Speaker
Description
A red LED and a green LED are wired in parallel and attached to a lamp cord. The LEDs are oriented in opposite directions, so when one of them is forward biased the other is reverse biased. When plugged in to a wall outlet, at any instant only one of the LEDs will be forward biased; this switches between red and green with a 60 Hz frequency. Waving the end of the cord back and forth (or twirling it) in a darkened room makes alternating red and green streaks visible, an indicator of the alternating voltage difference from the wall outlet. Careful observation shows that the red streaks are slightly longer than the green ones, an indicator of the different band gaps of the two LEDs. I will provide instructions (and make-and-take kits until they run out) for building this demonstration using inexpensive materials.
