March 3---Dust to Down light

Cooking hot dog at beginning of class. 

The hot dag get burned because the hot dog works like resistors.

hot dog with some LED, LED light up when parallel to the hot dog because it is voltage drop. LED not light when it is connected perpendicular to the hot dog because the two pins of LED has same voltage. 

Prelab

Based on ohms law we calculated voltage across the photocell resistance of 5K ohm will be 1.67V and resistance of 20K ohm will be 3.33V.

Analysis:

However we measured voltage when it is light is 0.64V and the voltage when it is dark is 2.74V. 

The percentage difference is (1.67-0.64)/1.67=59% for light condition and (3.33-2.74)/3.33=18%

Clearly the results are way off from the expected value. We expect that happened because of the actual resistance when it is light is much smaller than 5k ohm and the actual resistance at dark condition is slightly smaller than 20K ohm.And it is hard to control every time we take measurements we measure the voltage under same lighting condition, so we expect the value of resistance is different each time we take measurements. That could be the main source of error. 

Actual Circuit used in lab.

DEMO vedio

To improve this experiment, I think keep the photocell a fixed distance from a certain light source and measure the resistance and close all light and measure the resistance. Then make predictions and take measurements. That could reduce the influence by difference lighting condition since we just use hand to block some light during this experiment.