Video Transcript
The diagram shows a Coolidge tube
used for the production of X-rays. Which of the following physical
processes causes electrons to be emitted from the component labeled one?
Component one is what is known as a
cathode coil, which is essentially a metal coil that when the Coolidge tube is
turned on becomes negatively charged and emits electrons towards the anode target,
which is component two. So we are looking for the process
which causes electrons within the cathode coil to leave it, no longer being
constrained by the metal.
So let’s look at each of these
answers. (A) X-ray diffraction and (B)
electron diffraction both have to deal with diffraction. Diffraction is a phenomenon that
occurs with waves, either from light, like in X-rays, or for matter, like in
electrons. When waves approach a narrow
opening, diffraction occurs, causing the waves to bend in a characteristic way. Although the gaps in between the
individual loops of the cathode coil may be small, they are not causing diffraction
to occur. And diffraction is certainly not
the mechanism responsible for causing the electrons to leave the metal in the
cathode coil. Diffraction is doing nothing to the
electrons that would cause them to leave. So (A) and (B) cannot be it.
Answer (C) is Bremsstrahlung
radiation or breaking radiation. Bremsstrahlung is an important
process in creating X-rays in the Coolidge tube. Bremsstrahlung is produced whenever
an electron in the Coolidge tube slows down, emitting an X-ray in the process, with
an energy equal to the difference between its speed before and after it slows
down. In the case of a Coolidge tube,
this occurs when the electrons strike the anode target at the other end, producing
this Bremsstrahlung. So Bremsstrahlung, though it is
responsible for the X-rays in the Coolidge tube, is not responsible for the emission
of the electrons in the cathode coil. So (C) isn’t it either.
Answer (D), the photoelectric
effect is a phenomenon that occurs when a photon is absorbed by an electron within
an atom. This electron with its newfound
energy cannot stay in the energy level it was from and is ejected from the atom. While it may be possible for the
photoelectric effect to occur within the cathode coil, energizing the electrons
within it and causing them to leave, this is not the primary physical process which
causes the electrons to be emitted from the cathode coil. It is still possible even
potentially from the Coolidge tube’s own X-rays striking the cathode coil. But the amount of electrons that
would be emitted from this method are almost negligible compared to what happens
when you place a potential difference across the cathode coil. This potential difference is called
the thermionic potential difference. And when this potential difference
is present, it causes the cathode coil to heat up to very high temperatures. This thermal energy is transferred
to every part of every atom in the cathode coil, including the electrons. And when these electrons become
very energetic, they can leave the atom entirely, just like in the photoelectric
effect, except without having to wait for an incident photon. This process is called thermionic
emission or the thermionic effect.
So, out of all of these physical
processes, the one that causes the electrons to be emitted from the component
labeled one, the cathode coil, is (E) the thermionic effect.
