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cyclical phenomenaany repetitive or recurring social processes in which a sequence of events is followed by a similar sequence on completion. Numerous social processes are accepted as manifesting a cyclical pattern (e.g. the life cycle); other suggested cyclical patterns (e.g. historical cycles, the CIRCULATION OF ÉLITES) are more controversial.
Bourdon and Bourricaud (1989) identify an important general category of cyclical phenomena, i.e. those that result when ‘a process, in developing, causes a negative feedback to arise, which ends in a reversal of the process’. In ECONOMICS, the well-known cobweb theorem has this basis: producers tend to estimate future prices on the basis of current process, thus, they tend to produce an excess of products they think will be most profitable, and insufficient quantities of goods which they estimate will be less profitable, producing, when graphically expressed, a cyclical spider's web-like pattern of movements from one equilibrium position to another. A more straightforward example is provided by patterns of take-up of vaccination: high levels of vaccination lead to fewer illnesses due to a particular disease, leading to fewer vaccinations and a return of the disease, leading in turn to a renewal of take-up of vaccination. One attraction of conceptualizations of social reality as involving cyclical processes is that these can often be formulated mathematically although such models rarely manifest themselves in a pure form in social life.
in thermodynamics, a process in which a physical system (such as steam) returns to its initial state after undergoing a series of changes.
At the end of a cycle the thermodynamic parameters and characteristic functions of the system’s state (such as the temperature T, pressure p, volume V, internal energy U, and entropy S) once again assume their original values, and consequently the changes in them during the cycle are equal to zero (Δ £7 = 0, and so on). All changes arising as a result of a cycle occur only in the medium surrounding the system. In some sections a system or working body accomplishes positive work through its internal energy and the quantities of heat Qn gained from external sources, whereas in other sections of the cycle extrinsic forces perform work on the system, part of which goes to restore the system’s internal energy.
According to the first law of thermodynamics (the law of conservation of energy), the work accomplished in a cycle by or on a system (A) is equal to the algebraic sum of the quantities of heat (Q) received or given up in each section of the cycle (ΔU = Q - A = 0, A = Q). The ratio A/Qn (the ratio of the work accomplished by the system to the amount of heat received by it) is called the efficiency of the cycle.
A distinction is made between equilibrium cycles (or, more accurately, quasiequilibrium cycles), in which the states through which the system passes are close to equilibrium states, and nonequilibrium cycles, in which at least one of the sections is a nonequilibrium process. The efficiency is at a maximum in equilibrium cycles. A graphic representation of an equilibrium (reversible) Carnot cycle with maximum efficiency is shown in Figure 1.
A cycle is called direct if it results in the accomplishment of work on external bodies and in the transfer of a certain amount of heat from a hotter body (the heater) to a colder body (the cooler). A cycle that results in the transfer of a certain amount of heat from the cooler to the heater through extrinsic forces is called a reverse or cooling cycle.
Cycles have played a prominent role in physics, chemistry, and technology. The design of various equilibrium cycles was historically the first method of thermodynamic research. This method made possible, on the basis of analysis of the working cycle of an ideal heat engine (the Carnot cycle), the derivation of a mathematical expression for the second law of thermodynamics and the construction of the thermodynamic temperature scale. Many important thermodynamic relations (such as the Clausius-Clapeyron equation) were found by examining the corresponding
cycles. Cycles are used in technology as the working cycles of internal-combustion engines and of various thermal-power and refrigeration units.
REFERENCESKrichevskii, I. R. Poniatiia i osnovy termodinamiki. Moscow, 1962.
Kurs fizicheskoi khimii, 2nd ed., vol. 1. Edited by la. I. Gerasimov. Moscow, 1969.
ii. A complete process of starting at zero, passing through two maximums of opposite direction, and returning to zero again.
Each instruction takes a number of clock cycles. Often the computer can access its memory once on every clock cycle, and so one speaks also of "memory cycles".
Every hacker wants more cycles (noted hacker Bill Gosper describes himself as a "cycle junkie"). There are only so many cycles per second, and when you are sharing a computer the cycles get divided up among the users. The more cycles the computer spends working on your program rather than someone else's, the faster your program will run. That's why every hacker wants more cycles: so he can spend less time waiting for the computer to respond.
The use of the term "cycle" for a computer clock period can probably be traced back to the rotation of a generator generating alternating current though computers generally use a clock signal which is more like a square wave. Interestingly, the earliest mechanical calculators, e.g. Babbage's Difference Engine, really did have parts which rotated in true cycles.
cycle(1) A repeated event. For example, in a carrier frequency, one cycle is one complete wave. A complete charge or discharge of a battery is one cycle.
(2) A set of repeated events. For example, in a polling-based network, all attached terminals are accessed in one cycle. See machine cycle and memory cycle.