Lab Report Day Three - Hotdog, Dusk-to-Dawn Light

1. In the beginning of class, Professor Mason brings some hotdog, and try to cook the Hot Dog
a.) He attach a hotdog to a line cord and apply a 120 V potential across it.
We predict that nothing will happen. But based on the experiment,  it seems that we are wrong.

The hot dog is being slowly cooked.

Here is a cool video about a hotdog being cooked by applying 120 V potential. 

b.) Then, we have some parallel and some perpendicular LEDs on the hotdog. We are asked to predict what will happen when we apply a 120 V potential.
Answer is B. The LEDS parallel to the hotdog will light.

In the hotdog, when we apply a 120v potential on it, it will spread equally. When the LED is perpendicular to the hotdog, the potential has no difference, while when the LED is parallel to the hotdog, there is some potential difference existing. The potential difference makes the LED light up.

 

2.  Dusk-to-Dawn Light

In this lab, we are asked to build up a circuit which the LED in it only lights up when it is dark. Being able to do it, we need a bipolar junction transistor (BJT), and a photocell, which is a light sensible resistor. The current in the BJT can only flow when there is a voltage bigger than 2 V applying on it. When it is dark, the photocell has a high resistance, resulting of a high voltage. When it is light, it has a low resistance, resulting of a low voltage. Thus, the BJT acts like a switch.
This is the pre-lab of this experiment. We are asked to calculate the actual voltage across the photocell. When it is dark, it has a resistance of 20k, and has a voltage of 3.33v, which is higher than 2 v.  When it is light, it has a resistance of 5k, and has a voltage of 1.67v, which is lower than 2 v.

This is the set up of the experience. At room light, it does not light up.

When we cover the photocell with sleeves, it starts to light up.

 When it gets dark, it starts to light up.

 

 

 

 

 

 

This is the voltage across the resistor when there’s light. Our experimental value is 1.82 V, while our theoretical value is 1.67 V. 

% difference: (1.67-1.82)/1.67*100%=-8.98%

There is difference probably because the room light is darker than our theoretical light. 

 

 The voltage of it across the resistor when it is dark. 

Our experimental value is 2.60 V, while our theoretical value is 3.33 V. 

% difference: (3.33-2.60)/3.33*100%=21.9%

There is difference probably because the brightness in the sleeves is brighter than our theoretical light. 

 

Here is a video about our experiment.

 

Summary

Today we learn more with Kirchhoff’s Law. We do an amazing experiment to cook a hot dog with electricity, and let the LED light on the hot dog. We learn how night light works and talk about BJT.