Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Vibrator to generate continuous waves

Demonstration

Some ripple tank experiments previously done with wave pulses can be repeated with continuous waves, either plane or circular. Continuous straight waves are also used in other experiment collections in the Waves topic.

Apparatus and materials

Ripple tank and accessories

Motor mounted on beam, with beam support

Rubber bands, 2

Leads, one set, to motor

Dipper

Dry cells, 2

Rheostat

Health & Safety and Technical notes


Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately. 
 
Place the power supply for the lamp on a bench, not on the floor by the tank.

Power for the motor

The motor works well from a 1.5 volt cell in series with a 12-ohm rheostat. Two cells may be needed for the higher speeds but the motor then goes rather fast with the rheostat set at its minimum value. The polarity of the battery determines the direction of rotation, but that is immaterial. (If a battery with a higher e.m.f. is used, a rheostat with a correspondingly high resistance would be required.) 
 
Some manufacturers supply special power units for use with the ripple tanks. These provide the necessary voltage for the lamps and also a variable voltage output to drive the motors. They avoid the need for a transformer to light the lamp and a separate supply for the motors and some teachers may prefer to use them despite the extra cost. 

 

Procedure


Tank and wooden beam

To produce circular waves 
Take the wooden beam with the motor attached and hang it by two rubber bands of such length that the wood is above the water.

Attach a small spherical dipper to the vibrator by its L-shaped rod and adjust it so that the bottom of the sphere is about level with the surface of the water. 
 
At low frequencies, it is easy to see the waves; but at higher frequencies the persistence of vision obscures them. Blinking makes them visible. 
 
To produce straight waves 
Remove the dipper and re-adjust the height of the wooden beam so that the beam itself is about level with the surface of the water. When the beam is set vibrating, straight waves will travel across the ripple tank. If the beam is too deep in the water (or sitting on its glass bottom!) the ripples do not travel very far; if it is too shallow and the vibration is vigorous, the ripples are less distinct near the vibrator. 

 

Teaching notes


You will find that some experiments are best done with pulses only, as the reflections from continuous waves produce confusing patterns.  

For best results, the filament of the lamp should be parallel to the ripples. 

If the wooden rod does not vibrate enough, increase the eccentric loading on the shaft of the motor. 
 
This experiment was safety-checked in February 2006