Rüchardt experiment

The Rüchardt experiment is an experiment in thermodynamics that is used to determine the molar heat capacities and for gases. It is based on the fact that the temperature of a gas changes specifically to the pressure. The concrete test result is the isentropic exponent . It was named after the physicist Eduard Rüchardt (1888–1962). ${\ displaystyle c_ {p}}$ ${\ displaystyle c_ {V}}$ ${\ displaystyle \ kappa = {\ frac {c_ {p}} {c_ {V}}}}$

background

If a gas is compressed adiabatically , i. H. If there is no discharge of heat from the system, then as a result of the temperature increase, the pressure rises more strongly than with isothermal compression, in which the resulting compression work is dissipated as heat. The exponent with which the expansion of the gas due to the supply of heat can be calculated is called the isentropic or adiabatic exponent. This value is determined by the Rüchardt experiment.

An adiabatic and reversible change of state is isentropic ( entropy S remains the same, temperature T changes). In technology, an adiabatic change of state (e.g. in a steam turbine) is not isentropic, since friction, throttling and shock processes produce entropy.

experiment

Rüchardt experiment, glass cylinder with piston and Logger-Pro measuring devices

As a rule, a glass vessel (volume V) with a tubular opening at the top (pipe cross-section A) is available for the experiment. A piston (mass m) is dropped onto the open tube and hermetically seals the opening. The enclosed gas is initially compressed by the weight of the piston, which leads to an increase in temperature. As the process continues, the piston performs a harmonic oscillation on the gas cushion , which is dampened . The result is a rapid series of expansion and compression of the gas. The period T of the oscillation and the relative pressure p in the vessel are measured .

the equation

{\ displaystyle \ kappa = {\ frac {c_ {p}} {c_ {V}}} = {\ frac {4 \ pi ^ {2} mV} {A ^ {2} T ^ {2} p}} }

provides the isentropic exponent for this gas.

Web links

Internship instructions ( Memento from April 2, 2015 in the Internet Archive ) (Uni Graz; PDF; 205 kB)
Video: Determination of Cp / Cv according to Rüchardt . Institute for Scientific Film (IWF) 2004, made available by the Technical Information Library (TIB), doi : 10.3203 / IWF / C-14879 .