Which is the type of emission  that heats the metal to a sufficient temperature to enable the free electrons to leave the metal surface:?

Option 3 : Thermionic emission

Thermionic Emission:

• Thermionic emission is the liberation of electrons from an electrode by virtue of its temperature (releasing of energy supplied by heat).
• This occurs because the thermal energy given to the charge carrier overcomes the work function of the material.
• The charge carriers can be electrons or ions, and in older literature are sometimes referred to as thermions.
• 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 charge left behind is neutralized by the charge supplied by the battery as the emitted charge carriers move away from the emitter, and finally, the emitter will be in the same state as it was before emission.

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