Th2 - 36th International Physics Olympiad. Salamanca...

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36 th International Physics Olympiad. Salamanca (España) 2005 Th 2 Page 1 of 3 R.S.E.F. Th 2 ABSOLUTE MEASUREMENTS OF ELECTRICAL QUANTITIES The technological and scientific transformations underwent during the XIX century produced a compelling need of universally accepted standards for the electrical quantities. It was thought the new absolute units should only rely on the standards of length, mass and time established after the French Revolution. An intensive experimental work to settle the values of these units was developed from 1861 until 1912. We propose here three case studies. Marks are indicated at the beginning of each subquestion, in parenthesis. Determination of the ohm (Kelvin) A closed circular coil of N turns, radius a and total resistance R is rotated with uniform angular velocity ω about a vertical diameter in a horizontal magnetic field i B B r r 0 0 = . 1. (0.5+1.0) Compute the electromotive force ε induced in the coil, and also the mean power 1 P required for maintaining the coil in motion. Neglect the coil self inductance. A small magnetic needle is placed at the center of the coil, as shown in Figure F-1. It is free to turn slowly around the Z axis in a horizontal plane, but it cannot follow the rapid rotation of the coil. 2.
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Th2 - 36th International Physics Olympiad. Salamanca...

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