Thermoelectric-Photoelectric Thermionic Effects, Physics tutorial


We have observed that electric energy is readily converted into heat, however the inverse method was introduced in the year 1821 by T.J. Seebeck (1770 - 1831), who in reality found that a magnetic field surrounded through a circuit comprising of two metal conductors merely when the junctions among the metals were maintained at various temperatures. The generation of an e.m.f in a circuit having two dissimilar metals or semiconductors, if the junctions between the two are maintained at dissimilar temperatures is termed as Seebeck or thermoelectric effect. The magnitude of the e.m.f based on the nature of the metals and the difference in temperature.

The other method of generating electricity from other sources of energy is the photoelectric effect. This is the discharge of electrons from a substance exposed to electromagnetic radiation. An electron emitted from the substance by irradiation as an outcome of the photoelectric effect is termed as photoelectron. In the photoelectric effect, the energy of photons is changed into electrical energy.

Thermoelectric Effect:

Seebeck and Peltier Effects:

The Seebeck Effect explains a thermoelectric phenomenon through which temperature differences among the two dissimilar metals in a circuit transforms into an electric current.

Introduced in the year 1821, the Seebeck Effect is one of three reversible phenomena explaining similar processes associating to thermoelectricity, conductivity and temperature. The Peltier Effect was first noticed in the year 1834 and the Thomson Effect was first illustrated in the year1851.

The Seebeck Effect is termed for East Prussian scientist Thomas Johann Seebeck (1770-1831). In the year 1821, Seebeck introduced that a circuit made up of two different metals conducts electricity when the two places where the metals connect are held at various temperatures. Seebeck positioned a compass close to the circuit be built and observed that the needle deflected. He discovered that the magnitude of deflection rose proportionally as the temperature difference increased. His experiments as well noted that the temperature distribution all along the metal conductors didn't influence the compass. Though, changing the kinds of metals he employed did change the magnitude which the needle deflected.

In the year 1834, French scientist Jean Charles Athanase Peltier (1784-1845) explained the second closely associated phenomena, now termed as the Peltier Effect. In his experiment, Peltier modified the voltage between the metal conductors and discovered that the temperature at either junction changed proportionally. In the year 1839, German scientist Heinrich Lenz (1804-1865) expanded on Peltier's discovery and explained heat transfer at the junctions, based on the direction which the current flows all along the circuit. As such two experiments were mainly focused on various parts of the circuit and the thermoelectric effects, they are frequently termed to simply as the Seebeck-Peltier Effect or the Peltier-Seebeck Effect.

In the year 1851, British physicist William Thomson (1824-1907), later termed as the first Baron Kelvin, noticed that the heating or cooling of a single kind of metal conductor from the electrical current. The Thomson Effect explains the rate of heat created or absorbed in a current-carrying metal or other conductive material, subjected to the temperature gradient.

Thermocouple thermometers are the electrical engineering devices based on measuring the Seebeck Effect and the Peltier and Thompson effects. The thermometers work by changing the thermal potential difference to the electric potential difference.

Laws of Intermediate Metals and Intermediate Temperatures:

The given two laws have been established experimentally:

1) When A, B and C are three dissimilar metals, the thermoelectric e.m.f of the couple AC is equivalent to the sum of the e.m.f of the couples AB and BC over the similar temperature range.

It obeys that the junctions of a thermocouple might be soldered without influencing the e.m.f.

2) The e.m.f. of a thermocouple having junctions at temperatures θ1 and θ3 is the sum of the e.m.f of two couples of the similar metals having junctions at θ1 and θ2, and at θ2 and θ3, correspondingly.

Factors influencing the Thermo e.m.f.:

By computing the e.m.f produced by different thermocouples under various conditions, it is noticed that:

1) The e.m.f based on the metals of which the thermocouple is made up, being relatively big for iron-constantan thermocouples, for illustration and small for copper-iron at a specific temperature difference.

2) The e.m.f is not big for most of the thermocouples, amounting to not more than 40 to 50 mV for a temperature difference of around 1000oC.

3) The e.m.f is not directly proportional to the temperature difference however frequently increases to a maximum and then reduces, supposing approximately the shape of the parabola.

Thermoelectric Series:

For small values of temperature difference between the junctions, the metals can be arranged in the thermoelectric sequence as follows: Antimony, iron, zinc, copper, silver, lead, aluminum. Mercury, platinum-rhodium, platinum, nickel, constantan (60% copper and 40% nickel), bismuth.

If two of these metals are joined to form a thermocouple, the conventional current flows from the one earlier in the list to the other across the cold, junction; therefore the current flows from antimony to bismuth via the cold junction.

Photoelectric Effect:

Photoelectric effect is the emission of electrons from metal surfaces if electromagnetic radiation of high adequate frequency falls on them. The consequence is given through zinc if exposed to X-rays or ultraviolet. Sodium provides emission with X-rays, ultraviolet and all colors of light apart from orange and red, whereas preparations having caesium respond to infrared and also to high frequency radiation.

