Thermionic Electron Emission

Thermionic Electron Emission

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Thermionic electron emission is when electrons are released from a metal surface as it gets hotter. This discovery was made by Edmond Becquerel in 1853 and later rediscovered by Frederick Guthrie in 1873. He found that a negatively charged metal sphere would lose its charge if heated enough. Even though the electron wasn't known at the time and neither of these scientists knew how it worked, their discovery became very important later on.

Rutherford-Bohr model of the atom

Electrons in an atom are always moving, each with their own energy, as they circle around the atom's centre. The Rutherford-Bohr model of the atom, created by Niels Bohr and Ernest Rutherford, tells us that the electron's orbit depends on its kinetic energy. If an electron has more energy, it orbits further from the atom's centre.

We call these orbits energy levels, and they increase the further from the centre they are. If an electron gains energy, it can move up an energy level, and if it loses energy, it can move down an energy level. The Rutherford-Bohr model is helpful in understanding energy interactions in atoms, even though it doesn't explain everything about atomic structure. This model is how we can explain the discovery of thermionic electron emission made by Becquerel without him knowing about energy levels.

Thermionic electron emission theory

How does the Rutherford-Bohr model explain thermionic electron emission? When electron has enough kinetic energy to break free from the attractive force of the positively charged nucleus, electron emission happens. This can happen in different ways, but in thermionic electron emission, electrons gain energy as the temperature of the atom increases. The electrons move up energy levels away from the nucleus until they reach the metal's 'work function', which is the minimum energy needed to remove an electron from the material's surface. Once this happens, electron emission occurs, creating free electrons known as thermions.

It's important to know that the term 'thermion' can also refer to other charge carriers emitted by heating, such as ions.

This phenomenon was first discovered by Becquerel and Guthrie, but it was called the Edison effect for a while. Edison found that current flowed through a vacuum from a heated filament to a cooler metal surface while trying to improve his new incandescent lamp. This discovery eventually led to the invention of the cathode ray tube and the electron gun.

Thermionic electron emission and the electron gun

The electron gun is an electrical component that produces a focused beam of electrons based on the principles of thermionic electron emission. It is primarily used in the design of cathode ray tubes.

The electron gun has three main components: a metal filament cathode that emits free electrons when heated, electrodes that focus the resulting stream of electrons (often called a 'Wehnelt cylinder'), and a highly positive anode to accelerate the electrons to greater speeds.

A positive 'heating voltage' is applied across the cathode, while a much larger positive 'acceleration voltage' is applied to the anode. As the anode has a much greater positive voltage than the cathode, the negative electrons will accelerate towards it at greater speeds. The difference in voltage between the cathode and anode determines the acceleration of the free electrons in the beam.

The electron gun was a crucial component in the invention of the television. Before the advent of flat-screen TVs, television screens used beams from three electron guns to create the picture. This is why older televisions were much deeper; they needed sufficient space to accelerate the electrons to the screen.

Speeds of electrons in thermionic electron emission

To calculate the minimum speed of a free electron emitted from a metal filament, we can use the kinetic energy formula:

KE = (1/2)mv^2

where KE is the kinetic energy of the electron, m is the mass of the electron, and v is the velocity of the electron.

that the kinetic energy of the emitted electron is 4.9 x 10^-19 J and the mass of an electron is 9.1 x 10^-31 kg, we can plug these values into the formula:

4.9 x 10^-19 J = (1/2) x 9.1 x 10^-31 kg x v^2

Solving for v, we get:

v = √(2 x 4.9 x 10^-19 J / 9.1 x 10^-31 kg)

v ≈ 6.2 x 10^5 m/s

Therefore, the minimum speed of a free electron emitted from a metal filament is approximately 6.2 x 10^5 m/s.

As for the unit of energy used for electrons, it is the electron-volt (eV), which is the energy gained by a free electron accelerated through a potential difference of 1 volt. The formula for calculating the energy gained by an electron through a potential difference is:

E =V

where E is the energy gained by the electron, e is the charge of an electron (1.6 x 10^-19 C), and V is the potential difference.

Thus, an electron-volt is the amount of energy gained by an electron when it is accelerated through a 1-volt potential difference, and it is equal to 1.6 x 10^-19 J.

Thermionic Electron Emission

What is the thermionic emission of electrons?

Thermionic emission of electrons is the discharge of electrons from a metal surface due to its being heated.

Which electrons are escaping during thermionic emission?

When an atom absorbs heat energy, the electrons from its outer energy level are emitted.

How does thermionic emission work?

Thermionic electron emission occurs due to electrons of an atom gaining kinetic energy due to a rise in temperature. When an electron has a high enough kinetic energy, it is able to escape the attractive force of the nucleus.

How is thermionic emission used?

Thermionic emission is used in many areas, but most notably in cathode ray tubes. These devices were used in televisions and computer monitors for decades.

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