# Thermoelectric Effect: Short Questions and Answers Physics

#### 1. What do you mean by Seebeck Effect and why is it considered reversible?

Seebeck effect is the phenomenon in which heating a junction formed b a loop of two different metals gives rise to electromotive force and thus electric current through the whole loop. When the hot and cold junctions are interchanged, the direction of both electromotive force and electric current also changes. So, the Seebeck Effect is considered reversible.

#### What are the factors on which the thermal emf produced in a thermocouple depends? [HSEB 2061]

The first factor that the magnitude and direction of thermo emf (and current as well) depend on is the nature of the metals forming the thermocouple. If the metals forming the couple are far away in the thermoelectric series, the emf and current will be high and if they are near, it would be less.

The next factor is the difference in temperature between the two junctions. If the temperature difference is progressively made higher, the emf and current goes on increasing, reaches a maximum value, and then again decreases, till the value reaches zero.

#### 3. Why is Sb – Bi pair preferred as a thermocouple?

According to Seebeck, the thermo emf of a thermocouple goes on increasing as the metal took forming thermocouple is distant apart in the thermoelectric series. The metals antimony and bismuth are the most distant apart in the series. For a certain temperature difference between the two junctions, the emf produced with the Sb-Bi pair is the largest. Hence Sb-Bi pair is preferred as a thermocouple

#### 4. Why is a thermocouple not used to measure the temperature above its neutral temperature?

The variation of thermoelectric emf with temperature difference is not linear but follows a parabolic path. At neutral temperature, the emf is maximum and near that value, one value of the measured emf corresponds to two values of temperature. It may cause confusion in measurement. Therefore, the thermocouple is not used to measure the temperature above the neural temperature. And if done, there should be proper knowledge of the neutral temperature.

#### 5. What effects are caused by the thermoelectric effect in the preciseness of electric measurements? Explain.

In electrical or electronic circuits, different kinds of metals have to be used at different locations according to physical requirements. They form junctions of dissimilar metals at the connection points. In addition, some portions develop higher temperatures and some low because of the release of heat at various portions. This could give rise to thermo emf and current which interacts with the actual current, and cause errors in measurements.

#### 6. Why is lead used as a reference metal in thermoelectricity?

The Thomson effect of lead is almost zero. This means that no heat is evolved or absorbed when current is passed through an unequally heated lead material. Therefore, it is used as a reference metal for expressing thermo emf.

#### 7. Upon what factors does neutral temperature depend?

The temperature of the hot junction at which emf of the thermocouple is maximum is called Neutral Temperature. It depends upon the nature of materials forming the thermocouple.

#### 8. How many types of emf’s are present in a thermocouple?

Altogether four types of emf are present in a thermocouple, mentioned as

(a) Peltier emf at the cold junction.

(b) Peltier emf at the hot junction.

(c) Thomson emf along one wire of the thermocouple.

(d) Thomson emf along the other wire of the thermocouple.

#### 9. What is the cause of thermoelectric emf?

If two dissimilar metals are made in contact, the free electrons diffuse from the metal whose work function is low (or in which a number of free electrons are high) to the metal whose work function is high (or in which a number of free electrons are high). So, one of the metals will be positively charged and the other negatively charged. This forms a difference of potential across the junction. The diffusion continues until the potential difference across the junction becomes just enough to stop further diffusion. Such natural potential developed across the junction is called Contact Potential.

If both junctions are at the same temperature, the contact potentials are equal and opposite to each other. They cancel each other giving net env junction is heated more than the other, the hotter junction zero. If one will have higher contact potential than that with lower. This gives a net emf in the whole circuit which is the Thermoelectric EMF.

#### 10. How does the thermal emf change in a thermocouple when the temperature of the hot junction is changed? [HSEB 2054, 2063, 2066]

The thermoelectric emf does not show a regular increase or decrease along with the temperature. When the temperature difference between the hot and cold junction is zero, the potential differences Max EMF across both the junctions are equal and so there is no emf or current. When the temperature arises, the PD across the EMF hot junction becomes more than that at the cold junction, and the thermo-emf increases. This emf increases up to a maximum value and then starts to decrease. This effect is due to the thermal effect in the wires. The temperature at which the thermo-emf is maximum is called Neutral Temperature (θn). The temperature at which the emf decreases up to zero again is called Inversion Temperature (θi).

#### 11. What do you mean by the temperature of inversion? State the factors on which it depends.

The temperature of the hot junction of a thermocouple at which Thermo emf becomes zero & reverses its direction above this temperature is called Temperature of Inversion. It depends upon

(a) The nature of the materials forming thermocouple

(b) the temperature of the cold junction.

