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Run a plastic comb through dry hair, turn a tap until a smooth, pencil-thin stream flows without splashing, and hold the comb teeth close to the water. Before the plastic touches the liquid, the stream swerves sideways, curving toward the comb.
Many students assume an electric charge can only pull water if the water carries an opposite static charge. That assumption creates an immediate puzzle: if you switch from a negatively charged comb to a positively charged glass rod, does the water get pushed away?
Why does a charged comb attract water instead of repelling it?
Combing dry hair transfers electrons to the plastic, giving the comb a negative static charge. When placed near water, the comb's electric field acts on nearby water molecules.
Water (H₂O) is a bent molecule with a 104.5° bond angle. Because oxygen has an electronegativity of 3.44 while hydrogen measures 2.20 on the Pauling scale, oxygen pulls shared electrons closer. This gives oxygen a partial negative charge (δ⁻) and leaves hydrogens with partial positive charges (δ⁺), forming a permanent dipole moment of 1.85 Debyes (6.17 × 10⁻³⁰ C·m). Read how molecular shape determines dipole moments to see how molecular geometry shapes polarity.
In a uniform electric field, a dipole feels equal and opposite forces on both ends: it rotates via torque (τ = p × E) but experiences zero net translation. But a comb creates a non-uniform field that weakens with distance r. As the field turns the dipole, positive hydrogens rotate toward the negative comb, while negative oxygen faces away. Because the hydrogens sit in a stronger field, their inward attraction exceeds the outward repulsion on the oxygen. This field gradient produces a net attractive force, as detailed in finding the net force on a dipole in a non-uniform field.
What happens if you switch to a positively charged rod?
Rubbing a glass rod with silk leaves it positively charged. When held near the stream, the water bends toward the rod once again. It never repels.
In a positive field, water molecules simply pivot 180 degrees. The negative oxygen turns toward the rod, while positive hydrogens face away. Because oxygen now occupies the stronger field region, attraction again wins over repulsion. In both cases, the nearer pole of the aligned dipole sits in the stronger field, guaranteeing attraction.
Everyday tap water also exhibits a macroscopic conductive effect. In the *Journal of Chemical Education*, Maryam Ziaei-Moayyed and colleagues (2000) demonstrated that the stream acts as an electrical conductor. Tap water contains dissolved minerals and carbon dioxide that yield mobile ions. Because the stream connects to a grounded metal tap, electrostatic induction separates charges across the water column within microseconds, as analyzed in *The Physics Teacher* by Paul J. Dolan Jr. (2014). A negative comb repels negative ions into the plumbing and concentrates positive charge on the stream's near surface, while a positive rod does the reverse. Our review of how conductors distribute surface charges details this mechanism. Non-polar liquids like pure hexane show no deflection.
How can students use active prediction to master electrostatics?
Students master physics faster when they commit to predictions before testing them. A meta-analysis of 225 studies in *PNAS* by Scott Freeman and colleagues (2014) showed that active learning raises exam scores by roughly half a letter grade, while students under traditional lecturing were 1.5 times more likely to fail (failure rates dropped from 33.8% under lecturing to 21.8% with active learning).
You can turn this experiment into an active learning exercise:
Predict and commit: Write down whether a negative comb will attract or repel the stream, and why. Then write your prediction for a positive rod.
Establish laminar flow: Adjust the tap to produce a steady, transparent 1–2 mm stream without droplet break-up, preserving the conductive column.
Test both charges: Charge a comb on hair, bring it 15 mm from the stream, and observe the deflection. Repeat with a glass or acrylic rod rubbed with silk to verify that both polarities attract.
Model the gradient: A charged rod of linear charge density λ creates field E = λ / (2πε₀r). A dipole p aligned with the field feels attractive force F = p · (dE/dr) = −pλ / (2πε₀r²). Because the dipole rotates to align with E, the resulting force is strictly attractive regardless of the sign of λ.
When students find electrostatic field gradients and molecular dipoles easier to see than to read from equations alone, interactive models bridge the gap. The Fermi Simulation Lab provides interactive simulations for concepts that are easier to visualize than to read, allowing students to inspect field lines, switch charge signs, and observe molecular dipoles reorient in real time.
