Grade 10 physics Electricity and Magnetism – Electrostatics Notes
Physics — 3.0 Electricity & Magnetism
Subtopic 3.1: Electrostatics
Target age: 15 years (KCSE/Form 3 level)
Specific learning outcomes (after studying this subtopic):
- Explain the origin of electric charges in matter (atoms, protons, electrons, neutrons).
- Describe methods of charging conductors: contact (friction and conduction), induction and separation (polarisation and separation).
- Illustrate how charge distributes on conductors of different shapes (sphere, wedge/sharp point, hollow conductor).
- Explain parts and functions of a leaf electroscope.
- Show charging of an electroscope by contact and by induction.
- Appreciate uses of an electroscope and practical applications of static electricity in daily life.
- Know unit of charge (coulomb), elementary charge and Coulomb's law of electrostatics (magnitude and direction of force).
1. Origin of charge (simple)
- Atoms are made of a nucleus (protons + neutrons) and electrons in orbit. Protons are positive (+), electrons negative (−), neutrons neutral.
- Charge comes from an imbalance in numbers of protons and electrons. Gaining electrons → net negative charge; losing electrons → net positive charge.
- Elementary charge e = 1.6 × 10⁻¹⁹ C (C = coulomb). Objects usually carry many such charges. SI unit of charge: coulomb (C).
- Charge comes from an imbalance in numbers of protons and electrons. Gaining electrons → net negative charge; losing electrons → net positive charge.
- Elementary charge e = 1.6 × 10⁻¹⁹ C (C = coulomb). Objects usually carry many such charges. SI unit of charge: coulomb (C).
2. Coulomb's law (magnitude of electrostatic force)
- Two point charges q1 and q2 separated by distance r attract or repel with magnitude:
F = k * |q1 q2| / r² , where k ≈ 9.0 × 10⁹ N·m²/C²
- Direction: like charges repel, unlike charges attract. Force acts along the line joining the charges.
- Example (simple): Two charges each 1 μC (1×10⁻⁶ C) separated by 0.5 m:
F ≈ 9×10⁹ × (1×10⁻⁶ × 1×10⁻⁶) / (0.5)² ≈ 3.6×10⁻² N.
3. Methods of charging conductors
- Charging by friction (triboelectric): rubbing two different materials transfers electrons. Example: comb on dry hair, balloon on wool. The object that gains electrons becomes negatively charged.
- Charging by contact (conduction): a charged object touches a conductor; charges move until conductor reaches same potential. Example: touching a neutral metal sphere with a charged rod transfers some charge.
- Charging by induction: bringing a charged object near (but not touching) a neutral conductor causes charges to separate (polarisation). If the conductor is then earthed and the earth connection removed, the conductor can be left with net charge opposite to the approaching charged object.
- Separation / polarization: in insulators charges cannot move far, but dipoles form (local separation) — important in electrostatic attraction of small neutral objects (paper bits).
- Charging by contact (conduction): a charged object touches a conductor; charges move until conductor reaches same potential. Example: touching a neutral metal sphere with a charged rod transfers some charge.
- Charging by induction: bringing a charged object near (but not touching) a neutral conductor causes charges to separate (polarisation). If the conductor is then earthed and the earth connection removed, the conductor can be left with net charge opposite to the approaching charged object.
- Separation / polarization: in insulators charges cannot move far, but dipoles form (local separation) — important in electrostatic attraction of small neutral objects (paper bits).
4. The leaf electroscope — parts and functions
Ball
Leaf electroscope (simplified)
Main parts
- Metal ball or plate (top) — receives charge.
- Metal rod — conducts charge to the leaves.
- Two thin metal leaves (usually aluminium or gold) — show charge by diverging (both leaves get same sign and repel).
- Insulating container (glass) — prevents leakage to air; base may be insulated.
Functions
- Detect presence and sign of charge (by bringing a known charged object close and observing leaf movement).
5. Charging an electroscope (two methods)
- By contact (conduction): touch the metal ball with a charged rod. Some charge flows to the leaves → leaves diverge (both carry same sign). Removing the rod leaves electroscope charged.
- By induction (without contact): bring a charged rod near the ball. The leaves change position due to separation of charges (attract then repel). While rod is near, earth the electroscope briefly (touch with finger) so opposite charges flow in/out to ground. Remove finger, then remove rod: electroscope left with net charge opposite in sign to the rod.
- By induction (without contact): bring a charged rod near the ball. The leaves change position due to separation of charges (attract then repel). While rod is near, earth the electroscope briefly (touch with finger) so opposite charges flow in/out to ground. Remove finger, then remove rod: electroscope left with net charge opposite in sign to the rod.
Practical classroom activity: Rub a plastic ruler with dry cloth (friction). Touch the electroscope ball briefly → leaves diverge. Explain movement and try charging by induction and observe differences.
6. Distribution of charge on conductors (key points)
- In electrostatic equilibrium within a conductor, electric field inside is zero. So excess charge resides on the outer surface.
- For an isolated spherical conductor, charges spread uniformly on the outer surface (uniform surface charge density if isolated and conductor is symmetrical).
- On sharp points and edges (wedge-shaped or pointed conductors), charges concentrate more; surface charge density is higher, producing strong local electric fields. This is why corona discharge and sparking occur at sharp points.
- Hollow conductor: inside cavity (with no internal charges) field is zero and no net surface charge appears on inner surface unless charges are placed inside the cavity.
