Describe simple experiments to show the production of electrostatic charges by friction and to show the detection of electrostatic charges

4.2.1 Electric Charge

Learning Objective

Students will be able to describe simple experiments that (a) produce electrostatic charge by friction and (b) detect the presence and the sign of an electrostatic charge.

Key Concepts

• Two types of charge exist: positive and negative.
• Like charges repel; unlike charges attract.
• Electrostatic charge is transferred by friction, conduction or induction.
• Electron‑transfer model (friction): when two different materials are rubbed together, electrons move from the material with lower electron affinity to the one with higher affinity. The donor becomes positively charged, the acceptor becomes negatively charged.
• Charge is quantised: 1 C = 6.24 × 10¹⁸ e (where e = 1.602 × 10⁻¹⁹ C).
• Materials are either conductors (free electrons move easily) or insulators (electrons are tightly bound). This distinction is essential for understanding induction and conduction.
• Electric field (E): a region of space around a charge where another charge experiences a force. The direction of E is the direction a positive test charge would move.

Simple Electric‑Field Patterns (Cambridge syllabus)

• Point charge: radial field lines outward for a positive charge, inward for a negative charge.
• Charged conducting sphere: field outside behaves like a point charge at the centre; inside the field is zero.
• Parallel‑plate capacitor: uniform field between the plates, directed from the positive to the negative plate.

Experiment 1 – Producing Charge by Friction

Apparatus

Procedure

1. Fix the plastic rod on the insulating stand.
2. Rub the woollen cloth along the rod vigorously for about 10 s.
3. Hold a piece of paper a few centimetres from the rod (do not touch). Observe its motion.
4. Repeat the experiment using a glass rod rubbed with silk and record any differences.

Explanation (electron‑transfer model)

• During rubbing, electrons move from the material with lower electron affinity (wool or silk) to the material with higher affinity (plastic or glass).
• Resulting charges:

  • Plastic + wool → plastic gains electrons → negative; wool loses electrons → positive.
  • Glass + silk → glass loses electrons → positive; silk gains electrons → negative.

• The charged rod induces a separation of charges in the neutral paper; the opposite induced charge nearest the rod attracts the paper, causing it to move toward the rod.

Experiment 2 – Detecting Charge with a Simple Electroscope

Apparatus

Construction

1. Drill a tiny hole in the lid and insert the metal rod so that it hangs freely inside the jar.
2. Attach the two aluminium leaves to the lower end of the rod; they should just touch when the electroscope is neutral.
3. Seal the jar with the lid. The interior is now electrically isolated from the surroundings.

Procedure

1. Observe the leaves – they should be vertical and in contact (neutral state).
2. Bring the negatively charged plastic rod close to the metal rod without touching. Note any slight divergence (induced charge).
3. Touch the plastic rod briefly to the metal rod, then withdraw it. Record the new separation of the leaves.
4. Repeat steps 2–3 using the positively charged glass rod.

Explanation

• Induction: A nearby charged object polarises the metal rod. Opposite charges accumulate on the side nearest the external charge, causing a small temporary repulsion of the leaves.
• Conduction: Direct contact transfers electrons (negative charge) or removes them (positive charge) from the leaves. Like charges on each leaf repel, so the leaves spread apart.
• Sign of the charge:

  • Leaves spread after touching the plastic rod → the electroscope has acquired negative charge.
  • Leaves spread after touching the glass rod → the electroscope has acquired positive charge.

Experiment 3 – Detecting Charge with a Charged Pith Ball

Apparatus

Procedure

1. Tie the pith ball to the silk thread and let it hang freely.
2. Bring a negatively charged plastic rod close to the ball (no contact). Observe attraction.
3. Touch the rod to the ball, withdraw the rod, and then bring the same rod close again. Note whether the ball is attracted or repelled.
4. Repeat the whole sequence with the positively charged glass rod.

Explanation

• Before contact the ball is neutral; a nearby charged rod induces an opposite charge on the near side, so the ball is attracted regardless of the rod’s sign.
• After contact the ball acquires the same sign of charge as the rod. Consequently:

  • Like‑charged rod → repulsion.
  • Oppositely‑charged rod → attraction.

Extension Experiment – Conductors vs. Insulators

To emphasise the difference between conductors and insulators, test a metal spoon and a plastic spoon with a charged rod.

• Bring a negatively charged plastic rod close to each spoon without touching. Both will attract because of induction.
• Touch the rod to each spoon, then withdraw it. The metal spoon will quickly lose its charge when placed on a grounded surface (demonstrating conduction), whereas the plastic spoon will retain its charge for a longer time (demonstrating insulation).

Safety and Precautions

• Keep hands dry; moisture provides a conductive path to ground and can discharge the apparatus.
• Do not rub materials for excessively long periods – sparks may appear and could ignite flammable objects.
• Handle glass rods, jars and metal parts carefully to avoid breakage.
• Never point a charged rod at anyone’s face or at electronic devices.
• When using the electroscope, avoid touching the aluminium leaves with bare fingers; use a non‑conducting tool if adjustment is needed.

Summary

Friction between dissimilar materials transfers electrons, creating positive and negative static charges. The presence and sign of charge can be demonstrated with:

• Attraction of neutral objects (paper) to a charged rod (friction experiment).
• A simple electroscope – leaves diverge after charge is transferred by conduction; the direction of divergence tells the sign.
• A pith ball – shows induced attraction before contact and sign‑dependent attraction/repulsion after contact.
• Extension tests that highlight the behaviour of conductors versus insulators.

These low‑cost, classic investigations satisfy all Cambridge IGCSE requirements for “producing charge by friction” and “detecting electrostatic charge”.

Review Questions

1. Explain, using the electron‑transfer model, why rubbing a woollen cloth on a plastic rod makes the rod negatively charged.
2. What observation on the electroscope tells you that it has been positively charged?
3. Why does a neutral pith ball get attracted to both positively and negatively charged rods before it is touched?
4. Describe how you could use the electroscope to determine whether an unknown object carries a positive or a negative charge.
5. State two safety precautions that should be observed when performing these experiments.
6. Give one practical way to distinguish a conductor from an insulator using a charged rod.
7. Define the electric field and state its direction for a positive point charge.