Day 3 - Tuesday - March 3, 2015 - Dusk-to-Dawn Light

Today we started the day with a fun conceptual check about the drop in potential on circuit elements. Prof. Mason grabbed a hot dog with some LEDs and hooked them up as part of a circuit as shown below and asked which LEDs would turn on if any.

Counting from the left, the first, fourth and fifth LEDs are positioned so that their leads are parallel with the hot dog and the current. The second and third LEDs have their leads perpendicular to the hot dog and current. Remembering that potential drop occurs as current goes through a circuit element, it may seem at first that all LEDs should light up. However, the orientation of their leads makes a decisive difference.
The LEDs that have their leads perpendicular to the current receive current with the same amount of potential on both of their leads, meaning that there is no potential difference across them; hence, no current will flow through them. Meanwhile the LEDs with their leads parallel to the hot dog will receive currents with different potential. This means that there is a potential difference across them and current will flow and they will light up.
However, since they are getting their energy from the hot dog, they will receive very little because the hot dog has a high resistivity. This also means that the hot dog will get very hot as shown in the video below.

Then we worked out a problem.

Dusk-to-Dawn Light

This is the data. 

Below is a video of our circuit working