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A black and white image of scientist J. J. Thomson wearing a coat and oval shaped spectacles.
J. J. Thomson (credit: www.firstworldwar.com, via Wikimedia Commons)
A diagram of the glass apparatus that was used to discover the electron in J. J. Thompson’s experiment.
Diagram of Thomson’s CRT. (credit: Kurzon, Wikimedia Commons)
Image of a cathode ray tube on x axis between two inverted L shaped north and south pole magnets on y axis, with z axis as a wire carrying high voltage supply to the charging plates inside the C R T. Zoomed image of the charging plate area inside the C R T showing the intersection of magnetic field between the poles in red lines towards south pole on the y axis along with an electron beam in green color line with velocity v toward right on the x axis.
This schematic shows the electron beam in a CRT passing through crossed electric and magnetic fields and causing phosphor to glow when striking the end of the tube.

To see how the amount of deflection is used to calculate q e / m e size 12{q rSub { size 8{e} } /m rSub { size 8{e} } } {} , note that the deflection is proportional to the electric force on the electron:

F = q e E . size 12{F=q rSub { size 8{e} } E} {}

But the vertical deflection is also related to the electron’s mass, since the electron’s acceleration is

a = F m e . size 12{a= { {F} over {m rSub { size 8{e} } } } } {}

The value of F size 12{F} {} is not known, since q e size 12{q rSub { size 8{e} } } {} was not yet known. Substituting the expression for electric force into the expression for acceleration yields

a = F m e = q e E m e . size 12{a= { {F} over {m rSub { size 8{e} } } } = { {q rSub { size 8{e} } E} over {m rSub { size 8{e} } } } "." } {}

Gathering terms, we have

q e m e = a E . size 12{ { {q rSub { size 8{e} } } over {m rSub { size 8{e} } } } = { {a} over {E} } } {}

The deflection is analyzed to get a size 12{a} {} , and E size 12{E} {} is determined from the applied voltage and distance between the plates; thus, q e m e size 12{ { {q rSub { size 8{e} } } over {m rSub { size 8{e} } } } } {} can be determined. With the velocity known, another measurement of q e m e size 12{ { {q rSub { size 8{e} } } over {m rSub { size 8{e} } } } } {} can be obtained by bending the beam of electrons with the magnetic field. Since F mag = q e vB = m e a size 12{F rSub { size 8{"mag"} } =q rSub { size 8{e} } ital "vB"=m rSub { size 8{e} } a} {} , we have q e / m e = a / vB size 12{q rSub { size 8{e} } /m rSub { size 8{e} } =a/ ital "vB"} {} . Consistent results are obtained using magnetic deflection.

What is so important about q e / m e size 12{q rSub { size 8{e} } /m rSub { size 8{e} } } {} , the ratio of the electron’s charge to its mass? The value obtained is

q e m e = 1 . 76 × 10 11 C/kg (electron). size 12{ { {q rSub { size 8{e} } } over {m rSub { size 8{e} } } } = - 1 "." "76" times "10" rSup { size 8{"11"} } " C/kg"} {}

This is a huge number, as Thomson realized, and it implies that the electron has a very small mass. It was known from electroplating that about 10 8 C/kg size 12{"10" rSup { size 8{8} } " C/kg"} {} is needed to plate a material, a factor of about 1000 less than the charge per kilogram of electrons. Thomson went on to do the same experiment for positively charged hydrogen ions (now known to be bare protons) and found a charge per kilogram about 1000 times smaller than that for the electron, implying that the proton is about 1000 times more massive than the electron. Today, we know more precisely that

q p m p = 9.58 × 10 7 C/kg (proton), size 12{ { {q rSub { size 8{p} } } over {m rSub { size 8{p} } } } =9 "." "57" times "10" rSup { size 8{7} } " C/kg"} {}

where q p size 12{q rSub { size 8{p} } } {} is the charge of the proton and m p size 12{m rSub { size 8{p} } } {} is its mass. This ratio (to four significant figures) is 1836 times less charge per kilogram than for the electron. Since the charges of electrons and protons are equal in magnitude, this implies m p = 1836 m e size 12{m rSub { size 8{p} } ="1836"m rSub { size 8{e} } } {} .

Thomson performed a variety of experiments using differing gases in discharge tubes and employing other methods, such as the photoelectric effect, for freeing electrons from atoms. He always found the same properties for the electron, proving it to be an independent particle. For his work, the important pieces of which he began to publish in 1897, Thomson was awarded the 1906 Nobel Prize in Physics. In retrospect, it is difficult to appreciate how astonishing it was to find that the atom has a substructure. Thomson himself said, “It was only when I was convinced that the experiment left no escape from it that I published my belief in the existence of bodies smaller than atoms.”

Thomson attempted to measure the charge of individual electrons, but his method could determine its charge only to the order of magnitude expected.

Since Faraday’s experiments with electroplating in the 1830s, it had been known that about 100,000 C per mole was needed to plate singly ionized ions. Dividing this by the number of ions per mole (that is, by Avogadro’s number), which was approximately known, the charge per ion was calculated to be about 1 . 6 × 10 19 C size 12{1 "." 6 times "10" rSup { size 8{ - "19"} } " C"} {} , close to the actual value.

Questions & Answers

what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, College physics -- hlca 1104. OpenStax CNX. May 18, 2013 Download for free at http://legacy.cnx.org/content/col11525/1.1
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