All Form Two Physics Chapters

Chapter 1Static ElectricityRead below · Free
Chapter 2Current ElectricityContinue in the app
Chapter 3MagnetismContinue in the app
Chapter 4Nature and Reflection of LightContinue in the app
Chapter 5Refraction and Dispersion of LightContinue in the app
Chapter 6Optical InstrumentsContinue in the app

Lesson Objectives

By the end of this chapter, students should be able to:

Define static electricity and related key terms.
Explain how charges originate and how objects become charged by friction, contact and induction.
Distinguish between conductors and insulators and state the fundamental law of electrostatics.
Describe the structure and uses of a leaf electroscope.
Explain electrostatic potential and calculate with capacitors.
Describe how lightning forms and how lightning conductors protect buildings.

1. Key Terms and Concepts

Master these definitions. They are the language of this chapter.

Static electricity: The accumulation of excess electric charges on or within a material that does not conduct electricity.

Electron: A negatively charged subatomic particle; charge = −1.6 × 10⁻¹⁹ C.

Proton: A positively charged subatomic particle in the nucleus.

Neutral atom: An atom with equal numbers of protons and electrons, so it has no net charge.

Conductor: A material that allows the free flow of electrons, such as metals.

Insulator: A material that hinders the free flow of charge, such as plastic and rubber.

Electrostatic force: The force of attraction or repulsion between charged objects.

Fundamental law of electrostatics: Like charges repel; unlike charges attract.

Conservation of charge: The total charge of an isolated system remains constant.

Charging by friction: Transfer of electrons by rubbing two objects together.

Charging by contact (conduction): Charging an object by touching it with a charged object.

Charging by induction: Charging an object without touching it, by bringing a charged object near.

Electroscope: An instrument used to detect the presence and type of electric charge.

Potential difference (p.d.): The work needed to move a unit charge between two points, measured in volts (V).

Capacitor: A device that stores electric charge; two plates separated by a dielectric.

Capacitance (C): The charge a capacitor can store per unit potential difference: C = Q/V.

Farad (F): The SI unit of capacitance; 1 F = 1 C/V.

Dielectric: The insulating material between the plates of a capacitor.

Lightning conductor: A metal rod that gives a low-resistance path to earth to protect a structure.

Triboelectric series: A ranking of materials by their tendency to gain or lose electrons when rubbed.

2. Concept of Static Electricity

Static electricity results from the accumulation of electric charges on a material that does not conduct electricity (an insulator).

How Static Electricity is Generated

It is produced by the transfer of electrons between materials.
It happens when two objects touch and are then separated.
One material gains electrons and becomes negatively charged.
The other loses electrons and becomes positively charged.

Figure 1: A charged comb attracting small pieces of paper, a classic static electricity demonstration.

✏️ Activity 1.1: Charging an Object

Materials: a wool jumper and dry plastic sheets.

Place two transparent plastic sheets on a book.
Rub them with the wool jumper several times.
Separate them slowly and listen for a crackling sound.
Bring the sheets close and observe.
Rub the sheets separately and bring them together.

Quick check: What happened when the sheets were separated after rubbing?

A crackling sound was heard from small sparks as charges equalised. The sheets acquired opposite charges and attracted each other.

Quick check: Why did the sheets behave differently when rubbed separately?

Rubbed separately against the same material, both sheets gained the same type of charge, so they repelled each other.

Charges Acquired by Different Materials

Material	Rubbed with	Charge acquired
Ebonite	Woollen cloth or fur	Negative
Glass	Silk	Positive
Polystyrene	Woollen cloth or fur	Negative
Polythene	Woollen cloth or fur	Negative
Perspex	Woollen cloth	Positive
Cellulose	Woollen cloth	Positive

🔦 Spotlight — Fun Fact

The word “electricity” comes from the Greek word elektron, meaning amber. Ancient Greeks, around 600 BCE, found that rubbing amber with fur made it attract light objects. Thales of Miletus is credited with this first recorded observation of static electricity.

3. Origin of Charges

J.J. Thomson (1897) discovered the electron.
Ernest Rutherford (1909–1911) discovered the atomic nucleus, which is positively charged.
Atoms are normally neutral, with equal positive and negative charges.
An atom that loses an electron becomes a positive ion; one that gains an electron becomes a negative ion.
Charge on an electron = −1.6 × 10⁻¹⁹ C; charge on a proton = +1.6 × 10⁻¹⁹ C.

Atomic structure showing protons and electrons

Figure 2: Origin of charges: protons (positive) in the nucleus and electrons (negative) around it.

