Example experiments
Each example opens in the laboratory as your own copy: you can change anything without spoiling the original.
RC on the bench: generator and oscilloscope
A function generator with a square wave drives a low-pass filter made of a 1 kΩ resistor and a 100 nF capacitor; a two-channel analog oscilloscope shows the input and the output.
What to look at
The capacitor charges and discharges along an exponential curve: the time constant is τ = R·C = 100 µs. If the time base is too slow the curve looks like a square wave; if it is right you can see every edge being rounded off.
Try this
Turn TIME/DIV until you see a single edge; raise the generator frequency beyond the cutoff frequency (about 1.6 kHz) and watch the output shrink. Switch the oscilloscope loading off to see how much it changes the measurement.
RC low-pass filter with a square wave
The same filter, with no instruments on the sheet: a 1 kHz square-wave generator, R = 1 kΩ, C = 100 nF.
What to look at
The voltage across the capacitor rises towards the generator value and then falls, never following the edge: a capacitor does not allow instantaneous voltage changes.
Try this
Change C from 100 nF to 1 µF: the time constant becomes ten times longer, until the capacitor no longer has time to charge.
Series RLC at resonance
A sinusoidal generator drives a 10 mH inductor, a 100 Ω resistor and a 100 nF capacitor in series. The generator frequency is the resonant frequency, about 5 kHz.
What to look at
At resonance the reactances of L and C cancel and the current is at its maximum, limited only by R. The voltage across the capacitor is Q times the generator voltage (here Q ≈ 3.2): it is larger than the input, with no amplifier at all.
Try this
Move the frequency above and below 5 kHz and watch the capacitor voltage drop on both sides; set R to 20 Ω to raise Q and narrow the peak.
Half-wave rectifier with a capacitor
A 10 V peak, 50 Hz sinusoidal generator drives a 1N4007 diode that charges a 100 µF capacitor in parallel with a 1 kΩ resistor.
What to look at
The diode conducts only when the generator voltage exceeds the capacitor voltage, so the current flows in short pulses near the peaks; between pulses the capacitor discharges slowly into the resistor: that is the ripple voltage.
Try this
Reduce the capacitor to 10 µF: the ripple grows. Reduce the load resistor: the capacitor discharges faster and the ripple grows.
More experiments will arrive together with new instruments.