Plethysmographs are devices used for measuring changes in the volume of internal organs or limbs. In one form of this device, a rubber capillary tube with an inside diameter of 0.85 mm is filled with mercury at 20°C. The resistance of the mercury is measured with the aid of electrodes sealed into the ends of the tube. If 100 cm of the tube is wound in a helix around a patient's upper arm, the blood flow during a heartbeat causes the arm to expand, stretching the length of the tube by 0.0440 cm. From this observation and assuming cylindrical symmetry, you can find the change in volume of the arm, which gives an indication of blood flow. Take the resistivity of mercury to be 9.58 x 10-70. m. (Hint: The fraction by which the cross-sectional area of the mercury column decreases is the fraction by which the length increases because the volume of mercury is constant.) (a) Calculate the resistance of the mercury. (b) Calculate the fractional change in resistance during the heartbeat. AR R
Plethysmographs are devices used for measuring changes in the volume of internal organs or limbs. In one form of this device, a rubber capillary tube with an inside diameter of 0.85 mm is filled with mercury at 20°C. The resistance of the mercury is measured with the aid of electrodes sealed into the ends of the tube. If 100 cm of the tube is wound in a helix around a patient's upper arm, the blood flow during a heartbeat causes the arm to expand, stretching the length of the tube by 0.0440 cm. From this observation and assuming cylindrical symmetry, you can find the change in volume of the arm, which gives an indication of blood flow. Take the resistivity of mercury to be 9.58 x 10-70. m. (Hint: The fraction by which the cross-sectional area of the mercury column decreases is the fraction by which the length increases because the volume of mercury is constant.) (a) Calculate the resistance of the mercury. (b) Calculate the fractional change in resistance during the heartbeat. AR R
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Plethysmographs are devices used for measuring changes in the volume of internal organs or limbs. In one form of this device, a rubber capillary tube with an inside diameter of 0.85 mm is filled with mercury at
20°C. The resistance of the mercury is measured with the aid of electrodes sealed into the ends of the tube. If 100 cm of the tube is wound in a helix around a patient's upper arm, the blood flow during a
heartbeat causes the arm to expand, stretching the length of the tube by 0.0440 cm. From this observation and assuming cylindrical symmetry, you can find the change in volume of the arm, which gives an
indication of blood flow. Take the resistivity of mercury to be 9.58 x 10-7 2. m. (Hint: The fraction by which the cross-sectional area of the mercury column decreases is the fraction by which the length increases
because the volume of mercury is constant.)
(a) Calculate the resistance of the mercury.
(b) Calculate the fractional change in resistance during the heartbeat.
AR
R
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