4. Conductors and Insulators

	Conductors	Insulators
Free electrons	Have free electrons that let charge move	Lack free electrons
Charge distribution	Charge spreads evenly over the surface	Charge stays where it was placed
Examples	Copper, iron, aluminium, silver, gold	Plastic, wood, rubber, glass, dry paper, pure water

Charge distribution in conductor vs insulator

Figure 3: Charge spreads over a conductor but stays localised on an insulator.

How to Tell a Conductor from an Insulator

Note where the charges are first placed.
After a short time, check where the charges are.
If they have spread evenly, the material is a conductor; if they stay in place, it is an insulator.

Air as a Conductor

Air is normally an insulator. At very high voltage it can become a conductor, as in sparks and lightning. A spark discharges excess charge from your body; lightning is a very large discharge from thunderclouds.

Separation and Conservation of Charge

When two neutral objects are rubbed together, each becomes charged. For example, when rubber is rubbed with wool, electrons move from the wool to the rubber: the rubber becomes negative and the wool positive.

Law of Conservation of Charge: the total charge of an isolated system remains constant. Charge is not created or destroyed, only transferred.

Quick check: Why do charges stay on the surface of a conductor and not inside?

Like charges repel, so the free electrons move as far apart as they can and end up on the outer surface.

5. Electrostatic Force and the Fundamental Law

The interaction between static electric charges produces a force called the electrostatic force.

✏️ Activity 1.2: Existence of Electrostatic Force

Materials: plastic pen or comb, tissue paper and human hair.

Tear the tissue paper into small pieces.
Rub the plastic pen through your hair.
Bring the pen near the paper pieces without touching them.
Observe for about 15 seconds.

Observation: the pen picks up pieces of paper. After some time the paper may drop as the charge is shared or neutralised.

Fundamental Law of Electrostatics

Like charges repel; unlike charges attract.

Two negatively charged rods repel.
Two positively charged rods repel.
A positive and a negative rod attract.

Verification of fundamental law with rods

Figure 4: Like-charged rods repel; opposite charges attract.

🔦 Spotlight — Bend Water!

You can bend a thin stream of water with a charged comb. The comb induces opposite charge on the polar water molecules, so the stream is pulled towards it. Try it with a plastic comb and a slow tap.

6. Charging Objects

A neutral object has equal numbers of positive and negative charges. There are three main ways to charge an object.

A. Charging by Friction

Rubbing two objects together transfers electrons.
Only insulators can be charged effectively by friction, because conductors let the charge flow away.
Examples: combing hair makes the comb negative; rubbing a glass rod with silk makes the glass positive and the silk negative; a wool sweater crackles when removed.

The Triboelectric Series

Materials are ranked by their tendency to gain or lose electrons. Those higher in the series tend to become positive; those lower tend to become negative. Example order, from positive end to negative end:

Rabbit fur → Hair → Glass → Wool → Silk → Paper → Rubber → Polystyrene → Polyethylene → Teflon

B. Charging by Contact (Conduction)

A charged body touches an uncharged body and charge moves across.
Both bodies end up with the same type of charge.

Figure 5: Charging a metal sphere by contact with a charged ebonite rod.

C. Charging by Induction

Charging without touching. To charge a spherical conductor using a negatively charged rod:

Bring the negative rod near the uncharged sphere; electrons are repelled to the far side.
Connect the sphere to earth; excess electrons flow to earth.
Remove the earth connection.
Remove the rod. The sphere is left with a net positive charge.

Induction leaves an object with the charge opposite to that of the charged object used.

Quick check: Can conductors be charged by friction? Explain.

Not effectively when held in the hand, because any charge produced flows through the body to earth. A conductor keeps its charge only if it is well insulated from the ground.

🔦 Spotlight — Inventor

The electrophorus, invented by Alessandro Volta in 1775, makes repeated charges by induction. It was a forerunner of modern electrostatic generators.

7. Detection of Charges: The Electroscope

Leaf Electroscope

Structure:

A metal cap (usually brass) on a metal rod.
A metal plate with a thin metal leaf (gold or aluminium) attached at the lower end.
A glass or metal case to protect against air currents.
An insulating plug that supports the rod.

Figure 6: Structure of a gold-leaf electroscope.

Charging the Electroscope

By contact: touch the cap with a charged rod; the electroscope takes the same charge and the leaf rises.
By induction: bring a charged rod near the cap; the electroscope takes the opposite charge and the leaf rises.

Discharging

Earthing, such as touching the cap with a finger, lets the excess charge flow to earth and the leaf falls.

Using the Electroscope

Charge on electroscope	Charge brought near	Effect on leaf
+	+	Divergence increases
−	−	Divergence increases
+	−	Divergence decreases
+ or −	Uncharged body	Divergence decreases

Uses: test the sign of charge on a body, test insulating properties of materials, and detect the presence of charge.

