Explain how the pulse-echo technique works for medical imaging
What will be an ideal response?
A brief pulse of ultrasound is emitted by a transducer that transforms an electrical pulse into a sound-wave pulse. Part of the pulse is reflected as echoes at each interface in the body, and most of the pulse continues on. The detection of reflected pulses by the same transducer can then be displayed on the screen of a monitor. The time elapsed from when the pulse is emitted to when each reflection is received is proportional to the distance to the reflecting surface. The strength of a reflected pulse depends mainly on the difference in density of the two materials on either side of the interface and can be displayed as a pulse or as a dot. Each dot can be represented as a point whose position is given by the time delay and whose brightness depends on the strength of the echo. A two-dimensional image can then be formed out of these dots from a series of scans.
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_________is the increase in conductivity that results when photons excite electrons in a semiconductor or insulator into the conduction band.
Fill in the blank(s) with the appropriate word(s).
A 3 solar mass main sequence star is at the same distance as a 0.4 solar mass main sequence star. Which star appears brighter?
A. depends on the phase of the Moon B. the 0.4 solar mass main sequence star C. stars are approximately the same brightness D. the 3 solar mass main sequence star E. cannot tell with the information given
Describe two ways of detecting extrasolar planets
What will be an ideal response?
Each of two small non-conducting spheres is charged positively, the combined charge being 40 ?C. When the two spheres are 50 cm apart, each sphere is repelled from the other by a force of magnitude 2.0 N. Determine the magnitude of the smaller of the two charges
a. 1.4 ?C b. 1.1 ?C c. 2.0 ?C d. 3.3 ?C e. 17 ?C