Thermo Couple - Practice Questions & MCQ

Thermocouple

Thermocouple:  Two wires of different metals connected at two points to form two junctions. This thermoelectric device used to measure the temperature is called a thermocouple. If one junction of the thermocouple is at lower temperature and the other is at a higher temperature then a current starts flowing through the thermocouple. 

Seeback Effect - 

According to this when the two junctions of a thermocouple are kept and maintained at different temperatures, then a current starts flowing through the loop made by conductors known as thermo-electric current. Because of this potential difference will develop between the junctions which is called thermo electric emf which is of the order of a few micro-volts per degree temperature difference.

Seebeck arranged different metals in the decreasing order of their electron density. A few metals forming the series are as below.

Sb, Fe, Cd, Zn, Ag, Au, Cr, Sn, Pb, Hg, Mn, Cu, Pt, Co, Ni, Bi

• Neutral temperature: 

Keeping the temperature of cold junction constant and increasing the temperature of the hot junction, the emf increases and become maximum at a particular temperature. This temperature of the hot junction is called neutral temperature(). If the temperature is further increased the thermal emf start decreasing and at a particular temperature, thermal emf becomes zero. If the temperature is further increased the thermal emf start reversing. The temperature of the hot junction at which the thermal emf start reversing is known as inversion temperature()

$
\Theta_n=\frac{\Theta_i+\Theta_c}{2}
$

$\Theta_n=$ Neutral Temperature
$\Theta_i=$ Inversion Temperature
$\Theta_c=$ Cold Temperature

Thermo electric emf is given by the equation -

$
E=\alpha t+\frac{1}{2} \beta t^2
$

where $\alpha$ and $\beta$ are thermo electric constant ( $\mathrm{t}=\mathrm{temperature}$ of hot junction).
For E to be maximum at $\mathrm{t}=\mathrm{t}_n$, we will differentiate Electric field with respect to temperature of the hot junction and we get -

$
\frac{d E}{d t}=0 \text { i.e. } \alpha+\beta t_n=0 \Rightarrow t_n=-\frac{\alpha}{\beta}
$
 

If the temperature of hot junction increases beyond neutral temperature, then there is decrease in the thermo emf and at a particular temperature it becomes zero, if heat is supplied further, the direction of emf is reversed. This temperature of hot junction is called temperature of inversion (t_i).

$t_n=\frac{t_i+t_c}{2}$                                                                                            Here, t_c is the temperature of cold junction.