How Electron Emission Works: Mechanisms, Examples, and Importance
Electrons were discovered by J.J Thomson. In 1897, he performed an experiment in which he took an evacuated tube, called the cathode-ray tube, filled the gas inside it, and passed the electric charge to it. This experiment led to the discovery of electrons.
The emission of electrons from the metallic surface is called electron emission.
In Photoelectric emission, we learned the work function, which means the amount of energy required to emit electrons from the surface. The electrons can emit from the light and also from other surfaces.
In this article, we will learn electron emission and its types.
Free Electrons in Metals
In metals, the electrons in the outer shell called the valance electrons of the atoms are loosely bound. They are free to move easily within the metal but are not allowed to leave the surface of the metal. Such freely moving electrons are called free electrons.
The mass of electrons is 9.1 x 10-31 kg. The free electron in the metal at room temperature cannot move about freely inside the metal and leave its surface. The moment an electron comes out of the metallic surface, it gains a positive charge and pulls electrons toward itself. Thus the free electrons remain stuck with the metal by attractive forces of the surface called the restraining forces.
These attractive forces lead to a potential barrier. So, this barrier doesn’t allow electrons to leave the surface, that’s why some energy is offered to force these electrons to leave the surface. This energy is called the ‘work function’. So, what is the work function of the metal?
The work function is the minimum or the lowest amount of energy required by an electron to escape from the metallic surface by overcoming the attractive force with the surface of the metal.
The work function is symbolized as o and measured in eV, where eV stands for electron volt. So, eV is the energy required by an electron to escape from the metallic surface. According to the theory of electron emission, there are four types of electron emissions.
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Types of Emission of Electrons
The electron emission from the metallic surface is possible only when the energy supplied is greater than the work function of the metal. The energy required for the various types of electron emission can be supplied to the free electrons by the following physical processes:
Thermionic emission
Photoelectric emission
Secondary emission
Electric field emission
Thermionic Emission
What is thermionic? Therm in thermionic means ‘heat’ or ‘temperature’ and ionic means ions.
So, what is thermionic emission in Physics?
In this phenomenon, a metallic surface is heated for the emission of electrons. Here, the energy being supplied for the emission of electrons from the metallic surface is the thermal energy.
The electrons emitted from the surface are called the thermal electrons or thermions.
Here, the number of thermions emitted relies on the temperature of the metallic surface.
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Photoelectric Effect
Albert Einstein discovered the Photoelectric effect. Einstein was awarded the Nobel prize for discovering this phenomenon. In this type of emission, free electrons are released from the metallic surface when the light radiations of suitable frequency fall on the surface of the metal.
The energy passed to the free electrons for their emission is being supplied in the form of packets called the light photons or simply photons. The electrons emitting from the surface are called the photoelectrons. The name provided is because they are emitted by photons. Also, the number of photoelectrons emitted from the surface relies on the intensity of the incident light.
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Electric Field Emission
Field emission is also called the cold cathode emission. In this phenomenon, the emission of electrons from the metallic surface occurs under the application of a strong electric field.
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When free electrons leave the surface of the metal, the metallic surface acquires a positive charge and electrons get attracted to the positive charge because of which a strong electric field of about 108 V/m is applied to metal, it emits electrons.
Secondary Emission
Secondary emission is a phenomenon in which a large number of electrons emit when fast-moving electrons called the primary electrons strike the metallic surface.
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The fast-moving electrons are highly energetic, and when they fall on the surface, they transfer their energy to the free electrons (bonded by a strong attractive force with the metal) of the metal by which their energy becomes more than the work function of the metal and they release the electrons. Since the electrons are emitted with the help of primary electrons, that’s why the electrons emitted from the metal are called the secondary electrons.
Point to Remember
According to the order of electron emission, the emission of electrons from Cesium requires the lowest energy, while Platinum requires the highest energy to lose electrons.
FAQs on Electron Emission: Types, Process, and Applications
1. What exactly is electron emission?
Electron emission is the process where electrons are ejected from the surface of a material, usually a metal. This happens when the electrons gain enough energy to overcome the forces holding them to the surface. The minimum energy required for an electron to escape is called the work function of the material.
2. What are the main types of electron emission?
There are four primary ways to cause electron emission, based on the type of energy supplied to the electrons:
- Thermionic Emission: Electrons are emitted when the material is heated to a high temperature.
- Photoelectric Emission: Electrons are ejected when light of a suitable frequency shines on the material's surface.
- Field Emission: A very strong electric field is applied to the surface, which pulls the electrons out.
- Secondary Emission: Electrons are knocked out when the surface is hit by other high-energy particles, like other electrons.
3. Why is there a 'threshold frequency' in photoelectric emission?
In photoelectric emission, light behaves as packets of energy called photons. An electron can only be ejected if it absorbs a single photon with enough energy to overcome the material's work function. The energy of a photon is directly related to its frequency. The threshold frequency is the minimum frequency a photon needs to have just enough energy for this to happen. Below this frequency, no electrons are emitted, no matter how bright the light is.
4. What is the difference between thermionic and photoelectric emission?
The main difference is the source of energy used to eject the electrons. In thermionic emission, the energy comes from heat, which increases the thermal energy of the electrons until they can escape. In photoelectric emission, the energy comes from light, where individual photons transfer their energy to the electrons.
5. What do the laws of photoelectric emission tell us?
The laws of photoelectric emission are key observations about this phenomenon:
- For a given material, there is a minimum light frequency (the threshold frequency) below which no photoelectrons are emitted.
- Above the threshold frequency, the maximum kinetic energy of the emitted electrons depends only on the frequency of the light, not its intensity.
- The number of photoelectrons emitted per second is directly proportional to the intensity (brightness) of the incident light.
- The process is almost instantaneous; there is no significant time delay between the light hitting the surface and the electrons being emitted.
6. Where is electron emission used in real-world applications?
Electron emission is a fundamental principle behind many technologies. For example, thermionic emission was used in old vacuum tubes for radios and TVs. Photoelectric emission is used in devices like solar cells, automatic door sensors, and light detectors in cameras. Field emission is used in electron microscopes and some types of flat-panel displays.
7. How does a strong electric field cause electrons to be emitted?
In field emission, an extremely strong external electric field (around 10⁸ V/m) is applied near the metal's surface. This powerful field effectively pulls on the free electrons, giving them enough energy to overcome the surface barrier and 'tunnel' out of the material without needing any heat or light. This is sometimes called cold cathode emission.
