Experiment 4 - Crookes in action

The Crookes radiometer is a very neat scientific toy. The paddles inside the glass bulb move when the radiometer is exposed to light.

This toy has a series of paddles inside a glass bulb. The paddles are not connected to any motive power, and there is no motor or solar cells involved. Yet those paddles go around and around, seemingly for ever. It looks like perpetual motion, but really it is being powered by the Sun - or whatever light source is nearby. The glass bulb may prevent mechanical energy being used to turn the paddles, but it doesn't stop light, and in this form, energy is constantly streaming into the device.

Click the image to watch the video.

Thanks to www.gyroscope.com for permission to use their video.
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Experiment 4 - Crookes in action

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It used to be thought that the radiometer worked by the impact of light - photons have momentum because of their energy. But there isn’t enough light energy to push the paddles this way. And there’s something wrong with the way they move. (See book for more.)
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