![]() Which is to say that $X(t)$ is entirely independent of whatever happens in the past or future. In physics, we (mostly) deal with separable processes, entailing, One of the key features is that given a random time-dependent variable $X(t)$, and a set of times $t_1, t_2$ and so forth at which we measure $x_1, x_2 \dots, x_n$, there are a set of joint probability distributions, On the other hand, Brownian motion can be thought of as a more specific condition on the random motion exhibited by the system, namely that it is described by a Wiener stochastic process, which is made rigorous by probability theory and stochastic calculus. Random motion is a generic term which can be used to signify that a particular system's motion or behaviour is not deterministic, that is, there is an element of chance in going from one state to another, as oppose to say, for example, the classical harmonic oscillator. Ideally, if ones knows the positions and velocities of all molecules at a given time, we could in principle predict next configuration for both molecules and particles in suspension.īut if you consider thermal motion as an example of random motion, then Brownian motion is a more specific example of the general term "random motion". However the particles have to be small enough so that the effects of collisions with many molecules do not average to zero (or to values to small to matter).īoth molecular motion and Brownian motion can be called "random" (or not) depending of the meaning we associate with this concept of "randomness". So Brownian motion does not refer to the thermal motion of the molecules but is an effect of this molecular motion on particles much larger than one molecules. The motion is due to the random collisions between the molecules of fluid with the particles in suspension. If millions of these tiny circuits could be built on a 1-millimeter by 1-millimeter chip, they could serve as a low- power battery replacement.Brownian motion has a very specific meaning: the motion of small particles suspended in a fluid. The team's next objective is to determine if the DC current can be stored in a capacitor for later use, a goal that requires miniaturizing the circuit and patterning it on a silicon wafer, or chip. "What we did was reroute the current in the circuit and transform it into something useful." In fact, if no current was flowing, the resistor would cool down," Thibado explained. "People may think that current flowing in a resistor causes it to heat up, but the Brownian current does not. The team also discovered that the relatively slow motion of graphene induces current in the circuit at low frequencies, which is important from a technological perspective because electronics function more efficiently at lower frequencies. "This means that the second law of thermodynamics is not violated, nor is there any need to argue that 'Maxwell's Demon' is separating hot and cold electrons," Thibado said. That's an important distinction, said Thibado, because a temperature difference between the graphene and circuit, in a circuit producing power, would contradict the second law of thermodynamics. Though the thermal environment is performing work on the load resistor, the graphene and circuit are at the same temperature and heat does not flow between the two. ![]() With the diodes in opposition allowing the current to flow both ways, they provide separate paths through the circuit, producing a pulsing DC current that performs work on a load resistor.Īccording to Kumar, the graphene and circuit share a symbiotic relationship. Knowing this, Thibado's group built their circuit with two diodes for converting AC into a direct current (DC). In the 1950s, physicist Léon Brillouin published a landmark paper refuting the idea that adding a single diode, a one-way electrical gate, to a circuit is the solution to harvesting energy from Brownian motion. Thibado's team found that at room temperature the thermal motion of graphene does in fact induce an alternating current (AC) in a circuit, an achievement thought to be impossible. The idea of harvesting energy from graphene is controversial because it refutes physicist Richard Feynman's well-known assertion that the thermal motion of atoms, known as Brownian motion, cannot do work. The findings, published in the journal Physical Review E, are proof of a theory the physicists developed at the U of A three years ago that freestanding graphene-a single layer of carbon atoms-ripples and buckles in a way that holds promise for energy harvesting. "An energy-harvesting circuit based on graphene could be incorporated into a chip to provide clean, limitless, low-voltage power for small devices or sensors," said Paul Thibado, professor of physics and lead researcher in the discovery. ![]()
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