Question Video: Determining What Process Causes Electron Emission in a Coolidge Tube | Nagwa Question Video: Determining What Process Causes Electron Emission in a Coolidge Tube | Nagwa

# Question Video: Determining What Process Causes Electron Emission in a Coolidge Tube Physics • Third Year of Secondary School

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 I? [A] X-ray diffraction [B] Electron diffraction [C] Bremsstrahlung radiation [D] The photoelectric effect [E] The thermionic effect

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### 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.

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