magnetism

1)Find
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-radius semicircle in the figure .
The straight wires extend a great distance outward to the left and
carry a current
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.
Express your answer using two significant
figures.




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ans:


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= 5.9×10−5

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2.J. J. Thomson is best known for his discoveries about the nature of
cathode rays. Another important contribution of his was the invention,
together with one of his students, of the mass spectrometer. The ratio
of mass
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to (positive) charge

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of an ion may be accurately determined in a mass
spectrometer. In essence, the spectrometer consists of two regions: one
that accelerates the ion through a potential
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and a second that measures its radius of curvature in a
perpendicular magnetic field.
The ion begins at potential
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and is accelerated toward zero potential. When the
particle exits the region with the electric field it will have obtained a
speed
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a)With what speed
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does the ion exit the acceleration region?
Find the speed in terms of
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, and any constants.






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=
圖片參考：http://session.masteringphysics.com/render?tex=%5Csqrt%7B%5Cfrac%7BqV2%7D%7Bm%7D%7D

b)After being accelerated, the particle enters a
uniform magnetic field of strength
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(determined by observing where it hits on a screen--as
shown in the figure). The results of this experiment allow one to find
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in terms of the experimentally measured
quantities such as the particle radius, the magnetic field, and the
applied voltage.
What is
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?
Express
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, and any necessary constants.






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=
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i just confuse where the number 2 came from