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"Why" is generally a difficult question to answer, generally. But in this case it is really easy to draw a mental picture:
Imagine a complete set of billiard balls nicely ordered in their usual triangle formation. This quite literallyAs a analogy, this corresponds to something that is comparatively cold, (i.e., the atoms wrigglewiggle around relatively little; obviously theylittle and are more ordered when colder - of course, colder atoms are not standing still like the billiard balls).
Now the game starts, and a good player hits the balls so they are spread out over the whole table. This corresponds to the higher entropy of a warmer situation;situation, (i.e. the wrigglingdisorder of the warmer, more wiggling atoms increases).
Note that it does not matter how exactly the balls come to rest: any configuration of the balls is widely different from the original starting triangle, while there - this disorder corresponds to a higher entropy state. There is only one low entropy, highly ordered starting position. Also, itand many high entropy, disordered states after the breaking the triangle. It is very easy to create any chaotic configuration of the balls (just hit them with the cue however you like), while it. It is quite complicated to return themunlikely to produce an ordered state, like the triangle configuration if you only can use, (which corresponds to ordered, coherent, useful energy). Collision with the same cue is very unlikely to movereturn them around, notto the triangle frame (i.e. it is unlikely for atoms randomly colliding with each other to all move in the same direction).
Now, to relate to your question:
- The balls are the atoms.
- Balls in an ordered state (triangle) correspond to colder atoms.
- Balls in a chaotic state correspond to warmer atoms.
- Electrons correspond to the cue ball (in a wire with current flowing in it).
Conclusion/answerAnswer
Orderly electrons (currents) randomly hitting atoms easily make thosethe atoms they hit in a wire wiggle around more (heat them up);, which means it is easy to build an electric heater.
HeatedThe reverse, heated atoms somehow makingcausing electrons moving aroundto move in an ordered fashion, will not happen spontanouslyspontaneously because it is an extremely unlikely possibility. Therefore, there will never bewe cannot depend on an "accidental" current being generated from heat.