Section B Flashcards
State and explain how the hardness of an x-ray beam is controlled in an x-ray tube
Change anode voltage; electrons striking the anode will have a changed kinetic energy; therefore the x-ray photons will have a different frequency
Explain the purpose of the large uniform magnetic field in an MRI
Causes the nuclei to align, and causes the Larmor frequency to be in the RF region.
Define frequency deviation
The amount by which the frequency of the carrier wave changes, per every 1 volt change in the signal
Outline the advantages of AM over FM
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
Outline the advantages of FM over AM
Less electrical interference and noise (as frequency is harder to change than amplitude)
Greater bandwidth, so better quality of sound
Define noise
Random, unwanted power in a signal
Define regeneration:
The act of removing noise from a digital signal
Outline the advantages of digital over analogue transmission
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
Define sampling
The measurement of the analogue signal at regular time intervals
Define cross-talk
When the signal on one channel creates and undesired effect on the signal from another channel
Outline 3 causes of attenuation
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
Describe surface waves
diffract around surface of earth; (0-3MHz frequency range); long-wave and medium wave radio in the low frequency band
Explain the advantages of transmitting information at high frequencies
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
Outline the advantages of using optical fibres
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
Outline the advantages of using satellites
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)
Outline the disadvantages of using satellites
Satellite bandwidth becoming used up
Larger propagation delay than with terrestrial communication
To maintain connection satellite dish must always point towards satellite
Describe a geostationary orbit
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
Describe polar orbits
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
Define the decay constant (λ)
The probability than an individual nucleus will decay per unit time
Define activity (A)
The rate at which radioactive nuclei decay (A = -λN; A = N/t)
Outline the principles of CAT scanning
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
Outline the principles behind MRI
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
State and explain the function of the non-uniform magnetic field
The Larmor frequency depends on the magnetic field strength
So knowing field strength allows nuclei to be located,
By knowing frequency
Outline the advantages of MRI scanning
No harmful, ionizing radiation
Better soft-tissue contrast
Gives detailed 3-D image
No sensation/harmful side-effects for patient
Explain why transformers use alternating (rather than constant) voltage
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
Explain why absorption spectra contain a number of dark lines
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
Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal structures in the body
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
State the effect on the transmitted analogue signal from an ADC when the sampling rate is increased
Smaller step depth
more accurate/precise
State the effect on the transmitted analogue signal from an ADC when the number of bits is increased
Smaller step height
greater number of voltage levels
smaller voltage changes more easily seen
