When a metal is heated to a high temperature, electrons are emitted from its surface. The process is known as

Step 1: Analyze the question The question asks for the name of the process where electrons leave a metal surface because the metal is heated to a high temperature. The key word is heat.

Step 2: Explain the concepts Electrons in a metal are held in by the surface's attractive forces (the work function). Electrons can escape if they get enough energy. The energy can come from different sources, and each source gives a different process:

• Photoelectric effect (A): the energy comes from light (photons) striking the metal.
• Thermionic emission (B): the energy comes from heat. At high temperatures, electrons gain enough kinetic energy to overcome the work function and escape.
• Field emission (C): a very strong electric field pulls electrons out of the metal, even when it is cold.
• Secondary emission (D): fast-moving electrons bombard a surface and knock out other electrons.
• Radioactive emission (E): unstable nuclei emit particles such as beta particles. This has nothing to do with heating a metal.

Step 3: Apply to the question The electrons are released because of high temperature, so the process is thermionic emission. This is how the heated cathode of a cathode ray tube or a vacuum diode releases electrons.

Step 4: Answer The correct option is B. thermionic emission.

Exam tip: Match the energy source to the name:

• Thermo = heat → thermionic
• Photo = light → photoelectric
• Field = electric field → field emission
• Secondary = a bombarding electron → secondary emission