| | A Peltier device (Fig. 1) can be run in reverse of the heat pump mode (Phenomenon 5) in which case it functions as a heat engine—in this case as an electric generator (compare to the Stirling engine: Phenomenon 7). Placed between a hot and a cold body of water (Fig. 2), a voltage develops across the terminals of the device. If we attach a load to the device, it drives an electric process (Movie 1) whose power can be measured from voltage and electric current.
Interpretation The measured power is proportional to the square of the temperature difference between the hot and the cold water (Fig. 3). This is analogous to what we know of the power of an electric load resistor which is proportional to the square of the voltage. The energy released is proportional to temperature differences. If the flow of heat (*) is also proportional to the temperature difference, the resulting dependence of power on the temperature difference is what we observe in Fig. 3: Power depends upon the square of the temperature difference.
 (*) Note: The technical term for quantities of heat is entropy. What we call heat, and what is officially called entropy by engineers and physicsist is closely related to what scientisits before 1850 called caloric. When presenting phenomena in terms of standard language, it makes sense to keep using the word heat for what we normally call entropy.
| 
Figure 1
Figure 2 |
