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Q & A: Ohm's law and time reversal

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Most recent answer: 10/27/2014
Q:
The time reversal symmetry must break down for Ohm's law otherwise some other Laws Of Physics Would Be Violated.(which Ones)??
- tanima gupta (age 21)
sitapur,uttar pradesh,India
A:

Let's start with a picture of a circuit with a resistor in it, taken from the images on Wikipedia.

RC circuit Say that the capacitor C is initially charged up. Current I will flow through the resistor R draining off the charge. The resistor will heat up because the current dumps energy into the resistor at rate I2R. You end up with an uncharged C and with R slightly hotter than you started with. 

What would the time-reversed process be? An uncharged C would charge up as current flowed through R and as R cooled down! Since that event has never been observed, we say that nature doesn't behave symmetrically in time. 

The general rule for that universal asymmetrical behavior is that "entropy" increases. That means that systems drift toward conditions compatible with larger numbers of quantum states. In this case, taking the electrostatic energy out of the capacitor and distributing it among all the little internal parts of the resistor lets the resistor reach many more quantum states. The general rule of entropy increase is called the Second Law of Thermodynamics.

Mike W.


(published on 10/27/2014)

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