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1
The Electron – Learning Outcomes
 Discuss the properties of the electron and its location in
the atom.
 Discuss the etymology of the electron’s name.
 Discuss Millikan’s oil drop experiment.
 Solve energy problems in terms of electron-volts.
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Properties of the Electron
 It is a particle that orbits the nucleus of an atom.
 It has a very small mass (9.1 × 10−31 𝑘𝑔).
 It is negatively charged (−1.6 × 10−19 𝐶).
 Instead of the usual q, electron charge is usually
represented by e.
 This is also the smallest amount of charge in nature and
all other charges are integer multiples of this (we will
later see quarks as the exception to this).
 Named by an Irish guy, George J. Stoney (cause that’s
important).
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Millikan’s Oil Drop Experiment
 Millikan found the charge of an
electron by suspending charged
oil drops in an electric field.
 The electric force required was
equal to the weight of the
drops.
 He determined the charge to
be either 1.6 × 10−19 C or some
integer multiple of this.
 He concluded that this was the
charge on the electron.
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Electron-volt
 For many experiments, it is useful to talk about the
energy of individual particles rather than the total
energy.
 Using 𝑊 = 𝑞𝑉 and setting 𝑞 = 1, 1 eV is the energy
gained by an electron when accelerated through a
potential difference of 1 V. This is equal to 1.6 × 10−19 𝐽.
 e.g. an x-ray machine is rated at 35 kV. This means the
electrons gain 35 keV of energy when accelerated in
the machine.
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Electron-volt
 e.g. An electron is accelerated from rest through a
voltage of 3000 V. Find:
i.
the electrical potential energy it loses in eV.
ii.
the kinetic energy it gains in eV.
iii.
its final velocity.
 e.g. An electron strikes a phosphorescent screen with a
speed of 2 × 107 𝑚 𝑠 −1 . Through what voltage was it
passed?