The Electron
When first investigated in the 18th and 19th centuries, electrical and magnetic
phenomena were generally construed in terms of lethereal fluids, as were those
associated with heat and light. These fluids were thought to comprise minute mutually
repelling particles. Thus, heat was either thought to be vibrations in the fluid caloric or
an accumulation of this fluid in the interstices of materials. Light was either a flux of
particles emitted at high speed from luminous bodies or the vibrations of a ubiquitous
fluid zether. Electric fluids-electricir)*-flowed readily through metals and other
conductors but did not penetrate insulators such as paper and glass. Opinion was
divided 8s to whether there was just one electric fluid or two-a positive fluid and a
negutive fluid.
The possibility that electricity might not be a continuous fluid was first raised in
the middle of the 19th century following Faraday’s quantitative researches on
electrolysis. These showed the existence of a systematic relationship between the
amount of electricity passed through an electrolytic cell and the quantity of material
that undergoes chemical reaction (electrolysis) in the cell. Thus, the passage of a
certain amount of electricity-96,500 coulombs in modem terms-always liberates a
gramequivalent of substance from the electrolyte, whether it is the metal released at
the negative cathode or the non-metal at the positive anode. Putting aside his
misgivings about atomism, Faraday recognised that this suggested electricity might be
atomic in nature and that a natural indivisible unit of electricity exists. In 1891 Stoney’
suggested that this natural unit of electricity be called an electron.
On this view, every ion carries an integer multiple of this natural unit. For
example, a silver ion, Agf, carries a single natural unit of positive charge; a typical
copper ion, Cu++, carries two such units. Given that a gramequivalent of monovalent
ions comprises a mole, the magnitude of the natural unit of electricity, e, can be
calculated by dividing the 96,500 coulombs by Avogadro’s number, 6.02. 1023
The term electron is now used to designate the elementary particle that carries the
natural unit of negative charge which was first identified towards the end of the 19th
century in experiments on the conduction of electricity through gases at very low
pressures. At atmospheric pressure, gases do not usually conduct electricity. However,
at reduced pressures of O.5mmHg to lOmmHg and with applied potentials of several
thousand volts, they can be made to pass a current. These greatly reduced gas pressures
were first achieved at the end of the 19th century following the advances made at that
time in vacuum pump technology. The gases were contained in narrow glass tubes,
called dischaee tubes, into which suitabIe electrodes had been inserted. The passage
of the current is eccompanied by the appearance of striking colours in the tubes (Fig
1.1).
At still lower pressures, -0.02mmHg, the colours disappear but the glass tube itself
begins to glow with a green hue. An object placed in front of the cathode (the negative
electrode) casts a shadow on the opposite wall of the discharge tube (Fig 1.2). Certain
minerals, when placed in front of the cathode, fluoresce with brilliant colours. It
appears that something is being emitted from the cathode; this emanation was given
the name cathode rays.
In fbrther experiments it was found that the cathode rays were deflected by a
magnetic field as would a stream of negative charge (Fig 1.3a). Furthermore, a small
paddle wheel positioned between the electrodes rotated under their impact; switching
the polarity of the electrodes reversed the direction of the rotation (Fig 1.3b). These
two phenomena suggested the cathode rays might be negatively charged particles.
Nevertheless, many physicists at the time still considered them to be of an Ethereal
rather than a material nature
.
Convinced that the cathode rays were in fact charged particles of matter,
J.J.Thomson set out in 1897 to measure their velocity, v, and the ratio, q/m,between
their charge, q, and their mass, m. In one of the experiments he conducted, a narrow
collimated beam of cathode rays was aimed along the length of a very low pressure
glass discharge tube. After emerging from the hole in the anode, the beam passed
through a thin slit, between a pair of vertical coils and, finally, between the horizontal
parallel plates of a condenser. The green spot that appeared on the glass at the far end
of the tube indicated where the beam impinged upon it (Fig 1.4).
The cathode ray beam could be deflected vertically by two different means: (i)
magnetically; (ii) electrically:
(i) Magnetically-by passing a current through the coils. This induces a uniform
horizontal magnetic field, B, in the space between them which, in turn, exerts a
force, FB, on the cathode ray particles such that
FB = Bqv (1.2)
where v is the velocity of the particles. This force acts at right angles to the
direction of the particle's motion and so constitutes a centripetal force. Thus, as
they pass between the pair of coils, the cathode ray particles move in a circular
path-along the arc of a circle-such that
where r is the radius of the arc.
