The%20specific%20charge%20of%20the%20electron

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The specific charge of the electron

1. What is meant by specific charge?
2. Which equations are required to determine this?
3. Which experimental procedures can be used to
   determine the specific charge of an electron?

2
Quizzle

1. What is meant by the term specific charge (AKA
   charge to mass ratio)?
2. Which particle has the largest specific charge?
3. What will be deflected more in a magnetic field
   a proton or an electron?
4. What are the units of specific charge?
5. What is the specific charge of a helium nucleus
   (alpha particle)?
6. A sulphur atom has a nucleon number of 32.  What
   is the specific charge of an S2- ion?
7. How do you think this was determined by
   physicists in the good old days?

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Answers

1. e/m for an electron
2. Electron
3. Electron, lowest charge
4. Ckg-1
5. 4.85 107 C kg-1
6. 6.0 106 C kg-1
7. Fine beam tube type things

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J.J. Thomson
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1st experiment

• Deflection of cathode rays in an electrometer
  (charge measuring device)
• Charge is an inherent part of cathode rays it
  cannot be separated

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2nd experiment

• Realised that the cathode rays could be shielded
  from any electric fields by the gas surrounding
  them. Evacuated the tube, and found that the
  rays did now deflect.
• Cathode rays hold negative charge.

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3rd experiment

• Deflected cathode rays using a magnetic field.
• Adjusted until the deflection was equal to
  electrical deflection in experiment 2.
• Equated equations to find e/m.

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Significance

• Thomson found the specific charge of e to be much
  greater than H.
• Thomson produced three hypotheses
• Cathode rays are charged particles (which he
  called "corpuscles").
• These corpuscles are constituents of the atom.
• These corpuscles are the only constituents of the
  atom.

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Fine beam tube
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Measuring e/m with fine beam tube

1. Write an expression for the magnetic force on an
   electron and equate this to the centripetal
   force.
2. Write an expression equating the electrons KE to
   the work done by the accelerating potential (VA
   is the anode p.d.)
3. Eliminate the electron speed v from these
   equations to derive an expression for e/m in
   terms of VA, B (magnetic flux density), r (radius
   of curvature of beam)

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Finding B
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Question time

• How do the Helmholtz coils produce a circular
  trajectory for the electrons?
• Write an expression to show the magnetic force
  on an electron is equal to the centripetal force
• Rearrange this to give the specific charge
• What is the anode voltage of our fine beam tube?
• What is the specific charge on an electron?
• Using your answers to the previous questions,
  calculate the magnetic flux density produced by
  the Helmholtz coils
• E F/Q, E V/d, F Bev, F mv2 /r, eV ½ mv2