# Physics Lab - Electrical Circuits with Resistors and Capacitors

1. Mar 25, 2014

### mintsnapple

1. The problem statement, all variables and given/known data
Hello, I am having trouble with a physics lab report I'm working on. I'm going to try and be as clear as possible. Basically, I'm using different values of R and C in determining tau, the time scale in a circuit's behavior. I used a circuit with a resistor and capacitor and hooked it to an oscilloscope, and used the OpenChoice Desktop program to get 2500 data points in the capacitor voltage curve. My question is...I'm not really sure how to start plotting I guess. I have constant values of R and varying values of C and vice versa. How do I best start plotting when I have 2500 data points for each? And should I use a ln(V) vs T plot to find out what tau is? I'm sorry if this seemed vague, I can try and clarify more, but I really need help if possible! Thank you!

2. Relevant equations

3. The attempt at a solution

2. Mar 25, 2014

### Staff: Mentor

You should start off by plotting voltage against time. Which voltage? All 3! This is how the waveshape will occur in real life, so you need to get very familiar with it and its finer details so that you can henceforth picture it in your mind without needing to see it.

When that is all done, why not plot log(V) as you suggest!

Maybe also make another plot, you decide what results are worth presenting/summarizing in graphical form.

2,500 data points is a little over the top.

3. Mar 25, 2014

### mintsnapple

Thanks for the response. However I still don't quite understand...
Let's say I have datapoints for 11k ohms/.21 microfarads and another set for 16k ohms/.21 microfarads. What would be the best way to plot this?

4. Mar 25, 2014

### Staff: Mentor

Plot voltages vs. time as the capacitor charges up.

Have you thought about the useful measurements that you hope to be able to make on the plots?

5. Mar 25, 2014

### mintsnapple

6. Mar 25, 2014

### Staff: Mentor

Plotting ln v versus time is one of the graphs you can construct. Do this and see what the slope is.

But if you just plot voltage versus time, the slope of the steepest tangent to that exponential curve should also give time constant, tau.

7. Mar 25, 2014

### mintsnapple

I have lots of negative voltage points...Wouldn't plotting ln v be less effective then?

After plotting all these different graphs with different R and C values, how could I then make a conclusion on how tau is affected?

8. Mar 25, 2014

### Staff: Mentor

If all your points are of negative voltage, change their sign before taking the log. If only some are negative, then on the voltage axis plot ln(v - vi) where vi is the initial voltage. None of this will change the slope of the curve, and it's from the slope that you can read off $\Large\tau$.

You observe how the graphs change when C (and/or R) changes.

Last edited: Mar 25, 2014