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Physics II Lab w answers_Part_43

# Physics II Lab w answers_Part_43 - Fig 1 In this method...

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97 Fig 1 In this method, four pointed , collinear equispaced probes are placed on the plane surface of the specimen (Fig.1b). A small pressure is applied using springs to make the electrical contacts. The diameter of the contact (which is assumed to be hemispherical) between each probe and the specimen surface is small compared to the spacing between the probes. Assume that the thickness of the sample d is small compared to the spacing between the probes s (i.e., d << s). Then the current streamlines inside the sample due to a probe carrying current I will have radial symmetry, so that ° ± ² ³ ´ µ - = r r V E and from eqn.(1), - = J r r V ρ (2) If the outer two probes (l and 4) are current carrying probes, and the inner two probes (2 & 3) are used to monitor the potential difference between the inner two points of contact, then total current density at the probe point ‘2’ which is at a distance r from probe ‘1’ and r’ from probe ‘4’ can be written as, ° ° ± ² ³ ³ ´ µ - = r r r r I J d

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Physics II Lab w answers_Part_43 - Fig 1 In this method...

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