The effect was first observed by the German Physicist Heinrich Hertz in the year 1887 throughout the course of his experiments by the first simple radio transmitter. He noticed that a spark jumped more readily between the terminals of his high-voltage source if they were irradiated by means of ultraviolet rays. It was soon exhibited that negatively charged particles were emitted by means of some metals under irradiation. The charge-to-mass ratio of the particles was that of electrons, and physicists soon agreed that the particles were certainly electrons.

Einstein's Interpretation of the Photoelectric Effect:

The wave theory of light can provide no description for the existence of a threshold frequency. Emission of the electron would be expected as soon as adequate wave energy had been absorbed through the surface.

Einstein, in the year 1905, proposed that the description could be found when electromagnetic radiation were to be considered as made up of particles (photons) whose energy was associated to the frequency of the radiation. This was a growth of an earlier proposal through Planck (1902) who was trying to determine a mathematical basis for the curves of the continuous spectrum.

Einstein's photoelectric theory represents:

E = hf

For the energy of a quantum of radiation, that is, a single photon of frequency 'f':

h = 6.63 x 10-34Js, and is termed as Planck's constant.

When is symbolized the work function by Φ, the maximum kinetic energy by which the photoelectron emerges is:

1/2 mV2 = hf - Φ

Applications of Photoelectric Effect:

Two of the most significant applications of the photoelectric effect are the photoelectric cell (that is, or photocell) and solar cells. A photocell generally comprises of a vacuum tube having two electrodes. A vacuum tube is a glass tube from which nearly all of the air has been eradicated. The electrodes are two metal plates or wires. One electrode in a photocell comprises of a metal (that is, cathode) that will emit electrons if exposed to light. The other electrode (that is, anode) is given a positive electric charge as compared to the cathode. If light shines on the cathode, electrons are emitted and then attracted to the anode. The electron current flows in the tube from cathode to anode. The current can be employed to turn on a motor, to open a door, or to ring a bell in the alarm system. The system can be build up to respond to light, as explained above, or it can be responsive to the elimination of light.

Photocells are generally employed in factories. The items on a conveyor belt pass between a beam of light and a photocell. As each and every item passes the beam, it interrupts the light, the current in the photocell stops and a counter is turned on. By this process, the exact number of items leaving the factory can be counted. Photocells are as well installed on light poles to turn street lights on and off at dusk and dawn. Moreover, photocells are employed as exposure meters in cameras. They compute the precise amount of light entering a camera letting a photographer to adjust the lens of camera to the correct setting.

The Solar cells are machines or tools for transforming radiant energy (light) into the electrical energy. They are generally made up of specially prepared silicon which emits electrons whenever exposed to light. If a solar cell is exposed to sunlight, electrons emitted through silicon flow via external wires as a current.

Electron Volt:

In the Einstein equation:

1/2 mV2 = hf - Φ

Vs = (h/e) f - Φ/e

Here, we computed the maximum kinetic energy of the emitted electrons through noting the potential energy difference (Vse) that was equivalent to the electron kinetic energy. This process of finding out and expressing electron energies is principally convenient one and it recommends a new unit of energy. This new unit of energy is termed as the electron volt, eV that is stated as the amount of energy equivalent to the change in energy of one electronic charge if it moves via a potential difference of one volt.

Energies in joule's can be transformed or converted into electron volts by dividing by e = 1.60 x 10-19. In this case 'e' is not a charge however a conversion factor having the units of joules per electron volt.

Thermionic Emission:

The Thermionic emission, as well termed as thermal electron emission, is the method through which charge carriers like electrons or ions, move over a surface or some kind of energy barrier through the induction of heat. Charge carriers naturally restrain activity; though, in thermionic emission, thermal energy is introduced to the carriers, causing them to conquer such forces. The main reason behind the charge carrier's capability to carry out this action is as electrons and ions are mobile and unbound to the normal chains of the atomic structure which influence other particles. Conventionally, such charge carriers were termed to as 'thermions'.

One property of the thermionic emission theory is that the emitting area is sustained having a charge opposite to the original however equivalent in magnitude. This signifies that the location of the charge carrier prior to emission will produce a positive charge in case of electrons. Though, this can be modified by employing a battery. The emission is neutralized if the carriers are farther away from the area, resultant in no change to the original state.

Thermionic was first introduced by Frederick Guthrie in the year 1863. He was capable to recognize an alteration in the positive charge of a highly heated iron sphere that didn't take place when the object was negatively charged. Though, it wasn't till year 1880 that the science was readily harnessed through Thomas Edison. When working with his incandescent light bulbs, he observed that some areas remained darkened. This let him to recognize the flow of electrons due to heat, resultant in the formation of the diode.

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