Why Is It Negatively Charged To Receive An Electron?

In an X-ray tube, electrons move from negatively charged cathode to positively charged anode. The electron is negatively charged and the anode is positively charged. Therefore the anode attracts the electron. Due to this force of attraction, the electron is accelerated and, therefore, its kinetic energy increases. The kinetic energy of the electron is very high as it reaches the electron. The electron strikes an atom in the anode. There are two types of interactions - (1) interaction with an electron shell and (2) interaction with the atomic nucleus. The interaction with an electron shell produces a form of X-ray called characteristic radiation and the interaction with the atomic nucleus produces a form of X-ray called Bremsstrahlung radiation.

This is called characteristic radiation because it is characteristic of the material of the anode. The binding energy of an electron in an atom is defined as the minimum energy required to remove that electron out of the atom. This depends on the atomic number of the atom and the shell to which the electron belongs. As the incoming electron strikes an atom in the anode, the electron slows down and loses its kinetic energy. Some of the kinetic energy is converted into characteristic radiation and the remaining into heat.

If the incoming electron hits an atom in the anode close to the atomic nucleus, the electron is deflected and moves away from the atom. It is slowed down by the attractive force from the nucleus. This reduces the kinetic energy of the electron. The loss of kinetic energy of the electron appears as Bremsstrahlung radiation (German word for breaking radiation) and heat. Most of X-ray photons are in the form of Bremsstrahlung

The filament of the cathode is heated with the help of an electrial circuit. This is similar to how the filament of a light bulb is heated when current passes through it. "The filament is heated with the electric current passing through it, to the glowing temperature and the electrons are then expelled from the cathode".The process is known as thermionic emission. Thermionic emission occurs when thermal energy supplied to the charge carriers overcomes the binding potential of the atoms in the cathode. The binding potential is also known as the work function of the metal. The electrons in the outermost shell are ejected first because they have the lowest binding energy. A pure tungsten filament needs to be heated to a temperature of 2200 degrees or above to emit useful number of electrons for the production of x-ray. After emission, a charge that is equal in magnitude and opposite in sign to the total charge emitted is initially left behind in the emitting region. But if the emitter is connected to a battery, the battery produces a charge which is equal in magnitude and opposite in sign to the charge left behind. Thus the charge left behind is neutralized as the emitted charge carriers move away from the emitter, and finally the emitter is in the same state as it was before