RC Circuits 4/28/2014

Today we began RC circuits.  As the name suggests it is a resistor–capacitor circuit which consists of both resistors and capacitors.  We started with an experiment of charging and discharging capacitors. Prof Mason hooked up to a light bulb as a resistor.

When the circuit is hooked to the power supply, the capacitor begins to store charge. 

When the circuit is hooked without power supply, the capacitor begins to discharge.

The result light-bulb was initially very bright and gradually gotten dimmer as the capacitor charge/discharge.  This tells us that current and voltage is going down.  Next, we timed the amount of time from the initial lit until the light completely goes out using stopwatches.  They both turned out to be roughly 15s.

We then tried using two light bulbs as two resistors.  This time the time almost doubled and we discovered as resistance increases, the charge time got longer.  We say time is proportional to resistance

Next, we used not one, but two capacitors in series with one light bulb to measure the time.  The resulting time halved.  We conclude that since we too halved the capacitance, capacitance is also proportional to time.

This experiment warmed us up with the relationship between time, resistance, and capacitance.  For the primary experiment of the day, we were asked to create an RC circuit to measure the voltage graph produced when the silver capacitor is charged and discharged.  We

Hooking the resistor, power supply, and capacitor in series to prevent rapid charge time for better results.

 

Hooking the same setup in parallel for discharge 

We hooked up the capacitor to logger pro using a voltage measuring lead. However, before this we made a few predictions. Primarily that the relationship between Potential (Voltage) and time would be inversely related as seen below. After logging all of our data we had the following graphs:

From Minjie, RudyT, and Elias

From Dez, RudyC, and Eddie

These were the graph of voltage as a function of time that was created during the experiment.  It shows that a capacitor charges and discharges not linearly, but experimentally.  By looking at the discharge graph in red, we observed that B in the natural exponential equation.  We can also concluded that the A value should be our close to our original Voltage (from the power supply). Our final conclusion was that C was our charge time which should have been the same for both discharge and charge.

Calculations

We also can see the exponential nature as analyzed by Prof Mason's Mathematician VS Engineer story  that capacitors actually never becomes fully 100% charged or discharged.

Prof Mason's explanation of the same result found in our lab.