Coulomb's law experiments can be very expensive to purchase, but with a bit of ingenuity, the same results can be obtained at a fraction of the cost. Here, I used expanded polystyrene balls and a digital coulomb meter to measure the charge.

Investigating Coulomb’s Law with a Balance and Polystyrene Balls

Ever wondered how we can measure the invisible force between two electric charges? While the theory behind Coulomb’s Law might sound abstract, you can demonstrate it using a few surprisingly simple items: a Coulomb meter, a top pan balance, and two expanded polystyrene balls.

This DIY physics experiment helps students visualise how electric charges interact—and even lets you measure the force between them. Let’s take a closer look.

🧪 The Equipment You'll Need

Coulomb meter – to measure and verify the charge on each polystyrene ball.
Top pan balance – to detect tiny changes in weight, which actually represent the force between the charges.
Two lightweight expanded polystyrene balls – ideally coated to accept a static charge.
Insulating stand with a ring – to hold one ball in place directly on the balance pan.
Thin cotton thread – to suspend the second ball from above, ensuring minimal interference.
Charging source – such as a plastic rod and fur cloth, or Van de Graaff generator.
Vernier Calliper - to measure the diameter of the sphere we need the radius.

🧲 The Setup

Mount one polystyrene ball on the balance using a small insulating ring or non-conductive support.
Suspend the second ball from a retort stand using the cotton thread, allowing it to hang freely above the first.
Use your charging source to charge both balls with the same sign of charge (either both positive or both negative).
Use the Coulomb meter to verify the charge on each ball. It helps to ensure repeatability in the experiment.

📐 What Happens Next?

As the suspended ball is moved closer to the one on the balance, something interesting happens:

➡️ The reading on the balance increases.

This is because the two like charges repel each other. As the upper ball approaches, the lower ball experiences an upward electrostatic force. Since it is held in place, that upward push is transferred to the balance as a downward reaction force. The balance interprets this as an increase in weight.

The closer the charges get, the stronger the force becomes—just as Coulomb’s Law predicts.

🧮 Coulomb’s Law in Action

Coulomb’s Law is given by:

$F = \frac{k \cdot |q_1 \cdot q_2|}{r^2}$

Where:

$F$ = force between the charges (in newtons)
$q_1$ , $q_2$ = the magnitudes of the charges (in coulombs)
$r$ = distance between the centres of the charges (in metres)
$k$ = Coulomb's constant ≈ $8.99 \times 10^9 \, \text{Nm}^2/\text{C}^2$

Using the mass reading from the balance, you can calculate the electrostatic force:

$F = \Delta m \cdot g$

Where $\Delta m$ is the increase in mass recorded on the balance, and $g$ is the gravitational field strength (≈ 9.81 N/kg).

Now, knowing $F$ , $r$ , and the charges $q_1$ and $q_2$ from your Coulomb meter, you can verify Coulomb’s Law experimentally.

🔍 Tips for Accurate Results

Use lightweight balls so the forces are large enough to detect.
Ensure there’s no air movement in the room.
Use non-conductive tools when adjusting positions to avoid accidental discharges.
Repeat measurements at various distances and plot $F$ vs $1/r^2$ to see the inverse square law in action!

🧠 Why This Experiment Matters

This setup is more than just clever—it turns an abstract force into something measurable and visible. It's perfect for A-Level physics students exploring electrostatics, and it reinforces core experimental skills:

isolating variables,
careful measurement,
and interpreting mathematical relationships in real-world data.

Who knew two polystyrene balls could make Coulomb’s Law feel so real?

Next step: Try repeating the experiment with opposite charges. What changes? And how could you design an experiment to cancel the weight of one ball using electrostatic attraction? That’s physics for you—always pulling (or pushing) you into deeper thinking.

Try using spheres of different sizes.