Quick answer: Electric charge comes in positive and negative kinds, is conserved and is quantised in units of e = 1.60 × 10⁻¹⁹ C. This AP Physics 2 lab on coulomb's law 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 electric force between two like charges change as they are brought closer together? In this lab a balance measures the downward push of one charged sphere on another, and you use the readings to test the inverse-square law and to find the charge carried by each sphere.
Level: AP Physics 2 (algebra-based) · Curriculum: AP P2 Unit 10
Aim
To measure how the repulsive force between two like-charged spheres depends on their separation, and to find the charge on each sphere.
Equipment
- Top-pan balance reading to 0.001 g
- Two identical conducting spheres on insulating mounts, one resting on the pan and one on a rod above it
- Low-current high-voltage supply with protective resistor, or a Van de Graaff generator with a charging wand
- Ruler or metre rule and a retort stand
- Dry cloth and a dehumidified room if possible
Safety
Use only a supply with a protective resistor of at least 5 MΩ and never touch the terminals. Anyone with a heart condition or pacemaker must not take part. Discharge the spheres by touching them with an earthed lead before handling.
Method
- Place the lower sphere on the balance pan and tare the balance to zero.
- Charge both spheres equally by touching each to the same terminal, then isolate them.
- Hold the upper sphere above the lower one at r = 8.0 cm between centres. Record the balance reading Δm.
- Reduce r in steps to 6.0, 5.0, 4.0 and 3.0 cm, recharging when the readings drift.
- Calculate F = Δm g and 1/r² for each distance.
Results
Sample readings from Diego’s class in Texas are shown. Fill in the last two columns yourself.
| r (cm) | Δm (g) | F (N) | 1/r² (m⁻²) |
|---|---|---|---|
| 8.0 | 0.058 | ||
| 6.0 | 0.101 | ||
| 5.0 | 0.149 | ||
| 4.0 | 0.228 | ||
| 3.0 | 0.405 |
Analysis questions
Question 1 Convert to force
Use g = 9.81 m/s² to calculate F for the readings at r = 3.0 cm and r = 8.0 cm.Question 2 Calculate 1/r²
Convert each separation to metres and calculate 1/r² for all five readings.Question 3 Plot and describe
Plot F against 1/r² and describe the graph.Question 4 Gradient
Find the gradient using the 3.0 cm point and give its unit and meaning.Question 5 Charge on a sphere
Use the gradient to calculate the charge on each sphere.Question 6 Testing the power law
Use the 3.0 cm and 8.0 cm readings to find the exponent n in F ∝ 1/rⁿ.Question 7 Sources of error
Suggest two sources of error and say how each would change the readings.
