El radiòmetre

 

This is an instrument that can be used to measure indirectly the amount and intensity (strength) of sunlight. It is also an interesting "toy" to put on a window sill!
The radiometer is the coolest science toy around! A clear glass bulb, partially evacuated, contains 4 vanes or wings balanced on a virtually frictionless pin bearing. The vanes are black on one side, white on the other. When placed in sunlight or under a bright incandescent lamp, the vanes will spin around. The brighter the light, the faster the vanes will spin. It is very fragile!

 

Construeix-te’n un (si no vols comprar el de la Curie)

  1. Seal the small end of an eyedropper in a flame. After it cools, cut one inch off the end with a file.
  2. Punch a hole in the centre of a 2 inch square piece of light card so it fits snugly over the tip of the eye dropper.
  3. Remove the cardboard and make cuts shown by the solid lines.
  4. Bend the card down along the dotted lines to form vanes.
  5. Blacken the side of each vane by holding the card carefully over a candle. Mark one side with an "X"
  6. Stick the large end of a needle into a cork and place the cork in the glass jar.
  7. Set the eyedropper tip and cardboard vanes over the needle.
  8. Fasten the lid and try to remove as much air as possible with a vacuum pump.

Since most of the air has been removed from inside the radiometer, the paddle wheel or vanes are free to move. Normally the vanes would strike air molecules and be slowed or stopped. With the air removed, he slightest sunlight energy striking the vanes will start the paddle truning.

 

Student Materials | Science as Inquiry

Radiometer Experiment

How does solar heat reach the earth?

This simple toy provides a demonstration of heat transfer. What makes the vanes move?

Procedure

Place the radiometer in a bright light and observe what happens.

Try different light sources of different intensities. Cover the radiometer with a black cloth and look under the cover. What are the vanes doing?

Questions

1. Describe the motion of the radiometer vanes under the various conditions.

2. In terms of a particulate model of gases, explain what you have observed.

3. Light actually does exert a pressure. It is called radiation pressure. As a result, which way should the vanes turn from the pressure of light? How do you know that this pressure is much lower than the pressure produced by heated gas particles?

4. Using a particulate model for gases, explain why the radiometer would not work if the vanes were removed from the enclosure and the experiment repeated using air in the room.

5. Set up an arrangement where sunlight is passed through a prism. Place a radiometer in that region just outside of the red end of the spectrum where no visible light strikes the radiometer vanes. What do you observe? Explain this observation.

Transmisión del Calor por Radiación

Transmisión del Calor por Radiación- El Radiómetro de Crookes La Transmisión del Calor por Radiación no requiere de un medio material interpuesto entre la Fuente de Calor y el Receptor del mismo. En el Laboratorio de Física logramos reproducir esta situación, de por si bastante ideal, utilizando el Dispositivo denominado Radiómetro de Crookes. El mismo consta de una Ampolla de Vidrio Pyrex, donde se ha efectuado un vacío importante, que contiene en su interior un Molinete de cuatro aspas. Cada aspa se encuentra pintada de un lado con una Pintura Negra y del otro lado con una Pintura Aluminizada. Cuando se acerca una Fuente de Calor a la Ampolla el Molinete comienza a girar. La Pintura Negra absorbe más Energía Calórica que la Aluminizada. La Energía absorbida tiende a pasar del lugar de mayor concentración al de menor.Esto provoca una Cupla que hace que el Molinete gire. Cuando en Verano vestimos ropas oscuras, "sentimos" más el calor que con ropas claras. Por una razón similar se recubre el recapado asfáltico de las azoteas con una pintura aluminizada. Esta refleja más el Calor y atenúa el recalentamiento de la Loza. .