Class 11> Unit # 10: DC Circuits > Thermoelectricity & Thermocouple

Thermoelectricity & Thermocouple in Fire Alarm & Thermostats — Unit 10

Welcome to Talha's Physics Academy. In this lecture, we explore thermoelectricity, the structure and working of a thermocouple, the behavior of thermal EMF with respect to temperature variations, and practical applications in fire alarms and thermostats.

Video Lecture

1. What is Thermoelectricity?

Just as electricity can be generated using wind, hydroelectric dams, or solar energy, electricity can also be produced directly from heat energy.

• Definition: Thermoelectricity is the electrical energy generated as a result of the conversion of heat energy.
• Mechanism: This energy conversion takes place primarily with the help of a specialized device called a thermocouple.

2. The Thermocouple

A thermocouple is a device primarily used to measure temperature, which simultaneously converts thermal differentials into electrical signals (EMF or electric current).

• Construction: It consists of two dissimilar metal wires joined together at a single point (junction) where the temperature is to be measured.
• Functioning: Changes in the temperature of the metal wires stimulate a voltage change, producing a measurable thermal EMF.
• Setup: Typically features a cold junction (maintained at a constant lower temperature, such as in melting ice) and a hot junction connected via low-resistivity copper wires to a sensitive millivoltmeter.

3. Variation of Thermo-Electric EMF with Temperature

Using an iron-copper thermocouple setup (with one junction in a hot oil bath and the other in an ice bath):

• Initial State: When both junctions are at the same temperature (\(0^\circ\text{C}\)), the galvanometer shows zero deflection (thermal EMF is zero).
• Rising EMF & Neutral Temperature (\(T_n\)): As the hot junction temperature increases, thermal EMF rises until it reaches a maximum value at a specific temperature called the neutral temperature (\(T_n\)).
• Inversion Temperature (\(T_i\)): Heating beyond the neutral temperature causes the thermal EMF to decrease, eventually dropping to zero and reversing its polarity at a temperature known as the inversion temperature (\(T_i\)).

$$T_n = \frac{T_i + T_c}{2}$$

(Where \(T_n\) is the neutral temperature, \(T_i\) is the inversion temperature, and \(T_c\) is the cold junction temperature.)

4. Practical Applications

A. Fire Alarms

• Role of Thermistor: A thermistor is a variable resistor whose resistance changes with temperature (resistance decreases as temperature increases).
• Operation: When a fire breaks out, the local temperature rises, causing the thermistor's resistance to drop significantly. This triggers an NPN transistor switch, sending current through a battery-powered buzzer to produce an audible alarm.
• Circuit Components: Uses a 10 kΩ variable resistor, a diode for unidirectional current flow, and a capacitor to smooth out sudden resistance fluctuations.

B. Thermostats

• Definition: A thermostat ("thermo" = heat, "statos" = fixed/stationary) is a control device used to regulate appliances (such as irons, air conditioners, and refrigerators) according to temperature thresholds.
• Circuit Mechanism: Utilizes a voltage divider circuit comprising a thermistor and a variable resistor connected to the base of an NPN transistor. When temperatures cross specific preset threshold values, the circuit switches output indicators (such as LEDs) on or off to control appliances automatically.