#### 12. Define the temperature of inversion. If the temperature of the cold junction of a thermocouple is lowered, what will be the effect on it? (HSEB 2070)

The temperature of the hot junction, at which the direction of the termo-emf reverses, is called the temperature of inversion. The difference between the temperature of inversion and the neutral temperature for a thermo-couple is equal to the difference between the temperature of the cold junction and the neural temperature. Hence if we decrease the temperature of the cold junction, the difference between the temperature of the cold junction and neutral temperature increases since the neutral temperature is constant for a given thermocouple. So, the difference between the temperature of inversion and the neutral temperature also increases i.e. the temperature of inversion increases.

#### 13. What do you mean by the Peltier effect? How is it caused?

When two different metals are joined, contact potential difference arises at the junctions because of different free electron concentrations in them. This implies that at the junctions one metal will be at a higher potential than the other. For example, in the case of Fe-Cu thermocouple, Fe will be at a higher potential than Cu. Therefore, with the circuit as in the figure, current flows from Fe to Cu in the right junction. That is from high potential to low potential. So, energy is released at this junction, making it. Hence, this junction becomes hot.

Similarly, at the left junction, current flows from Cu to Fe. That is from low potential ‘to high potential, which requires energy. This energy is used up from the already existing heat at this junction. So it gets colder than before

#### 14. What is Peltier’s coefficient?

Peltier’s coefficient is defined as the amount of heat energy absorbed/evolved per second at a junction of a thermocouple when a unit current is passed through it. it is denoted by It and is measured in J/ C. The value of it depends upon the nature of two metals forming a thermocouple and the temperature of the junction.

#### 15. How does the Seebeck effect different from the Peltier effect?

Seebeck effect is the phenomenon in which heating a junction formed by a loop of two different metals gives rise to electromotive force and thus electric current through the whole loop, whereas in the Peltier effect, allowing current through the same loop causes heating of one junction and cooling of the other. In the Seebeck effect, heat energy transforms into electrical energy and in Peltier, electrical energy transforms into heat.

#### 16. What is the Thomson effect?

When two ends of a conductor are maintained at different temperatures, an emf is established across it. The absorption or evolution of heat along the length of a conductor on passing a current through it, when its ends are kept at different temperatures is known as the Thomson effect.

#### 17. What is Thomson’s coefficient?

Thomson’s coefficient is defined as the emf that exists between the two ends of an unequally heated conductor of the uniform area of cross-section, having a temperature difference of 1K between them. It is denoted by σ and is measured in V/K.

#### 18. What are the positive and negative of Thomson’s effects?

Positive Thomson Effect: In Thomson’s effect, there are some substances in which if the current is passed from hot junction to cold junction, heat is evolved, or the overall temperature rises. When current is passed from the cold junction to the hot junction, heat is absorbed, or the temperature decreases. This effect is called the Positive Thomson Effect. Substances that show a positive Thomson effect are Cu, Ag, Zn, etc.

Negative Thomson Effect: There are some substances in which if the current is passed from the hot junction to the cold junction, heat is absorbed, or the temperature decreases. Similarly, when current is passed from the cold junction to the hot junction, heat is evolved or the temperature increases. Such a phenomenon is called the Negative Thomson Effect. The substances which show a negative Thomson effect are Fe, Pt, Co, etc.

#### 19. Discuss the cause of Thomson’s effect.

When two ends of a conductor are maintained at different temperatures, the two ends will be at different potentials. That is one end will be at a higher potential than the other end. Now, if a steady current is maintained in the conductor in such a way that the current passes from the end at the higher potential to the end at a lower potential, energy is evolved (released) and heat is generated in the conductor. However, if the current passes from the end at the lower potential to the end at higher potential, energy is absorbed from the conductor. This means the temperature decreases and cooling is observed

#### 20. What are the uses of the thermoelectric effect?

Thermoelectric effect is used in:

(a) Measuring high temperatures.

(b) Generating emf.

(d) Refrigeration (Cooling).

#### 21. Does thermoelectric effect obey the Law of conservation of energy? [HSEB 2068]

Yes, any of the three thermoelectric effects obey the law of conservation of energy. In the Seebeck effect, heat energy absorbed from external sources is converted into electrical energy, making the total energy constant. It is con  Peltier effect one junction evolves heat (showing an increase in temperature), whereas the other junction absorbs heat (showing a decrease in temperature). The evolved heat is released to the atmosphere where the heat that is absorbed is obtained from the atmosphere. Similarly, in the Thomson effect, electrical energy is converted into heat ever again making total energy constant.

#### 24. What is thermopile?

Thermopile is a device used for the detection and measurement of the intensity of heat radiation. It is based on the Seebeck Effect. It consists of a large number of Sb-Bi thermocouples connected in series so that their thermal emf gets multiplied. Thermopile is very sensitive & can detect a very small amount of heat radiation such as a match stick lighted at a distance of 250 m from the thermopile.