= Peltier effect
{c}
The <Peltier effect> is <heat transfer> associated with an <electric current> crossing a junction between materials with different <Peltier coefficients>. At fixed junction <temperature>, its contribution changes sign when the <conventional current> reverses. A junction can absorb <heat> from its surroundings for one direction and release it for the other. <Joule heating>, proportional to the square of the <electric current>, accompanies this transport but does not reverse sign. Consequently, observing a warm device alone does not identify the <Peltier effect>.
This makes the <Peltier effect> useful as an electrically controlled <heat pump>: reversing the supply can switch an object's thermal role without changing any plumbing. In <thermoelectric cooling>, however, the useful cooling is the junction's absorbed <heat> minus returning <heat conduction> and its share of <Joule heating>. Increasing <electric current> eventually makes cooling worse. The distinction between transported <heat> and net cooling is the central practical lesson of the <thermoelectric cooler energy balance>.
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