The specific charge
of the electron
What is meant by specific charge?
Which equations are required to determine this?
Which experimental procedures can be used to
determine the specific charge of an electron?
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Quizzle
What is meant by the term specific charge (AKA
charge to mass ratio)?
Which particle has the largest specific charge?
What will be deflected more in a magnetic field; a
proton or an electron?
What are the units of specific charge?
What is the specific charge of a helium nucleus
(alpha particle)?
A sulphur atom has a nucleon number of
32. What is the specific charge of an S2- ion?
How do you think this was determined by
physicists in the good old days?
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Answers
e/m for an electron
Electron
Electron, lowest charge
Ckg-1
= 4.85 × 107 C kg-1
= 6.0 × 106 C kg-1
Fine beam tube type things
J.J. Thomson
1st experiment
O Deflection of cathode rays in
an electrometer (charge
measuring device)
O Charge is an inherent part of
cathode rays; it cannot be
separated
2nd experiment
O 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.
O Cathode rays hold negative charge.
3rd experiment
O Deflected cathode rays using
a magnetic field.
O Adjusted until the deflection
was equal to electrical
deflection in experiment 2.
O Equated equations to find
e/m.
Significance
O Thomson found the specific charge of e to
be much greater than H+.
O Thomson produced three hypotheses:
1. Cathode rays are charged particles (which
he called "corpuscles").
2. These corpuscles are constituents of the
atom.
3. These corpuscles are the only constituents
of the atom.
Fine beam tube
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 electron’s
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)
Finding B
Question time
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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