(ii) Electrically-by connecting the condenser plates to a potential source; this
produces a vertical electric field, E, in the space between them that exerts a
vertical force, FE, on the cathode ray particles such that
FE = Eq (1.4)
Initially, Thomson applied just the magnetic field. This had the effect of moving
the green spot down from P to Q. He then activated the electric field and adjusted the
potential between the condenser plates until the field was just sufficient to return the
green spot from Q to P. At this point, the two forces, FB and FE, cancelled each other
out, such that:
FE = FB
or qE = Bqv =mv2/r
From which, the particles' charge to mass ratio is given by
Thomson knew the value of the magnetic field, B, from the dimensions of the coils
and the current flowing through them, of the arc radius, r, from the dimensions of the
apparatus and the distance PQ and of the electric field, E, from the potential applied
between the condenser plates and the distance between them. Substituting these in
equation (1.6) gave a value, in modem SI units, of -2-10 power 11 C/kg for the charge to mass
ratio of the cathode ray particles. Thornson repeated the experiment using different
metal cathodes and with different gases in the low pressure tube, but in each case he
found approximately the same value for the ratio.The value found by Thomson for the
charge to mass ratio of the cathode ray
particles was three orders of magnitude greater than the largest previously known value
of this ratio, namely, that found for the aqueous hydrogen ion, H(aq). This could be
attributed either “to the smallness of m or the largeness of q, or a combination of these
two”. Thomson opted for the “smallness of m” and assumed that the magnitude of the
charge, q, carried by the cathode ray particles was equal to the smallest charge known
to be carried by any ion, i.e., q = -1.6-10 power -19 C. On this assumption, he calculated the
mass of the cathode ray particles and obtained the result, astonishing at the time, that
their mass was only Kg36 that of the hydrogen atom, itself the smallest of all atoms.*
In an attempt to verify Thomson’s assumption, direct measurements of the
magnitude of the charges carried by gaseous ions were made. Although these
experiments suffered initially from many sources of error, their results appeared to
confirm Thomson’s supposition. The issue was finally and unequivocally settled in
1906 when, in a series of accurate and careful experiments, Millikan8 measured the
magnitude of the electrical charges carried by both positively and negatively charged
oil droplets. In none of the hundreds of measurements he made, did he detect a droplet
that carried a charge whose magnitude was less than 1.6.1O power-l9 C , nor one that carried
a charge whose magnitude was a fractional multiple of this amount. In every case, the
magnitude of the charges was an integer multiple of e = 1.6.10power-19 C . The clear
inference was that electrical charge-positive or negative-always appears as an
integer multiple of the natural unit e.
The cathode ray particles Thomson had discovered are the elementary particles we
now call electrons; the electron was the first elementary particle to be discovered.
Electrons carry the natural unit, -e, of negative charge. They are the smallest of the
three constituent particles--protons, neutrons and electrons--of ordinary matter.
Though electrons have no discernible internal structure or dimensions, they
nevertheless possess an intrinsic angular momentum (spin) and an associated magnetic
moment.
Cathode rays were not the only emanations observed in discharge tubes. If the
cathode in the discharge tube was pierced, rays were seen to emerge from the hole in
the direction away from the anode (the positive electrode), i.e., in the opposite
direction to the cathode rays (Fig 1.5). These emanations were called canal rays; they
were found to be positively charged.
Having identified the cathode rays, Thomson proceeded to investigate the nature of
the canal rays. These too proved to be particles of matter. By measuring the ratio
between their charge and their mass he identified them as positive ions of the gas
present in the discharge tube. He construed that these positive ions were produced in
the space between the cathode and the anode from the bombardment of the gaseous
atoms by the cathode rays (electrons). Since they carried a positive charge, they were
attracted to and accelerated towards the negative cathode, passing through the hole that
had been pierced in it and emerging from the other side. In one notable experiment, in
which the gas in the discharge tube was a pure sample of the noble gas neon, Thomson
discovered two different species of positive ions in the canal rays whose masses
differed by about 10%. The ions could only be neon ions, the purity of the sample
ensured this. This discovery of different species of neon ions first demonstrated the
existence of atoms that share the same chemical identity but have different masses;
what we now refer to as isotopes.
In 1932, a particle was discovered that is identical to the electron in all its
properties except that it is positively and not negatively charged; it was designated the
unti-electron or positron. When a positron and electron collide, the pair of particles
self-destruct, releasing a burst of high frequency electromagnetic radiation- Y rays.
In fbrther experiments it was found that the cathode rays were deflected by a
magnetic field as would a stream of negative charge (Fig 1.3a). Furthermore, a small
paddle wheel positioned between the electrodes rotated under their impact; switching
the polarity of the electrodes reversed the direction of the rotation (Fig 1.3b). These
two phenomena suggested the cathode rays might be negatively charged particles.
Nevertheless, many physicists at the time still considered them to be of an Ethereal
rather than a material nature
.Convinced that the cathode rays were in fact charged particles of matter,
J.J.Thomson set out in 1897 to measure their velocity, v, and the ratio, q/m,between
their charge, q, and their mass, m. In one of the experiments he conducted, a narrow
collimated beam of cathode rays was aimed along the length of a very low pressure
glass discharge tube. After emerging from the hole in the anode, the beam passed
through a thin slit, between a pair of vertical coils and, finally, between the horizontal
parallel plates of a condenser. The green spot that appeared on the glass at the far end
of the tube indicated where the beam impinged upon it (Fig 1.4).
