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Charges, fields, and potentials (MC)

Page history last edited by Joe Redish 5 years, 4 months ago

6.2.3.P3

 

A. Two charged parallel plates are used in many devices like a mass spectrometer or a cyclotron to speed up ions or elementary particles. The two parallel plates are raised to different voltages, which results in an electric field between the plates. Charged ions drift through a small hole in one of the plates and are accelerated by the field between the plates. The figure on the right shows a C14+ ion about to drift into the region between the plates. As it is accelerated, it will pass through the points marked a and b. At which point will the magnitudes of the electric field, the electric potential, the force felt by the ion, and the acceleration of the ion be greater? Check one answer from each column. (The plates are close enough together that they can be approximated as infinitely large and the holes through which the ions enter and leave are small enough that their effect on the field may be ignored.)

E Field

Potential

Force

Acceleration

____ Ea > Eb

____ Va > Vb

____ Fa > Fb

____ aa > ab

____ Ea = Eb

____ Va = Vb

____ Fa = Fb

____ aa = ab

____ Ea < Eb

____ Va < Vb

____ Fa < Fb

____ aa < ab

____ Not enough info
         given to tell

____ Not enough info
         given to tell

____ Not enough info
         given to tell

____ Not enough info
         given to tell

 

 

B.Two long molecules have unbalanced charges at the end so the field they produce looks like that of the indicated + and – charges at the ends of the molecules. The rest of the molecules’ charges essentially cancel and can be ignored.

 

B.1 If a positive ion is located somewhere along the y-axis of this diagram, the electric potential the ion will feel as a result of the charges on the molecules will be

  1. Positive all along the y axis.
  2. Negative all along the y axis.
  3. Zero all along the y axis.
  4. The sign will depend on where you are along the y axis.
  5. There is not enough information to decide.

 

B.2 The y-component of the electric force the positive ion feels from the two molecules while located along the y axis will

  1. Point in the upward direction all along the y axis.
  2. Point in the downward direction all along the y axis.
  3. Be zero all along the y axis.
  4. Point in different directions depending on where it is along the y axis.
  5. There is not enough information to decide.

 

B.3  The x-component of the electric force the positive ion feels from the two molecules while located along the y axis will

  1. point in the rightward direction all along the y axis
  2. point in the leftward direction all along the y axis
  3. be zero all along the y axis
  4. point in different directions depending on where it is along the y axis
  5. not enough information to decide

 

Joe Redish 8/16/12

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