A) positrons
B) photons
C) electrons
D) protons
Correct Answer: C
Solution :
Key Idea: Metals have large number of free electrons. In metals some electrons get detached from their parent atoms and move randomly within the metal. These are called free electrons or conduction electrons. The atoms of the metal from which these electrons are detached are left in the form of positive ions. So, under normal conditions free electrons in the metal can not leave the metal due to attraction by positive ions. They require additional energy. This is called thermionic emission. Thermions are known as electrons.
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