The cathode ray beam could be deflected vertically by two different means: (i)
magnetically; (ii) electrically:
(i) Magnetically-by passing a current through the coils. This induces a uniform
horizontal magnetic field, B, in the space between them which, in turn, exerts a
force, FB, on the cathode ray particles such that
FB = Bqv (1.2)
where v is the velocity of the particles. This force acts at right angles to the
direction of the particle's motion and so constitutes a centripetal force. Thus, as
they pass between the pair of coils, the cathode ray particles move in a circular
path-along the arc of a circle-such that
where r is the radius of the arc.
(ii) Electrically-by connecting the condenser plates to a potential source; this
produces a vertical electric field, E, in the space between them that exerts a
vertical force, FE, on the cathode ray particles such that
FE = Eq (1.4)
Initially, Thomson applied just the magnetic field. This had the effect of moving
the green spot down from P to Q. He then activated the electric field and adjusted the
potential between the condenser plates until the field was just sufficient to return the
green spot from Q to P. At this point, the two forces, FB and FE, cancelled each other
out, such that:
FE = FB
or qE = Bqv =mv2/r
From which, the particles' charge to mass ratio is given by
Thomson knew the value of the magnetic field, B, from the dimensions of the coils
and the current flowing through them, of the arc radius, r, from the dimensions of the
apparatus and the distance PQ and of the electric field, E, from the potential applied
between the condenser plates and the distance between them. Substituting these in
equation (1.6) gave a value, in modem SI units, of -2-10 power 11 C/kg for the charge to mass
ratio of the cathode ray particles. Thornson repeated the experiment using different
metal cathodes and with different gases in the low pressure tube, but in each case he
found approximately the same value for the ratio.The value found by Thomson for the
charge to mass ratio of the cathode ray
particles was three orders of magnitude greater than the largest previously known value
of this ratio, namely, that found for the aqueous hydrogen ion, H(aq). This could be
attributed either “to the smallness of m or the largeness of q, or a combination of these
two”. Thomson opted for the “smallness of m” and assumed that the magnitude of the
charge, q, carried by the cathode ray particles was equal to the smallest charge known
to be carried by any ion, i.e., q = -1.6-10 power -19 C. On this assumption, he calculated the
mass of the cathode ray particles and obtained the result, astonishing at the time, that
their mass was only Kg36 that of the hydrogen atom, itself the smallest of all atoms.*
In an attempt to verify Thomson’s assumption, direct measurements of the
magnitude of the charges carried by gaseous ions were made. Although these
experiments suffered initially from many sources of error, their results appeared to
confirm Thomson’s supposition. The issue was finally and unequivocally settled in
1906 when, in a series of accurate and careful experiments, Millikan8 measured the
magnitude of the electrical charges carried by both positively and negatively charged
oil droplets. In none of the hundreds of measurements he made, did he detect a droplet
that carried a charge whose magnitude was less than 1.6.1O power-l9 C , nor one that carried
a charge whose magnitude was a fractional multiple of this amount. In every case, the
magnitude of the charges was an integer multiple of e = 1.6.10power-19 C . The clear
inference was that electrical charge-positive or negative-always appears as an
integer multiple of the natural unit e.
The cathode ray particles Thomson had discovered are the elementary particles we
now call electrons; the electron was the first elementary particle to be discovered.
Electrons carry the natural unit, -e, of negative charge. They are the smallest of the
three constituent particles--protons, neutrons and electrons--of ordinary matter.
Though electrons have no discernible internal structure or dimensions, they
nevertheless possess an intrinsic angular momentum (spin) and an associated magnetic
moment.
Cathode rays were not the only emanations observed in discharge tubes. If the
cathode in the discharge tube was pierced, rays were seen to emerge from the hole in
the direction away from the anode (the positive electrode), i.e., in the opposite
direction to the cathode rays (Fig 1.5). These emanations were called canal rays; they
were found to be positively charged.
Having identified the cathode rays, Thomson proceeded to investigate the nature of
the canal rays. These too proved to be particles of matter. By measuring the ratio
between their charge and their mass he identified them as positive ions of the gas
present in the discharge tube. He construed that these positive ions were produced in
the space between the cathode and the anode from the bombardment of the gaseous
atoms by the cathode rays (electrons). Since they carried a positive charge, they were
attracted to and accelerated towards the negative cathode, passing through the hole that
had been pierced in it and emerging from the other side. In one notable experiment, in
which the gas in the discharge tube was a pure sample of the noble gas neon, Thomson
discovered two different species of positive ions in the canal rays whose masses
differed by about 10%. The ions could only be neon ions, the purity of the sample
ensured this. This discovery of different species of neon ions first demonstrated the
existence of atoms that share the same chemical identity but have different masses;
what we now refer to as isotopes.
In 1932, a particle was discovered that is identical to the electron in all its
properties except that it is positively and not negatively charged; it was designated the
unti-electron or positron. When a positron and electron collide, the pair of particles
self-destruct, releasing a burst of high frequency electromagnetic radiation- Y rays.
|
Post a Comment