Relaxation (physics)

Relaxation designated in the natural sciences the transition of a system through relaxation to its ground state or in a state of equilibrium (often after an excitation or an external disturbance ).

The relaxation time (more precisely, relaxation time constant ), the characteristic time in which a system (usually exponentially ) approaches the steady state. Clearly, the system has moved noticeably to the duration of relaxation time to its equilibrium state; after a period of three to six relaxation time constants can usually start from a largely closed relaxation. The inverse of the relaxation time constant is referred to as a relaxation rate.

The distinction of " relaxation " and " relaxation time " makes sense, since in experiments to observe or quantify the relaxation of the free choice of time during which you can relax a system as " relaxation time " is called.

Mathematical Description

If the relaxation of a variable from the initial value to the asymptotic end value follows an exponential law:

Then the associated relaxation time and the relaxation rate.

After a relaxation time constant (), the size has approached the final value of up to 36.8 %, compared to 13.5 % and by up to 5.0%; that is, the system is at this time to 95% ( or almost complete) relaxes.

In the case of more complicated (for example, stretched - exponential ) time dependence, one can define the relaxation time as

Examples

• Said heat transfer with the thermal relaxation time; This describes how fast the temperature of a body adapts to the new ambient temperature. Here, the mass of the body, the specific heat capacity, the heat transfer coefficient, and the interface.
• The magnetization relaxation in nuclear magnetic resonance (NMR) or electron spin resonance (ESR) with the relaxation times and the longitudinal and transverse magnetization.
• The charging and discharging process of the capacitor of an RC element in the electronics with the relaxation time, the time constant # see capacitor.
• The Violent relaxation of the kinetic energy of a star or galaxy cluster gas so that adjusts a thermal equilibrium.
• The timing of a chemical reaction as measured by the relaxation method, see also kinetics (chemistry).
• The temporal decrease in stress at constant strain in the strength of materials, see relaxation test.

Other meanings

In solid-state physics and surface chemistry, the presence of altered atomic distances at or near the solid surface is referred to as (surface) relaxation. This is not a dynamic relaxation process in accordance with the description given above.