0 energy; so each state is one of negative energy

Question 42

Why is it important that the target material in an X-ray tube is a good conductor?

Answer 42

Most of the incident electrons energy is converted to heat; to dissipate that heat requires a good comductor

Question 43

Why is it important that the target material in an X-ray tube has a high melting point?

Answer 43

The target will become v hot bcus of the incident electrons energy that’s converted to heat so if it is not to melt, target needs a high melting pt

Question 44

Both copper and aluminum are good conductors of heat and have high melting pts. Why is copper, but not aluminum, used in X-ray tubes?

Answer 44

Transitions between levels must be large enough to be in the X-Ray energy range; copper is, aluminum is not.

Question 45

The continuous X-ray spectrum is produced by a process called Braking radiation. What is this?

Answer 45

Incident electrons are rapidly decelerated as they approach the target atoms. They lose their KE in a series of collisions. This loss of kinetic energy is converted to X-Rays.

Question 46

Why is a computer essential in CT Scanning, but not in conventional X-Ray photographs?

Answer 46

A computer is required to build up a 3D image in tomography; a conventional X-Ray photograph gives only a 2D image and requires no imaging software