Physics Lab · At-Home Setup

Charge on a Plate — Step-by-Step Setup Guide

The foolproof version of Lab 2 (Option #1): measure the electric force between two charged plates with the IOLab, then a model reveals the charge. Follow the diagrams top to bottom.

⚠️ THE #1 RULE — READ FIRST

Your charged plates MUST be insulators (plastic lid, styrofoam plate, plastic plate). Insulators hold the static charge you rub onto them.

Do NOT use aluminum foil as a charged plate. Foil is a conductor — it won't build up charge by rubbing, and the instant you touch it the charge drains away through your hand. A foil plate reads ~0 force and the lab fails.

➜ Use two insulating plates of the same size, and only touch them by the edges / a taped-on handle, never the flat charged face.

THE BIG IDEA

Rub two plates and they pick up static charge. Held close and parallel, the charges create an electric force between the plates that the IOLab force sensor reads directly. The lab's parallel-plate model then converts that force into the hidden charge Q:

F = 2k·Q² / A → Q = √( F · A / (2k) )

F = measured force (N) A = plate area (m²) k = 9×10⁹

Gather your materials

IOLab device + USB dongle
2 insulating plates, same size
(plastic lid + styrofoam/plastic plate)
Tape (for handles + mounting)
A ruler / tape measure
A wool sweater, fleece — or carpet
The Excel calculator (optional)
Chrome + iolab.science/iolabweb

Best plates: two styrofoam plates (hold charge great, ~25¢). Also fine: plastic lids, plastic picnic plates, plastic clamshell packaging. Avoid: anything paper/fiber/foil/metal.

Build the two plates + measure the area A

Both plates flat + the same size. Tape a small handle so you never touch the face.

Make sure both plates are flat and the same size. Tape a small insulating handle tab to the back of each so you can hold and move them without touching the charged face (touching = instant charge drain).

Measure the plate area A = length × width, converted to m² (example: 20 cm × 20 cm = 0.20 m × 0.20 m = 0.04 m²). Write it down — the formula needs it.

Set up the IOLab + CALIBRATE the Force sensor

Open iolab.science/iolabweb in Chrome and connect. In the sensor list, check Force (uncheck everything else).

Calibrate it — with nothing pushing on the probe, zero the Force sensor so it reads 0 N at rest. The slides specifically remind you: do not forget to calibrate.

Why calibrate? The force you're measuring is tiny — a stray offset would swamp it. Zeroing first makes the reading trustworthy.

Mount plate #1 on the force probe

Plate #1 is attached to the IOLab's force probe. Plate #2 stays in your hand.

Attach plate #1 to the IOLab's force-sensor probe so its flat face points outward. This is the plate the IOLab "feels" the force on.

Charge BOTH plates hard

More charge = bigger, easier-to-read force. Handle by the tab only.

Rub the face of each plate hard on the carpet or a wool sweater — many firm strokes, right before measuring. Hold each plate by its handle/edges the whole time. Don't let the two plates touch each other.

Record: bring plate #2 close, read the force

Hold plate #2 parallel and steady. The force sensor reads F in newtons.

Click Record and let it read a flat baseline (0 N) for ~2 seconds. Then bring plate #2 parallel to plate #1, CLOSE (~2–3 cm), and hold it steady by the handle. Hold ~3–5 s, then Stop. Highlight the flat plateau → set Trendline = Mean → read the Mean force (N). Record it.

Screenshot each trial with the Mean showing (Win+Shift+S, crop to just the graph — no phone photos). That's your data + your figures.

Repeat: close, far, and 3–5 trials

Recharge both plates, then measure again at a FAR gap (~8–10 cm) and a middle gap. Do 3–5 trials at each distance, recharging before each. This is the close-vs-far comparison the slides ask for.

What to expect: the parallel-plate model says the force is roughly independent of distance (a uniform field). Real finite plates won't be perfect — that gap between model and reality is a great uncertainty discussion point (not a failure).

Calculate the charge Q

Q = √( F · A / (2k) )

Plug each trial's Mean force F and your plate area A into the formula (or the Excel calculator) to get the charge Q, then average your trials. A sensible static charge lands in the range of ~0.01–1 µC (millionths of a coulomb).

✓

Foolproofing — if something goes wrong

Reads ~0 N? → charge harder, get closer, confirm both plates are insulators (not foil!), and you're not touching the faces.
Charge dies too fast? → dry room, work quickly, more strokes.
Noisy / spiky graph? → hold the plate still, brace your hands.
Force pegs at the top (railing)? → too much charge / too close → back off a little so the plateau sits mid-scale.
Keep the same # of strokes each trial for consistency.

Uncertainty / error sources (for the report)

Charge leakage — charge fades while you measure (biggest one).
Edge effects — the plates aren't infinite, so the field isn't perfectly uniform.
Plates not perfectly parallel or not perfectly aligned.
Unknown / uneven charge — you don't know exactly how much charge, or that it's uniform.
Distance & area measurement precision.
Humidity affecting charge retention.
Force-sensor resolution — the electric force is small.

NEVER write "human error." The slides literally say it means "you admitted you screwed up and we'll grade accordingly." Name a specific physical cause instead (charge leakage, edge effects, humidity, etc.) — those are the real, gradeable sources.

🔁 Backup methods (if the force is too small to measure)

A) High-Gain electrometer (uses your foil — the way you already got working data)

Here foil is used correctly: not as a charged plate, but as a sensor. Charge a plastic lid (that's the "plate" you estimate). Wire aluminum foil → G+ and G− → GND. Select the High Gain sensor, Rezero with nothing near, set Smoothing = 5 and Trendline = Mean. Bring the charged lid near the foil and hold still (moving it makes it spike; too close makes it rail/saturate at the top). Read the flat plateau (mV).

Note: this gives a voltage, so you estimate charge via the handout's V→Q calibration (q = C·V) — it does not use F = 2kQ²/A. Confirm your instructor accepts this method.

B) Balloon balance method (no IOLab)

Balance one charged balloon over another and use m·g = k·q²/r². Produces a photo + measurements, not an IOLab graph. → Open the balloon setup guide →

4 things that make or break it: ① Insulator plates only (never foil as the charged plate). ② Charge hard, handle by the edges. ③ Dry room, work fast — charge leaks. ④ Calibrate the force sensor first.

Plate / force-sensor method (Lab handout Option #1, model F = 2kQ²/A) · pairs with your report + Excel calculator · alternatives: High-Gain foil sensor · balloon balance.