What’s Wrong With This RC Circuit Experiment?
It’s pretty much a classic physics experiment. Take a resistor and a capacitor and connect it to a power supply. As the capacitor gains charge, the total current decreases. Eventually, the capacitor will get mostly charged (technically it doesn’t get to 100 percent).
But if you choose appropriate values of R (the resistance) and C (the capacitance), this thing will charge slowly enough that students can record the voltage as a function of time. With that, they can show that the following equation works.
In this expression (just to be clear), Vb is the battery voltage.
If the battery, resistor and capacitor are all in series (one single loop), then the loop rule should still work. That means that if you measure the three voltages (power supply, capacitor, resistor), the following should be true.
So, how about measuring these three voltages to confirm the loop rule WHILE charging the capacitor? Yup. Let’s do it. The power supply (Vb) should be constant, so I just need to record the voltage across the capacitor and resistor over some time interval. I used my phone to video the two voltmeters — then I can just record the voltages for some time values.
Here’s what I get. Oh, I guess I should say that the resistor is 10 MΩ and the capacitor is 5 μF with around 12 volts…
