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Thermionic Emission
· Thermionic emission is when you heat a
metal, giving the free electrons thermal
energy. When the free electrons have
enough thermal energy they break free
from the surface of the metal.
· Once the electrons have been emitted,
they can be accelerated by the electric
field of the electron gun.…read more

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Electron Guns
· An electron gun contains a heating coil, that
heats the metal cathode, which emits electrons.
The electric field, set up via a high voltage,
causes the electrons to be accelerated towards
the anode, which has a hole in it.
· Some of the electrons pass through this hole,
making a narrow beam of electrons. The
electrons then travel at a constant velocity, as
there is no electric field (no force) acting on them
when they leave the anode.…read more

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Electron Microscopes
· An electron microscope uses an electron
gun. In the electron gun the stream of
electrons is focused on the sample, using
an electric field. The electrons have wave
like properties, so show fine detail.
· The electron wavelength is related to its
momentum via the de Broglie equation:
= h/…read more

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The Electron Volt
· The kinetic energy that a particle of charge
Q gains when its accelerated through a
potential difference of V volts is QV joules.
· If you replace the Q by the charge of a
single electron, e, you get ½mv^2=eV
· Electron Volt: 1 electron volt is the kinetic
energy carried by an electron after it has
been accelerated through a p.d of 1V.…read more

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· A particle accelerator (linac) is a long, straight tube,
containing a series of electrodes.
· A high frequency, alternating current is applied to the
electrodes, so their charge is constantly altered between
positive and negative.
· The a.c. is timed so that the particle is always attracted
to the next electrode, and repelled by the one behind.
· The particles speed increases as it passes through the
· The high energy particles leaving the linac then collide
with a target at the end of the tube.…read more

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