units 22-25

Question 1

What is a photon?

Answer 1

A photon is a quantum of electromagnetic energy.

Question 2

What is an electronvolt?

Answer 2

An electronvolt is the energy gained by an electron travelling through the potential difference of 1 Volt.

Question 3

What is the photoelectric effect?

Answer 3

The photoelectric effect is the emission of electrons from a metallic surface when electromagnetic radiation is incident on it.

Question 4

What is threshold frequency?

Answer 4

Threshold frequency is the minimum frequency required to release electrons from the surface of a metal.

Question 5

What is threshold wavelength?

Answer 5

Threshold wavelength is the longest wavelength of the incident electromagnetic radiation that would eject electrons from the surface of a metal.

Question 6

What is the work function of a metal?

Answer 6

The work function of a metal is the minimum amount of photon energy required to remove an electron from its surface.

Question 7

What is the de Broglie wavelength?

Answer 7

The de Broglie wavelength is the wavelength of a moving particle, which is dependent on its momentum.

Question 8

What are photon energies?

Answer 8

Photon energies are the energy of a photon, absorbed or emitted, as a result of an electron making a transition between two energy levels E1 and E2.

Question 9

What evidence supports the particulate behaviour of electromagnetic radiation?

Answer 9

The photoelectric effect provides evidence for the particulate behaviour of electromagnetic radiation.

Question 10

What evidence supports the wave behaviour of particles?

Answer 10

Electron diffraction provides evidence for the wave behaviour of particles.

Question 11

What is mass defect of a nucleus?

Answer 11

It is equal to the difference between the mass of the individual separate nucleons and the mass of the nucleus when the nucleons are separated.

Question 12

What is binding energy of a nucleus?

Answer 12

It is the minimum energy needed to separate its nucleons to infinity.

Question 13

What is fission?

Answer 13

It is the process in which a massive nucleus splits to form two or more smaller fragments (A>56).

Question 14

What is fusion?

Answer 14

It is the process by which 2 very light nuclei join together to form a heavier nucleus (A<56).

Question 15

What is radioactive decay?

Answer 15

The spontaneous emission of particles/radiation by an unstable nucleus.

Question 16

What does SPONTANEOUS mean in the context of decay?

Answer 16

It happens unprovoked and cannot be controlled how and when it occurs. It is not affected by environmental factors, chemical reactions or external factors like temperature or pressure; nor by the presence of other nuclei.

Question 17

What does random mean in the context of decay?

Answer 17

It is impossible to predict when a nucleus in a sample is going to decay.
Constant probability of decay of a nucleus
Fluctuations in count rate provide evidence that nuclear decay is random

Question 18

What is decay constant?

Answer 18

It is the probability that an individual nucleus will decay per unit time interval.

Question 19

What is activity (A) of a radioactive sample?

Answer 19

It is the number of decay per unit time.

Question 20

What is half-life (t1/2) of an isotope?

Answer 20

It is the mean time taken for half of the active nuclei in a sample to decay.

Question 21

What final result do nuclear reactions lead to?

Answer 21

They always lead to greater/higher nucleon stability and release of kinetic and gamma energy.

Question 22

What is attenuation?

Answer 22

The gradual decrease in the intensity of a beam of x-rays as it passes through matter.

Question 23

What is half-thickness of a material?

Answer 23

The thickness that will reduce the transmitted intensity of an X-ray beam of a particular frequency to half of its original value.

Question 24

How is X-ray generated?

Answer 24

Electrons are accelerated by an applied p.d. When electrons hit the target, they rapidly accelerate and some of the energy is converted to x-rays.

Question 25

What is contrast in x-ray images?

Answer 25

The difference in the degrees of blackening.

Question 26

What is specific acoustic impedance?

Answer 26

The product of density and the speed of ultrasound in the medium.

Question 27

What is a tracer?

Answer 27

A substance containing radioactive nuclei that can be introduced into the body and is then absorbed by the tissue being studied.

Question 28

What is annihilation?

Answer 28

Occurs when a particle interacts with its antiparticle so that their mass is converted into energy. During the process, mass-energy is conserved.

Question 29

How does CT Scanning work?

Answer 29

The purpose of CT scanning is to produce a 3-dimensional image of structure/body In a CT scanner, the X-ray source is rotated around the patient. Detectors are set up around the patient so many scans are taken from many different angles. From the beam intensity detected in the different directions, an image of a narrow slice throughout the body is built up. This is repeated for many slices and the images of consecutive slices are combined to form a 3D image

Question 30

What is the piezoelectric effect?

Answer 30

Applied voltage causes a piezo-electric crystal to contract or expand. An applied stress causes an induced emf across the crystal.

Question 31

How is ultrasound generated?

Answer 31

An alternating p.d. across a quartz crystal causes it to vibrate (piezoelectric effect) Resonance occurs when frequency of p.d. matches natural frequency of crystal The natural frequency of crystal is in ultrasound range

Question 32

How is ultrasound detected?

Answer 32

Pulses of ultrasound are incident on a quartz crystal. The waves make the crystal oscillate, generating an alternating e.m.f. which is detected.

Question 33

How is ultrasound used?

Answer 33

Ultrasound pulses are generated and detected by quartz crystal The pulses are reflected at boundaries between different materials and detected by ultrasound generator Gel is used to minimise reflection at skin The time delay between generation and detection gives information about the depth of the material. The intensity of reflected wave gives information about the nature of the boundary Degree of reflection depends upon impedance of two media at boundary ## Footnote Depth of material relates to speed of sound in material.

Question 34

+What role does gel play in ultrasound?

Answer 34

Gel is used to minimize reflection at skin.

Question 35

How does a PET scan produce an image?

Answer 35

A tracer containing a radioactive isotope is injected into the body and decays via β+ decay (positron emission). The emitted positron meets an electron and annihilation occurs, which produces 2 gamma photons. The two gamma photons travel in opposite directions. The gamma photons are detected outside body by detectors. The gamma photons arrive at the detectors at different times. The time difference can be used to determine the location of production of the gamma rays. The produced image shows the tracer concentration in specific tissues.

Question 36

+What does the produced image in a PET scan show?

Answer 36

The tracer concentration in specific tissues.

Question 37

What is the luminosity of a star?

Answer 37

The total power of radiation emitted by a star.

Question 38

What is a standard candle in astronomy?

Answer 38

An astronomical object of known luminosity.

Question 39

What is radiant flux intensity?

Answer 39

It is defined as the radiant power passing normally through a surface per unit area.

Question 40

What does Wien’s displacement law state?

Answer 40

In blackbody radiation, the wavelength of maximum intensity is inversely proportional to the thermodynamic temperature.

Question 41

What is redshift?

Answer 41

It shows that wavelengths (of spectral lines) are greater than their known values, indicating that stars are moving away from Earth.

Question 42

What does Hubble’s Law describe?

Answer 42

The speed of recession of a galaxy (from observer) is directly proportional to the distance of that galaxy from the observer.

Question 43

What is evidence for the Big Bang?

Answer 43

1. All parts of Universe are moving away from each other (Redshift) 2. More distant objects are moving away faster (Hubble) Hence these ideas tell us that matter must have been close together/very dense in the past.