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ADVANCED Physics

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Lesson 10

RELATIVE ADVANTAGES OF AM TRANSMISSION

RELATIVE ADVANTAGES OF FM TRANSMISSION

The transmitters can serve a large area because

of the capability to broadcast long-wave (LW),

medium-wave (MW) and short wave (SW) wavebands

over very large distances.

Has better sound quality compared with AM transmission

It is simpler and cheaper to broadcast than using

FM transmission.

It can avoid the interference of noise better than with AM

transmissions because noise affects amplitude and not the

varying frequency.

The human ear can detect the sound transmitted

because the maximum frequency that can be broadcast

is 4.5kHz and it lies within the frequency range that is picked

up by the human ear and it is adequate for speech.

This is not the case with FM transmissions.

higher bandwidth (for the same range of broadcast frequencies)

is broadcast than in AM transmission because the FM waveform

has additional side frequencies that are multiples of the

information signal frequency.


FM receivers capture stronger signals and eliminate the weaker

signals.


Needs a smaller number of transmitters that can share the same

waveband than for AM


Can be used to serve small local areas


Lesson 15


Nuclear Magnetic Resonance Imaging (NMRI)

Nuclear Magnetic Resonance

  • Property of atomic nuclei is ‘spin’.

  • Spin enables the atomic nuclei to behave as if they were small magnets.

  •  The nuclei have odd numbers of protons and/or neutrons e.g. hydrogen, carbon and phosphorus.

  • Alignment is not perfect and nuclei rotate about the direction of the magnetic field, due to their spin. This rotation is known as precession.

  • The spinning about the direction of the magnetic field has a frequency known as the frequency of precession or the Larmor Frequency.

  • The frequency depends on:

    • The nature of nucleus 

    • Magnitude of Magnetic Flux Density.

  • This frequency is in the radio-frequency (RF) band with a wavelength shorter than about 10cm. A short time after the (resonance) the incident pulse has ended, the nuclei will return to their equilibrium state, emitting RF radiation.

  • Relaxation time: the short time between the end of the RF pulse and the re-emitting of the radiation.

  • NMRI is oftenly/simply abbreviated to MRI (Magnetic Resonance Imaging).

  • Since hydrogen is abundant in body tissues and fluids, hydrogen is the nucleus used in MRI.

  • A Magnetic Field is a region of space  where a magnet will experience forces.

  • On de-excitation, the nuclei give off radio frequency waves that can be detected and analyzed.



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