| Source | 6 VDC |
| R1 | 1.2 kΩ |
| C1 | 0.1 µF |
| SW1 | slide switch |
The cables at the bottom are a 6 VDC source. Set up the Field Meter and measure:
Now look at DC on the oscilloscope. It takes the meter’s place. A scope draws voltage (up) against time (across). Set it up:
What does 6 VDC look like on the scope?
The cables are now a function generator: 6 V RMS sine at 1 kHz. Still no generator drawn, only the wires. Set up the Field Meter and measure:
The Field Meter’s AC voltage is RMS: the steady voltage that would heat a resistor the same amount. It is not the peak.
For this step the oscilloscope takes the meter’s place. A scope has one probe with a hook tip, and a short ground lead with a clip. Set it up:
Then read the wave. The meter read 6 V RMS. The scope shows the whole wave:
Vmax is the positive peak: how far the wave rises above 0 V. It is about…
Vmin is the negative peak: how far the wave drops below 0 V. It is about…
Vpp is peak-to-peak: Vmax − Vmin, the whole height from the bottom of the wave to the top. It is about…
What RMS means. An AC voltage never sits still, so which number is “the” voltage? RMS (root-mean-square) is the steady DC voltage that would heat a resistor just as much. It is what the Field Meter showed in step 3, and what the 120 V of a wall outlet means.
| Vpp from the scope | 16.97 V |
For a sine wave:
Why 0.354? Half of Vpp is the peak, and RMS is 0.707 of the peak: ½ × 0.707 ≈ 0.354.
On DC, only resistance limited the current. On AC, the capacitor or inductor pushes back too. Three ideas, all measured in ohms (Ω):
- Resistance R ohms, Ω
- Opposition from a resistor. The same on DC and AC, at any frequency. R1 is 1.2 kΩ.
- Reactance X ohms, Ω
- Opposition from a capacitor (XC) or inductor (XL) to a current that keeps reversing. It changes with frequency: XC = 1 / (2π f C) shrinks as f rises; XL = 2π f L grows. It stores and returns energy instead of turning it into heat.
- Impedance Z ohms, Ω
- The total opposition of R and X together. They are out of step, so they do not simply add: Z = √(R² + X²).
Then Ohm’s law works with Z in place of R: I = VS ÷ Z.
Impedance Z combines…
Every quantity in this lab has a symbol and a unit. Reactance and impedance share the resistor’s unit:
| Quantity | Symbol | Unit |
|---|---|---|
| Voltage | V | volts, V |
| Current | I | amps, A |
| Resistance, reactance, impedance | R, X, Z | ohms, Ω |
| Frequency | f | hertz, Hz |
| Capacitance | C | farads, F (0.1 µF = 0.1 × 10−6 F) |
| Inductance | L | henries, H (250 mH = 0.25 H) |
Reactance and impedance are measured in…
The circuit:
| Frequency f | 1 kHz |
| C1 | 0.1 µF |
| R1 | 1.2 kΩ |
| VS | 6 V RMS |
Work it out a line at a time with the calculator. Fill in each blank; the next line opens when this one is right.
Measure the AC drop across R1 and across C1. They peak at different times, so they add as a right triangle, not a receipt. Set up the Field Meter and measure:
Your two readings are the short sides of a right triangle. The source, VS = 6 V, is the long side.
Add them two ways. Fill in each blank; the next line opens when this one is right.
Which one matches the 6 V source?
Slide the frequency up and down and watch the readouts. Then answer:
As the frequency rises, what happens?