Dental Radiology Foundation Chain & Physics Practice Test

The main idea is thermionic emission: the filament serves as the electron source when it is heated. When current passes through the tungsten filament, it becomes extremely hot. That heat provides enough energy for electrons to overcome tungsten’s surface work function, allowing them to escape into the vacuum. These emitted electrons form the cathode beam and are accelerated toward the anode by the tube’s high voltage, producing X-rays when they strike the target. The filament’s job is not to emit photons or to actively focus electrons; nor is its primary role to dissipate heat, though it does get very hot. Tungsten is used because it has a very high melting point and remains stable at high temperatures, making a reliable source of electrons for the X-ray tube.