Download Thermionic emission

NCEA Physics
Thermionic emission
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Thermionic emission
Aims:
•To understand that electrons are ‘boiled off’ hot metal
filaments and this is called thermionic emission.
•To know that a beam of electrons is equivalent to an
electric current and perform simple calculations
involving the rate of flow of electrons and the current,
given the electronic charge.
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Thermionic emission
Aims:
•To understand that an electron beam, or a stream of
charged ink drops, can be deflected by the electric field
between parallel charged metal plates.
•To understand the principles of a simple electron gun
with a heated cathode and accelerating anode.
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Thermionic emission
In normal atoms orbiting
electrons are held in place by
the force of attraction of the
positive nucleus.
However when some metals
are heated to high
temperatures the energy
gained by the outermost
electrons is enough to break
free of the atom.
We say that the electrons have
been ‘boiled off’ by
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thermionic emission.
Electron beams
•Electrons boiled off a hot metal filament by
thermionic emission can be channelled into a thin
beam.
•These beams of electrons are used in electron guns
that can be found in televisions and computer
monitors.
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Beam deflection
Here we can see
that a beam of
electrons is being
repelled by the
magnet above the
tube.
Moving electrons
generate a magnetic
field.
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Picture of ink jet printer
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Ink droplets
•Tiny droplets of ink can be easily charged.
•They are so small that when they fall through a large
electric field they can be attracted or repelled.
•The electric field is usually made by two big metal
plates one positive and one negative.
•Some ink jet printers use this movement of droplets
by an electric field to place ink onto paper.
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Electron guns
Electrons emitted by
Glass tube
thermionic emission can
be used in a variety of
ways. The emitted
electrons can be attracted
towards a positively
charged plate called an
anode. This sort of
Electron gun device, called an electron
Fluorescent
gun, can be used in
coated screen
televisions and computer
monitors.
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Electron guns
Glass tube
Electron gun
Fluorescent
coated screen
When the electrons
hit the flourescent
coated screen they
give off a flash of
light.
If there are a large
enough number of
flashes we can see a
picture on the screen.
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Controlling the gun
While a high voltage is needed to accelerate the electrons a
lower voltage can be used to control the path of the
electrons. Magnets or plates can be used to deflect the
electrons onto a screen coated with fluorescent paint. 11
Deflection tube
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Heated cathodes emit electrons
The first part of an electron gun is a small electric
circuit that is capable of ‘boiling off’ electrons by
thermionic emission. The voltage and current in this
circuit do not necessarily have to be very large.
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Cathodes
are
negative
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Accelerated by an anode
•Electrons emitted by thermionic
emission will soon jump back
onto the atoms they came from.
•However a large positive voltage
can be used to attract the free
electrons away from the metal
that they came from.
•This accelerating voltage needs
to be very large often 100’s or
even 1000’s of volts.
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Anodes
are
positive
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Deflecting the electrons
A large pair of plates, one positive and one negative, can
now be used to deflect the beam of electrons up and down.
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Vacuum tube
The electron beam is sealed inside a vacuum tube to stop
ionisation and deflection of the beam by other particles.
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Electronic charge
The charge on a single electron is 1.6  10-19 Coulombs.
Current = Charge / time
I=Q/t
Amps = Coulombs / seconds
How many electrons flow to make 1A of current?
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Coulombs
One coulomb is equal to the number of electrons
multiplied by the charge on each electron.
Charge = Number of electrons × Electronic charge
Charge = Number of electrons × ( 1.6 × 10-19 )
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Charge example 1
How many electrons are in a charge of 2 C?
Electrons = Charge  ( 6.25  1018 )
Electrons = 2  ( 6.25  1018 )
Electrons = 1.35  1019
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Summary – Thermionic emission
•Electrons have negative charge and flow along wires in a
current.
•Current = Electrons × Electronic charge / time
•Electrons can be ‘boiled off’ a hot metal filament in a
process called thermionic emission.
•In an electron gun these electrons are attracted to an
anode and can also be deflected onto a screen.
•In ink jet printers small drops of ink are charged and
then deflected onto paper to form letters and shapes.
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