- For an isolated spherical conductor, charges spread uniformly on the outer surface (uniform surface charge density if isolated and conductor is symmetrical).
- On sharp points and edges (wedge-shaped or pointed conductors), charges concentrate more; surface charge density is higher, producing strong local electric fields. This is why corona discharge and sparking occur at sharp points.
- Hollow conductor: inside cavity (with no internal charges) field is zero and no net surface charge appears on inner surface unless charges are placed inside the cavity.
Sphere
Charges spread evenly over outer surface.
Sharp point / wedge
Charge concentrates at point → high field → easier discharge.
7. Applications of electrostatics (everyday & industrial)
- Electrostatic spray painting / spray gun: paint droplets are charged so they are attracted to the oppositely charged or grounded object → even coating, less waste. Common in vehicle workshops and furniture making.
- Photocopiers (xerography): use charged drum and toner particles (charged) to form images on paper (common in schools and offices).
- Fingerprinting (forensics): charged powders can stick to charged residues; electrostatic methods can reveal prints on some surfaces.
- Electrostatic precipitators: used in industry to remove dust particles from exhaust gases by charging particles and collecting them on plates. Important for air pollution control.
- Lightning: giant electrostatic discharge between charged clouds and earth. Lightning rods (conductors) provide safe path to ground; earthing (grounding) is essential for electrical safety.
- Photocopiers (xerography): use charged drum and toner particles (charged) to form images on paper (common in schools and offices).
- Fingerprinting (forensics): charged powders can stick to charged residues; electrostatic methods can reveal prints on some surfaces.
- Electrostatic precipitators: used in industry to remove dust particles from exhaust gases by charging particles and collecting them on plates. Important for air pollution control.
- Lightning: giant electrostatic discharge between charged clouds and earth. Lightning rods (conductors) provide safe path to ground; earthing (grounding) is essential for electrical safety.
Kenyan context note: Electrostatic spray guns are used in spray-painting vehicles and furniture in Nairobi workshops. Photocopiers are widely used in schools and offices across Kenya. Awareness of lightning safety is important during rainy seasons across the country.
8. Lightning and safety measures
- Avoid standing under tall isolated trees during a storm. Stay indoors or in a metal-roofed car (car as Faraday cage).
- Install lightning rods and ensure proper earthing for buildings, especially tall structures and schools.
- Stay away from metal objects, electrical appliances, and plumbing during thunderstorms. Unplug sensitive electronics to avoid surges.
- Install lightning rods and ensure proper earthing for buildings, especially tall structures and schools.
- Stay away from metal objects, electrical appliances, and plumbing during thunderstorms. Unplug sensitive electronics to avoid surges.
9. Suggested classroom activities (safe, low-cost)
- Friction experiment: Rub a balloon or plastic ruler with dry wool or hair and pick up small paper bits. Observe attraction and explain electron transfer.
- Electroscope investigation: Build a simple electroscope from an aluminium can lid, copper wire and aluminium foil leaves. Test charging by contact and by induction; record leaf angle qualitatively.
- Charge distribution demo: Use two metal models (sphere and cone) connected to a Van de Graaff or high-voltage source (teacher demonstration only) to show corona or corona related glow at sharp points (if available). Alternatively, use diagrams and photos.
- Photocopier demo: Show how toner particles (charged) adhere to paper. Discuss advantages and limitations.
- Field trip / video: Visit a local workshop that uses spray painting (with teacher permission) or show video demonstrating electrostatic precipitators in factories.
- Electroscope investigation: Build a simple electroscope from an aluminium can lid, copper wire and aluminium foil leaves. Test charging by contact and by induction; record leaf angle qualitatively.
- Charge distribution demo: Use two metal models (sphere and cone) connected to a Van de Graaff or high-voltage source (teacher demonstration only) to show corona or corona related glow at sharp points (if available). Alternatively, use diagrams and photos.
- Photocopier demo: Show how toner particles (charged) adhere to paper. Discuss advantages and limitations.
- Field trip / video: Visit a local workshop that uses spray painting (with teacher permission) or show video demonstrating electrostatic precipitators in factories.
10. Summary
- Charge arises from electrons and protons; electrons move in conductors. Objects can be charged by friction, contact or induction.
- Coulomb's law gives the force between charges. In conductors, excess charges sit on the surface and concentrate at sharp points.
- Electroscopes are useful detectors of charge. Electrostatics has useful applications (spray painting, photocopiers, dust removal) but also hazards (lightning) that require safety measures.
- Coulomb's law gives the force between charges. In conductors, excess charges sit on the surface and concentrate at sharp points.
- Electroscopes are useful detectors of charge. Electrostatics has useful applications (spray painting, photocopiers, dust removal) but also hazards (lightning) that require safety measures.
Revision questions
- Explain how rubbing a balloon on hair results in the balloon sticking to a wall.
- Describe step-by-step how to charge an electroscope by induction and explain why leaves diverge.
- Why do charges concentrate at sharp points on a conductor? Give one practical consequence.
- State Coulomb's law and solve: two point charges +2 μC and −3 μC are 0.2 m apart. Find the force between them (magnitude and type).
- List three everyday uses of electrostatics and explain one in detail.
Notes prepared for Kenyan secondary-school learners (age ~15). Simple diagrams and demonstrations above are intended for classroom use with teacher supervision.