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NEB Class 12 Physics Thermoelectric Effects Note Handwritten in PDF
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Seebeck Effect
When two different metals are joined at two junctions and the junctions are at different temperatures, an EMF is generated in the circuit. This is called the thermoelectric EMF or Seebeck effect.
Key facts:
- Discovered by Thomas Seebeck in 1821
- The EMF depends on the temperature difference between junctions and the pair of metals used
- The current that flows is called thermoelectric current
- Common pairs: Copper-Iron, Bismuth-Antimony
Variation of thermoelectric EMF with temperature: As the temperature of hot junction increases (keeping cold junction at 0°C):
- EMF increases to a maximum at neutral temperature (T_n)
- Then decreases back to zero at inversion temperature (T_i)
- Beyond T_i, EMF reverses direction
Neutral temperature: T_n = average of hot and cold junction temps = (T_cold + T_i)/2
For Cu-Fe: T_n ≈ 275°C, T_i ≈ 550°C (when cold junction is at 0°C)
Thermocouple
A thermocouple is a pair of dissimilar metal wires joined at one end (measuring junction) and connected to a measuring device at the other end (reference junction).
Working:
- Seebeck effect generates EMF proportional to temperature difference
- EMF is calibrated against temperature using a known reference
- The measuring device reads temperature directly in modern thermocouples
Advantages over mercury thermometer:
- Works at extreme temperatures (−200°C to +2000°C) mercury freezes and boils within a narrow range
- Very fast response time
- Can measure temperature at a specific point
- Can be used in harsh environments (vacuum, corrosive gases)
- Can be connected to electronic data loggers
Types of thermocouples: K-type (Chromel-Alumel, general purpose), J-type (Iron-Constantan), T-type (Copper-Constantan for low temperatures)
Peltier Effect
The reverse of the Seebeck effect. When electric current is passed through a junction of two dissimilar metals, heat is either absorbed or released at the junction, depending on the direction of current.
Key facts:
- Discovered by Jean Peltier in 1834
- One junction becomes hot, the opposite junction becomes cold
- Reversing current reverses which junction heats and which cools
Applications of Peltier effect:
- Peltier coolers (TEC modules): Solid-state refrigerators with no moving parts, used in:
- CPU coolers in computers (silent, compact)
- Portable mini-fridges
- Cooling of laser diodes, infrared sensors
- Wine coolers
- Thermoelectric generators (TEG): Using waste heat (from car exhaust, industrial furnaces) to generate electricity
Thermopile
A thermopile consists of many thermocouples connected in series, all with their hot junctions on one side and cold junctions on the other.
Why thermopile? Each thermocouple generates a small EMF. Connecting N thermocouples in series multiplies the EMF by N → much more sensitive.
Uses:
- Infrared thermometers (non-contact temperature guns — used for COVID screening)
- Heat flux sensors (measuring heat flow in walls, solar panels)
- Night vision and thermal imaging (detects infrared radiation from warm objects)
- Power generation from body heat (experimental wearable devices)
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Frequently Asked Questions
What is Seebeck effect in Class 12 Physics?
When two dissimilar metals are joined at two junctions held at different temperatures, an EMF is generated causing current to flow. This is the Seebeck effect. The EMF depends on temperature difference and the pair of metals used. It is the basis of thermocouples used for temperature measurement.
What is neutral temperature and inversion temperature Class 12?
As hot junction temperature increases (cold junction fixed at 0°C), thermoelectric EMF increases to maximum at neutral temperature (T_n) then decreases to zero at inversion temperature (T_i). Beyond T_i EMF reverses direction. For copper-iron thermocouple: T_n ≈ 275°C, T_i ≈ 550°C when cold junction is at 0°C.
What is the advantage of thermocouple over mercury thermometer?
Thermocouple works from −200°C to over 2000°C mercury thermometer only works from −39°C to 357°C. Thermocouple has very fast response time (milliseconds). Can measure temperature at a precise point. Can be connected to electronic data loggers. Works in vacuum and corrosive environments where mercury thermometers fail completely.
What is Peltier effect in Class 12 Physics?
When electric current passes through a junction of two dissimilar metals, heat is absorbed at one junction (cooling) and released at the other (heating). This is the Peltier effect the reverse of Seebeck effect. Reversing current direction reverses which junction heats and which cools. Used in solid-state refrigerators and CPU coolers.
What is the difference between Seebeck effect and Peltier effect?
Seebeck effect: temperature difference between junctions generates EMF and current (heat → electricity). Peltier effect: passing current through junction causes heating or cooling (electricity → heat difference). They are exact reverses of each other. Seebeck is used in thermocouples for measurement; Peltier is used in cooling devices.
What is a thermopile and its uses?
Thermopile is multiple thermocouples connected in series all hot junctions on one side, all cold junctions on other. Series connection multiplies total EMF making it much more sensitive than a single thermocouple. Uses include infrared thermometers (non-contact temperature guns), heat flux sensors, night vision devices, and thermal imaging cameras.
What is thermocouple and how does it work Class 12?
Thermocouple has two dissimilar metal wires joined at the measuring junction and connected to a meter at the reference junction (kept at known temperature, usually 0°C). Seebeck effect generates EMF proportional to temperature difference. The EMF is calibrated to read temperature directly. More accurate and versatile than glass thermometers.
Which thermoelectric topics are most important for Exam?
Most important: Seebeck effect explanation (2 marks), neutral and inversion temperature definition (2 marks), Peltier effect and its difference from Seebeck (2 marks), thermocouple working and advantages (2 marks), thermopile uses (2 marks). These are mostly short 2-mark questions but appear regularly in NEB board papers.
