Antimony and bismuth are usually used in a thermocouple, because

Options

A. 1.a constant thermo e.m.f. is produced
B. 2.higher thermo e.m.f. is produced
C. 3.a negative thermo e.m.f. is produced
D. 4.lower thermo e.m.f. is produced

Answer

To solve the question regarding why antimony and bismuth are usually used in a thermocouple, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Thermocouples**:
A thermocouple is a device that converts thermal energy into electrical energy. It consists of two different metals joined at one end, which generates a voltage (thermoelectric voltage) when there is a temperature difference between the two junctions.

2. **Seebeck Coefficient**:
The Seebeck coefficient (S) is a measure of the thermoelectric voltage generated per unit temperature difference. It is expressed in microvolts per Kelvin (µV/K). Different materials have different Seebeck coefficients.

3. **Values of Seebeck Coefficient**:
- For antimony (Sb), the Seebeck coefficient \( S_1 = 47 \, \mu V/K \).
- For bismuth (Bi), the Seebeck coefficient \( S_2 = -72 \, \mu V/K \).

4. **Calculating the Difference**:
To find the effective Seebeck coefficient for the thermocouple made of antimony and bismuth, we calculate the difference:
\[ \Delta S = S_1 - S_2 = 47 \, \mu V/K - (-72 \, \mu V/K) = 47 + 72 = 119 \, \mu V/K. \]

5. **Significance of the Difference**:
The larger the difference in Seebeck coefficients between the two materials, the higher the thermoelectric voltage (thermo emf) produced. Since antimony has a high positive value and bismuth has a significant negative value, their combination yields a large voltage output.

6. **Conclusion**:
Therefore, antimony and bismuth are used in thermocouples because they produce a higher thermo emf due to the significant difference in their Seebeck coefficients.

### Final Answer:
Antimony and bismuth are usually used in a thermocouple because they produce a higher thermo emf due to the large difference in their Seebeck coefficients.

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