Electron emission is the process when an electron escapes from a metal surface. Every atom has a positively charged nuclear part and negatively charged electrons around it. Sometimes these electrons are loosely bound to the nucleus. Hence, a little push or tap sets these electrons flying out of their orbits.
There are free electrons inside a metal surface. If these electrons are not bound to any nucleus, why don’t they escape the metal surface? This is because metals are neutral and if an electron escapes the surface, the surface gets a positive charge. This will attract the electron back to the surface and prevent it from escaping. As a result, a barrier forms near the surface. We call it the surface barrier. So the free electrons are “free” only inside the metal. To get them out of the metal surface, we need some force to overcome this surface barrier.
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Inside a metal
As we said earlier, inside metals electrons are free to roam around but to escape the surface, they need to overcome an electric force or potential. The energy for this may be supplied from outside causing the electrons to be emitted from the surface of the metal. The electrons are inside a well. They can move freely inside the well but to take them out of it, we have to provide them with some energy.
We call this the finite potential well. The energy required to liberate these electrons from the potential well (metal surface) is known as the work function of the metallic surface. Once an electron gets energy equal to the work function, it overcomes the potential well and is free to leave the metal surface.
Types of Electron Emission
The process of emission happens in the following steps:
- Step – 1: Delivery of Energy equal to or greater than the work function to the metal surface.
- Step – 2: The electron absorbs the energy. Thus it escapes the metal surface.
Depending on how you deliver the energy to the metal surface, the emission is of different types.
The emission is a thermionic emission if the energy responsible for it is in the form of heat energy.
The electric field has an electric potential energy associated with it. If a charge ‘q’ is present in a potential V, then E =qV. The emission is a field emission if the energy responsible for it is in the form of electric energy.
Light consists of packets of energy called photons. The Plank-Einstein relation E = hν gives the energy of a photon beam of wavelength ‘ν’. If the frequency of the photons is greater than a specific value known as the threshold frequency, then electrons are emitted from the metal surface. This is the photoelectric effect. These electrons are the photoelectrons.
Solved Examples For You
Two photons, each of energy 2.5eV are simultaneously incident on the metal surface. If the work function of the metal is 4.5eV, then from the surface of metal
- Two electrons will be emitted.
- Not even a single electron will be emitted.
- One electron will be emitted.
- More than two electrons will be emitted.
Solution: B) Not even a single electron will be emitted.
Since the energy of each photon (2.5 eV) is lesser than the work function (4.5 eV), There will not be any emission of electrons. Both the photons will be absorbed and will excite the electrons. But the electrons will still remain bound to the metal.