Presentation on theme: "Balkan Summer Institute 2011 (BSI2011)"— Presentation transcript:
1 Balkan Summer Institute 2011 (BSI2011) Seminar for teachers BSS2011: Trends in Modern Physics (BSS2011)Measuring the Surface Tension of Water by Light Diffraction on Capillary WavesLjubisa NesicFaculty of Science, Nis, SerbiaandDragisa NikolicGrammar School, Pirot, Serbia
3 Surface tensionThe cohesive forces among the liquid molecules are responsible for this phenomenon
4 Surface tensionThis creates some internal pressure and forces liquid surfaces to contract to the minimal areaSurface tension is measured in newton per meter (N/m) or joules per square meter (J/m2)
5 Surface tension and capillarity Capillary action, or capillarity, is the ability of a liquid to flow against gravity where liquid spontaneously rises in a narrow spaceThe adhesive forces among the liquid molecules and solid surrounding surfaces are responsible for this phenomenon
7 Surface tension - contraction to the minimal area If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and forces of adhesion between the liquid and container act to lift the liquid.
9 Measuring of surface tension Static (ordinary) methodinvolves the measurement of the force to be exerted on a piece of metal as it is pulled from a liquid.Butit cannot be used to continuously measure the surface tension.It is also difficult to use in some environments.
10 Capillary waves Type of the surface waves on water Waves - result of a generating forceblowing wind, vessels moving, volcanic eruptions, submarine earthquakes, …, stones, …An additional force - restoring force - acts to return the surface of the water to its original flat level.two different restoring forcesgravity (gravity waves)surface tension (capillary waves - ripples)
11 wave form moves and with it, energy is transmitted Wave directionWavewave energy – NOT the water particles – moves across the surface of the seawave form moves and with it, energy is transmittednearly all water movements are turbulent (non-laminar)
14 As more energy is transferred to the water, the waves will grow in size As the waves grow, gravity will become the restoring force and the waves will be gravity waves.Speed of the waves u?Many approachesAnalytical mechanics, conservation of energy, dimensional analysis, …
15 Dispersion relation ug= / T = / k is the function of: wavelength , gravitational field strength g, density of water u=const g The variation of wave speed with wavelength is called dispersion and the functional relationship is called the dispersion relation.
16 Dispersion relationIf surface tension is taken as a dominant restoring force, the waves are capillary, and their speed will depend on density, wavelength and the surface tension (): u=const
17 Capillary and Gravity waves Capillary waves- wavelength < 1.73 cmGravity waves- wavelength > 1.73 cm
19 ExperimentA laser beam is aimed at a reservoir containing water, with ripples induced by a wire attached to a speaker driven by a function generator. The surface ripples act as a diffraction grating causing the interference spots on the screen.