Quick answer: Electric potential is potential energy per unit charge, V = U/q, measured in volts. This AP Physics C lab on electric potential gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
How does the potential change from one electrode to the other, and does the slope of that graph give the field? You will map V along a salt-water tray and test whether lines of equal potential run across the tray, using only a small battery and a voltmeter.
Level: AP Physics C (Mechanics and E&M, calculus-based) · Curriculum: AP PC:E&M Unit 9
Aim
Map the potential between two electrodes in a conducting solution and find the electric field from the gradient of V.
Equipment
- Shallow plastic tray, about 20 cm × 10 cm
- Two metal strip electrodes (stainless steel)
- Salt water (a teaspoon of salt in 500 mL)
- Four 1.5 V cells in a holder (6.0 V) and leads
- Digital voltmeter with a pointed probe
- Ruler or paper scale under the tray
Safety
Use only the 6.0 V battery, never mains electricity, near water. Wipe up spills and wash your hands after handling the salt solution and electrodes.
Method
- Fix the electrodes at the two short ends of the tray, 20 cm apart, and add water 1 cm deep.
- Connect the battery across them, + to the left electrode.
- Dip the voltmeter probe in the water on the centre line at x = 4 cm from the + electrode and read V relative to the − electrode.
- Repeat every 2 cm up to x = 16 cm.
- At x = 10 cm move the probe 2 cm sideways, both ways, and record V again.
- Switch the battery off between readings to limit electrolysis.
Results
Sample measurements, centre line, voltmeter negative lead on the − electrode:
| x from + electrode (cm) | 4 | 6 | 8 | 10 | 12 | 14 | 16 |
|---|---|---|---|---|---|---|---|
| V (V) | 4.9 | 4.3 | 3.8 | 3.1 | 2.5 | 1.9 | 1.3 |
Sideways check at x = 10 cm: 3.1 V on the centre line, 3.1 V at 2 cm left and 3.0 V at 2 cm right.
Analysis questions
Question 1 Plot the graph
Plot V against x and describe its shape.
Question 2 Gradient and field
Use the gradient to calculate the field in the middle of the tray, in V/m.
Question 3 Direction of E
State the direction of the field in the tray and justify it from your graph.
Question 4 Compare with theory
Calculate Vbattery/d for the electrodes and compare it with your measured field.
Question 5 Equipotential check
Use the sideways readings to explain what they show about the lines of constant V.
Question 6 Uncertainty
The voltmeter reads to ±0.1 V and the probe position to ±2 mm. Estimate the uncertainty in the field.
Question 7 Reducing error
Propose two changes that would make the measured field more reliable, and say why each helps.
