Tell me more ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.
up vote 1 down vote favorite

Ok this is a really silly question, but I was really curious as to why this wouldnt work. I'm just starting my electricity and magnetism course, so I really dont know much about electricity. I know this is not possible, but why? alt text

share|improve this question
The name "The Hedgehog From Hell Industries" clearly indicates that this is a cheap prank. – Bernhard Heijstek Jan 20 '11 at 4:31
1  
Well I know, but it still got me thinking.. – maq Jan 20 '11 at 4:51
I don't think that this question belongs here. – gigacyan Jan 20 '11 at 8:11
It is too basic so I close it (hopefully) to prevent downvotes, yet I believe it should stay here as a reference, especially with sb1's answer. – mbq♦ Jan 20 '11 at 9:42
1  
Ever heard from "Baron Münchhausen"? He pulled himself out of a swamp gripping his own hair. In earnest: there are some gadgets offerd to be fed by the USB-connector, which are nonsense, because the USB will feed .5 Amp maximum. I think this cabe is a joke on this or the silliness of certain "cable buyers" in general. – Georg Jan 20 '11 at 12:37
show 1 more comment

closed as off topic by mbq♦ Jan 20 '11 at 9:42

Questions on Physics Stack Exchange are expected to relate to physics within the scope defined in the FAQ. Consider editing the question or leaving comments for improvement if you believe the question can be reworded to fit within the scope. Read more about closed questions here.

1 Answer

up vote 6 down vote accepted

Sorry, but your self charging laptop can not work since it will violate the second law of thermodynamics. A computer processes information and during computation it generates heat. This heat is dissipated through the cooling system. You need continuous supply of usable energy from an external source to run the laptop.

In general, the second law of thermodynamics asserts that it is impossible to create a perpetual motion machine of the second kind. All real machines have efficiency less than 100% which means output power is less than input power. The difference is lost/ wasted as heat or other useless forms of energy. You must have noticed your laptop's temperature is high and making noisy sound during a heavy computation.

I hope you got the point.

share|improve this answer
3  
Hi sb1, don't you primarily need the first law of thermodynamics, the conservation of energy here? ;-) – Luboš Motl Jan 20 '11 at 6:54
1  
abosolutely lubos ;-) but i thought, it would be trivial to mention this ;-) – user1355 Jan 20 '11 at 7:38
Although, I suppose the first law by itself would allow you to charge your computer using its own USB port as long as the computer didn't do anything else ;-) but then it wouldn't really be a computer, of course. – David Zaslavsky♦ Jan 20 '11 at 10:42
One does not need any thoughts on "information" here. The whole thing can be wieved as a accumulator of, say 12 Volt, a step-down stage making 5 Volts from those 12 Volts with an efficency of less than 100 % and then a feedback cable from 5Volt USB to the charger input which requires a bit above 12 Volt. (I assumed the laptop has a 12 Volt accumulator, maybe those laptops have 15 Volt, but this does not change the nonsensicality) – Georg Jan 20 '11 at 12:49

Not the answer you're looking for? Browse other questions tagged or ask your own question.