The same current running in the same direction

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Unformatted text preview: I Magnetic field R Figure 2: Helmholtz coils. The same current running in the same direction through both coils produces a uniform magnetic field in the shaded center region between the two coils. Physics 7B Charge-to-mass: e/m p. 6 We won’t ask you to show it here, but you should know that (1) the first derivative, dBtot/dz evaluated at the midpoint between the coils is zero by symmetry; (2) the second derivative, d2Btot/dz2, is also zero if the coils are separated by a distance equal to their radius R. Because we want as uniform a field as possible, Helmholtz coils are separated by just this distance R. Parallax Errors Close one eye and hold up a ruler between you and a far wall. Now move the ruler towards or away from your eye without moving your head, so that the ruler just covers the wall from end to end. If you didn’t know better, you’d think that you had just measured the length of the wall to be the same as that of the ruler. This is a parallax error, which can occur when a measuring stick is not placed directly against the object it is measuring. (If you had put the ruler right up against the far wall, you’d immediately see your mistake.) Since the circling electron beam is encased in a glass bulb, we can’t put a measuring stick directly up a...
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This note was uploaded on 11/19/2010 for the course LECTURE 1 taught by Professor Yildiz during the Fall '10 term at University of California, Berkeley.

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