Section B

Question 1

State and explain how the hardness of an x-ray beam is controlled in an x-ray tube

Answer 1

Change anode voltage; electrons striking the anode will have a changed kinetic energy; therefore the x-ray photons will have a different frequency

Question 2

Explain the purpose of the large uniform magnetic field in an MRI

Answer 2

Causes the nuclei to align, and causes the Larmor frequency to be in the RF region.

Question 3

Define frequency deviation

Answer 3

The amount by which the frequency of the carrier wave changes, per every 1 volt change in the signal

Question 4

Outline the advantages of AM over FM

Answer 4

Greater area covered for the same power output (whereas FM is line-of-sight);
Smaller bandwidth for each signal means that there can be more frequencies in any given range
Cheaper and simpler radio sets

Question 5

Outline the advantages of FM over AM

Answer 5

Less electrical interference and noise (as frequency is harder to change than amplitude)
Greater bandwidth, so better quality of sound

Question 6

Define noise

Answer 6

Random, unwanted power in a signal

Question 7

Define regeneration:

Answer 7

The act of removing noise from a digital signal

Question 8

Outline the advantages of digital over analogue transmission

Answer 8

Digital can be amplified and restored (so can be transmitted over longer distances, more accurately)
More easily stored and processed due to modern technology
Built-in safeguards mean part of signal can be re-sent if there is an error in reception

Question 9

Define sampling

Answer 9

The measurement of the analogue signal at regular time intervals

Question 10

Define cross-talk

Answer 10

When the signal on one channel creates and undesired effect on the signal from another channel

Question 11

Outline 3 causes of attenuation

Answer 11

In wires: Voltage drop due to resistance; energy lost due to electrical heating
In radio waves: absorption of energy through which wave travels
In optic fibres: light scattered or absorbed by irregularities in glass structure

Question 12

Describe surface waves

Answer 12

diffract around surface of earth; (0-3MHz frequency range); long-wave and medium wave radio in the low frequency band

Question 13

Explain the advantages of transmitting information at high frequencies

Answer 13

As frequency increases, bandwidth increases; therefore better quality signal.
As frequency increases, maximum distance travelled by the wave increases. This is because at higher frequencies waves have higher energy, so attenuation decreases

Question 14

Outline the advantages of using optical fibres

Answer 14

Less of a delay between transmission and reception than with satellite
Large bandwidth - so can carry large amounts of information simultaneously
Low attenuation per unit length, so fewer repeaters and regenerators needed
No electrical interference or cross-talk

Question 15

Outline the advantages of using satellites

Answer 15

Coverage area exceeds that of terrestrial system

Uses super-high- and ultra-high frequencies (SHF and UHF) so has a high bandwidth (large amounts of info per unit time)

Question 16

Outline the disadvantages of using satellites

Answer 16

Satellite bandwidth becoming used up
Larger propagation delay than with terrestrial communication
To maintain connection satellite dish must always point towards satellite

Question 17

Describe a geostationary orbit

Answer 17

Same orbital time period (T = 24 hours) as the earth
Rotates in same direction as Earth (west to east)
Remains stationary relative to Earth’s surface

Question 18

Describe polar orbits

Answer 18

Travel from pole to pole
Are at a smaller height above the Earth than geostationary, so can determine details (e.g. for espionage)
Have a shorter orbital period than Earth

Question 19

Define the decay constant (λ)

Answer 19

The probability than an individual nucleus will decay per unit time

Question 20

Define activity (A)

Answer 20

The rate at which radioactive nuclei decay (A = -λN; A = N/t)

Question 21

Outline the principles of CAT scanning

Answer 21

X-ray images are taken from different angles
In the same plane
These images are combined to give an image of a ‘slice’
Images of successive slices are combined
By computer
To form a 3-dimensional image
Which can be rotated/viewed from any angle

Question 22

Outline the principles behind MRI

Answer 22

A large, uniform magnetic field is applied along the body
Nuclei precess
An RF pulse is applied
at Larmor frequency
causes hydrogen nuclei
to resonate/absorb energy / excitate
After relaxation time, nuclei return to equilibrium state (de-excitate)
Each de-excitating electron emits an RF pulse
Pulses detected and processed by computer to display an image
Calibrated non-uniform field allows nuclei to be located

Question 23

State and explain the function of the non-uniform magnetic field

Answer 23

The Larmor frequency depends on the magnetic field strength
So knowing field strength allows nuclei to be located,
By knowing frequency

Question 24

Outline the advantages of MRI scanning

Answer 24

No harmful, ionizing radiation
Better soft-tissue contrast
Gives detailed 3-D image
No sensation/harmful side-effects for patient

Question 25

Explain why transformers use alternating (rather than constant) voltage

Answer 25

Alternating voltage gives rise to changing magnetic flux in core
Changing flux links the secondary coil
E.m.f only induced in secondary coil when flux is cut

Question 26

Explain why absorption spectra contain a number of dark lines

Answer 26

Electrons in gas interact with photons
If photon energy = energy difference between electron levels,
an electron will transition to a higher energy state
When electrons de-excitate, photons will be emitted in all directions

Question 27

Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal structures in the body

Answer 27

Pulse of ultrasound
produced by piezo-electric crystal
Gel medium between skin and transducer used to reduce reflection at skin
Wave reflected from boundaries between media
Reflected pulse detected by ultrasound transmitter
Reflected wave processed and displayed
Intensity of reflected pulse gives information about density of structures
Time delay gives information about depth of boundary

Question 28

State the effect on the transmitted analogue signal from an ADC when the sampling rate is increased

Answer 28

Smaller step depth

more accurate/precise

Question 29

State the effect on the transmitted analogue signal from an ADC when the number of bits is increased

Answer 29

Smaller step height
greater number of voltage levels
smaller voltage changes more easily seen