Quick check: The leaf of a charged electroscope falls when an unknown object is brought near. What does this show?

The object has the opposite charge to the electroscope (or is uncharged). Opposite charge reduces the net charge on the leaf side, so the leaf falls.

8. Electrostatic Potential

An electrostatic force field exists around any charge.
The work needed to move a unit positive charge against this field is the electrostatic potential.
Potential difference is measured in volts: 1 volt = 1 joule per coulomb.
Positive potential: electrons flow from earth to the body.
Negative potential: electrons flow from the body to earth.
Electrons (negative charges) move from regions of lower potential to regions of higher potential.

9. Capacitors

A capacitor stores electric charge. It has two conducting plates separated by an insulator (the dielectric), such as ceramic, plastic film, air, paper or mica.

Capacitance: C = Q ÷ V, where Q is the charge stored and V the potential difference across the plates.

SI unit: farad (F). One farad is the capacitance when 1 C of charge gives a p.d. of 1 V.

Unit	Value
1 mF	10⁻³ F
1 µF	10⁻⁶ F
1 nF	10⁻⁹ F
1 pF	10⁻¹² F

Example. A capacitor of 200 µF has a p.d. of 10 V. Find the charge.
Solution: Q = CV = (200 × 10⁻⁶) × 10 = 2 × 10⁻³ C = 2 mC

Types of Capacitors

Paper or plastic: metal foil with waxed paper or plastic film.
Mica: very stable, used in precision circuits.
Electrolytic: high capacitance, but polarity sensitive.
Air-filled (variable): used in radio tuning.

Charging and Discharging

When charging, the charge on the capacitor increases and levels off at a maximum. When discharging, the charge decreases exponentially.

Combining Capacitors

Arrangement	Formula	Example
Series	1/C = 1/C₁ + 1/C₂ + 1/C₃	10 µF, 20 µF and 30 µF in series: C = 60/11 ≈ 5.45 µF
Parallel	C = C₁ + C₂ + C₃	20 µF + 20 µF = 40 µF

Factors Affecting Capacitance

Area of plates (A): C ∝ A, so larger area gives larger capacitance.
Distance between plates (d): C ∝ 1/d, so smaller distance gives larger capacitance.
Dielectric: a material with higher permittivity gives larger capacitance.

Quick check: A capacitor is charged and then disconnected from the battery. What happens to its charge and voltage if the plate separation is increased?

The charge stays constant because the capacitor is isolated. Capacitance falls (C ∝ 1/d), so the voltage V = Q/C rises.

🔦 Spotlight — Supercapacitors

Supercapacitors store very large charge and release it quickly. They are used in regenerative braking in electric vehicles and can charge in seconds.

10. Charge Distribution on a Conductor

Charge on a conductor lies on its outer surface.
Its distribution depends on the shape of the conductor.
Charge concentrates at sharp points (high charge density).
In a hollow conductor, charge is only on the outer surface.

Experimental observations: an electroscope leaf rises more when a proof plane touches a sharp point than a flat surface, and no charge is found inside a hollow charged conductor.

This is why lightning rods have sharp points: the high charge density at the tip ionises the air and helps discharge the cloud safely.

11. Lightning and Thunderstorms

How Lightning Forms

Ice crystals and water droplets collide inside storm clouds.
Positive and negative charges separate: the lower part of the cloud becomes negative and the upper part positive.
The ground below becomes positively charged by induction.
When the charge difference is large enough, the air breaks down as an insulator.
A large current flows, ionising air molecules; this gives the flash of light (lightning) and rapid expansion of air (thunder).

The temperature in a lightning channel can reach about 27,000 °C, hotter than the surface of the Sun.

Lightning Protection

A lightning conductor has:

Copper spikes (or a pointed rod) at the top.
A thick copper rod or cable running down the building.
A copper plate buried deep in the earth.

Mode of action: a negatively charged cloud passes overhead; the sharp spikes become positively charged by induction; charge flows safely to the ground. The discharge path is through the conductor and not through the building.

🔦 Spotlight — Safety Tip

Never shelter under a tree in a thunderstorm; tall trees act as natural lightning rods. Outdoors, the safest place is a low ditch, away from tall objects. Lightning can strike the same place more than once.

12. Chapter Summary

Static electricity is the accumulation of excess charge on insulators.
Charge is conserved; electrons are transferred, not created.
Like charges repel; unlike charges attract.
Three charging methods: friction, contact and induction.
Conductors allow free electron movement; insulators do not.
The leaf electroscope detects the presence and sign of charge.
Capacitors store charge; C = Q/V; capacitance depends on area, plate separation and dielectric.
Lightning is a huge electrostatic discharge; lightning conductors give it a safe path to earth.

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