03/01/2016

03/01/2016: More on Kirchhoff's Law, Current Division, and Voltage Divider

I. Introduction:

Today we started out with "sausage". Professor Mason got a good-looking sausage. However, he wanted to use it as a conductor. We had to predict what happen to the sausage if we let the current run through it

II. A Strange Conductor:

1. Setup

Here was the scheme. Basically it was just the sausage connected directly to a power supply of 12V

Here was our prediction. We belived that at the moment Professor Mason hooked up the wire with the sausage, the current with respect to time would be zero. Let it wait for a couple of seconds, the current would get to a constraint limit, Since there was a certain amount of resistance of the sausage, there would be some dissipated power P=I^2R that would slowly cook the sausage

2. LED bulb connected to the sausage:
Here is the question: There were two ways to put the Led bulb, either parallel or orthogonal to the sausage's plane
a. Prediction:

Since the orthogonal way of setup would make the two metal pieces had the same potential, which means that there was no potential difference. We can conclude that orthogonal bulb would not be lighted up, but the parallel light bulb had the potential difference, therefore, it would be lighted up

Experimentally, smoke was coming out of the sausage. Therefore, our conclusion of dissipated power was correct. It was hard to look at, but actually the parallel bulbs were light up, and the orthogonal did not, which means that our conclusion was correct

II. Kirchhoff's Law:
We were given some problems about Kirchhoff's Law

Here is the problem. There was two currents going toward the middle branch. We set up the KCL accordingly, which means that the current in the middle branch would be the sum of the left and right branches. There was one variable and one equation, therefore, we could find the value of the variable.

Here we applied Kirchoff at loop number 1 to solve for i_0 and V_0

Here is the setup of voltage divider

Given the V_in and V_out, we had to find the resistance for R_1 and R_2

More problem on Kirchoff's rule

Pre-lab Photo-sensor circuit. Here we were given a thermistor and a photocell. Our goal is to find the base voltage of the BJT (theoretically and experimentally)

Here the voltage V_b (no light is when 5kohm_=4.49V. Compare to 3.33V theoretically
and V_b(with light 20kohm)=2.51 compare to 

Here is our circuit. We were testing the current in the circuit

V. Here are some videos for future reference:

VI. Review:
Today we went over the voltage divider. We did go over the concept of analyzing the circuit using the node method. For the lab, we set up the circuit with photocell and a BJT to test for change in voltage under the light and low-light condition. We did a better job set up the circuit this time. The data values are smaller compared to the theoretical calculation