Lab: RC Circuits

A resistor and capacitor in series with a battery gives you a circuit where current starts high and dies away, and capacitor voltage starts at zero and climbs toward the battery voltage. A bulb wired in series flashes bright at the instant the switch closes, because current is largest when the capacitor is empty and offers no opposition yet. As charge piles up on the plates, it pushes back against incoming charge, current drops, and the bulb fades even though the battery voltage never changed.

To watch this happen, you log capacitor voltage with a voltage probe or multimeter many times per second, since the whole process can finish in a second or less for typical lab resistors and capacitors. Plotting voltage against time gives a curve that rises steeply then levels off, following V(t) = V0(1 - e^−t/RC). Current, found either with a separate probe or calculated from the resistor's voltage drop divided by R, gives a mirror-image curve that starts high and decays toward zero as I(t) = I0·e^−t/RC.

Swapping in a resistor three times as large stretches the time constant RC by the same factor three, so the whole charging process takes three times as long, but the capacitor still ends up at the same final voltage, since that depends only on the battery. This lab sits alongside other time-constant work in the course, like RC discharging, and reinforces the same skill used in projectile and radioactive decay graphs: recognizing an exponential shape and extracting a rate constant from it rather than reading off a straight line.