IGCSE CP3 – Electrostatic Charging | Edexcel International GCSE Physics

Theory — Electrostatic Charging by Friction

When two insulating materials are rubbed together, electrons transfer between them — leaving one material positively charged and the other negatively charged.

How Charging by Friction Works

All materials contain equal numbers of protons (+) and electrons (−). When two insulators are rubbed together, friction causes electrons to transfer from one material to the other.

The material that gains electrons becomes negatively charged (more electrons than protons)
The material that loses electrons becomes positively charged (fewer electrons than protons)
Only electrons move — protons are fixed in the nucleus and cannot transfer
Total charge is always conserved: charge gained by one = charge lost by the other

Polythene vs Perspex Rods

When rubbed with a cloth (e.g. wool or silk):

Polythene rod — gains electrons from the cloth → becomes negatively charged
Perspex (acetate) rod — loses electrons to the cloth → becomes positively charged

This is determined by the triboelectric series — the order in which materials tend to lose or gain electrons when rubbed.

Like and Unlike Charges

Like charges (++ or −−) → REPEL each other Unlike charges (+−) → ATTRACT each other

A charged rod also attracts neutral objects (like small pieces of paper). This is because the charged rod causes charge separation (polarisation) in the neutral object — the opposite charges are pulled closer, producing a net attraction.

Why Only Insulators Hold Static Charge

In conductors (metals), electrons move freely — any charge given to a conductor spreads across it and quickly leaks away through the user's body to earth. In insulators (polythene, glass, rubber), electrons cannot move freely, so the charge stays on the surface where it was deposited.

Procedure

Equipment

Polythene rod · Perspex (acetate) rod · Wool or silk cloth · Gold-leaf electroscope · Small pieces of paper · Water tap · Inflated balloon · Flat wall surface

1

Zero the electroscope

Touch the metal plate of the electroscope briefly with your finger. This earths it and removes any residual charge. The gold leaf should hang straight down alongside the central stem.

2

Charge the rod by rubbing

Rub the polythene or perspex rod vigorously with the wool cloth for about 20–30 seconds. The rod is now charged — polythene becomes negative, perspex becomes positive.

💡 Keep the cloth dry. Moisture conducts charge away. Do this on a dry day for best results.

3

Bring rod near the plate

Hold the charged end of the rod close to (but not touching) the metal plate of the electroscope. Observe: the gold leaf rises and separates from the stem — showing the rod is charged.

4

Explain the observation

Charge from the rod induces charge on the plate by electrostatic induction. This charge flows down to the stem and leaf, which both carry the same sign of charge and repel each other.

💡 The larger the charge on the rod, the greater the angle of the gold leaf deflection.

⚡ Select a demonstration, choose a rod material, then press ▶ Rub to charge it. Observe what happens!

Demonstration

Rod Material

Cloth

Charge State

Rod Charge

Uncharged

Rub rod with cloth to charge

Readings

MaterialPolythene

ClothWool

Charge sign—

Electrons—

Questions

Question 1

A polythene rod is rubbed with a wool cloth. Explain, in terms of electron movement, why the rod becomes negatively charged. Why does the cloth become positively charged?

When the polythene rod is rubbed with the wool cloth, friction causes electrons to transfer from the cloth to the surface of the polythene rod. The rod gains electrons — it now has more electrons than protons — so it has a net negative charge. The cloth loses electrons — it now has fewer electrons than protons — so it has a net positive charge. Note: only electrons move; protons remain fixed in the nuclei and cannot transfer between materials. The total charge of the rod + cloth system remains zero — charge is conserved.

Question 2

A charged polythene rod is brought near (but not touching) the plate of a gold-leaf electroscope. The gold leaf rises. (a) What does this tell you about the charge on the rod? (b) Explain why the leaf rises even though the rod does not touch the plate.

(a) The rising leaf shows that the rod is charged (either positive or negative — the electroscope detects charge but not its sign using this method alone). (b) The charged rod induces a charge on the metal plate by electrostatic induction: the rod's charge repels same-sign charges in the metal downward into the stem and gold leaf, while attracting opposite charges to the plate. Since the stem and leaf now carry the same sign of charge, they repel each other — the leaf rises and diverges from the stem. The greater the charge on the rod, the greater the angle of divergence.

Question 3

A negatively charged polythene rod is held near a thin stream of water from a tap. The stream bends towards the rod. A positively charged perspex rod is then held in the same position. Predict what happens and explain why.

The stream also bends towards the perspex rod. Water (H₂O) is a polar molecule — each molecule has a slightly positive end (the hydrogen atoms, δ+) and a slightly negative end (the oxygen atom, δ−). When the negatively charged rod is nearby, the δ+ ends of the water molecules are attracted toward the rod, causing the stream to deflect toward it. When the positively charged rod is nearby, the δ− ends of the water molecules are attracted toward the rod — again causing deflection toward it. In both cases the stream bends toward the rod, because polar molecules always align so the attracted end faces the rod. A charged rod always attracts a water stream, regardless of the sign of the charge.

Question 4

A student charges a balloon by rubbing it with wool and holds it against a wall. Explain why it sticks, and why it eventually falls off after a few minutes.

The rubbed balloon gains electrons and becomes negatively charged. When held against the wall, the balloon's negative charge repels electrons in the wall surface away from the contact region, leaving behind a region of positive charge on the wall. The attraction between the negative balloon and the positive region of the wall holds the balloon in place. Over time, the charge gradually leaks away — through the air (especially on a humid day, when water molecules in the air provide a conduction path) or through the wall material. As the balloon's charge decreases, the electrostatic attraction weakens until it can no longer overcome gravity, and the balloon falls.